&reftitle.setup;
&reftitle.required;

`PHP 5.3.6` or newer. Some advanced functionality requires `PHP 5.4.0` or newer.

The `mysqlnd_ms` replication and load balancing plugin supports all PHP applications and all available PHP MySQL extensions (mysqli, mysql, PDO_MYSQL). The PHP MySQL extension must be configured to use mysqlnd in order to be able to use the `mysqlnd_ms` plugin for mysqlnd.

&reference.mysqlnd-ms.configure; &reference.mysqlnd-ms.ini;

## Plugin configuration file (>=1.1.x)

### Changelog: Feature was added in PECL/mysqlnd_ms 1.1.0-beta

The below description applies to PECL/mysqlnd_ms >= 1.1.0-beta. It is not valid for prior versions.

The plugin uses its own configuration file. The configuration file holds information about the MySQL replication master server, the MySQL replication slave servers, the server pick (load balancing) policy, the failover strategy, and the use of lazy connections.

The plugin loads its configuration file at the beginning of a web request. It is then cached in memory and used for the duration of the web request. This way, there is no need to restart PHP after deploying the configuration file. Configuration file changes will become active almost instantly.

The PHP configuration directive `mysqlnd_ms.config_file` is used to set the plugins configuration file. Please note, that the PHP configuration directive may not be evaluated for every web request. Therefore, changing the plugins configuration file name or location may require a PHP restart. However, no restart is required to read changes if an already existing plugin configuration file is updated.

Using and parsing JSON is efficient, and using JSON makes it easier to express hierarchical data structures than the standard `php.ini` format.

### Converting a PHP array (hash) into JSON format

Or alternatively, a developer may be more familiar with the PHP `array` syntax, and prefer it. This example demonstrates how a developer might convert a PHP array to JSON.

    <?php
    $config = array(
      "myapp" => array(
        "master" => array(
          "master_0" => array(
            "host"   => "localhost",
            "socket" => "/tmp/mysql.sock",
          ),
        ),
        "slave" => array(),
      ),
    );

    file_put_contents("mysqlnd_ms.ini", json_encode($config, JSON_PRETTY_PRINT));
    printf("mysqlnd_ms.ini file created...\n");
    printf("Dumping file contents...\n");
    printf("%s\n", str_repeat("-", 80));
    echo file_get_contents("mysqlnd_ms.ini");
    printf("\n%s\n", str_repeat("-", 80));
    ?>

&example.outputs;

    mysqlnd_ms.ini file created...
    Dumping file contents...
    --------------------------------------------------------------------------------
    {
        "myapp": {
            "master": {
                "master_0": {
                    "host": "localhost",
                    "socket": "\/tmp\/mysql.sock"
                }
            },
            "slave": [

            ]
        }
    }
    --------------------------------------------------------------------------------

A plugin configuration file consists of one or more sections. Sections are represented by the top-level object properties of the object encoded in the JSON file. Sections could also be called configuration names.

Applications reference sections by their name. Applications use section names as the host (server) parameter to the various connect methods of the mysqli, mysql and PDO_MYSQL extensions. Upon connect, the mysqlnd plugin compares the hostname with all of the section names from the plugin configuration file. If the hostname and section name match, then the plugin will load the settings for that section.

### Using section names example

    {
        "myapp": {
            "master": {
                "master_0": {
                    "host": "localhost"
                }
            },
            "slave": {
                "slave_0": {
                    "host": "192.168.2.27"
                },
                "slave_1": {
                    "host": "192.168.2.27",
                    "port": 3306
                }
            }
        },
        "localhost": {
            "master": [
                {
                    "host": "localhost",
                    "socket": "\/path\/to\/mysql.sock"
                }
            ],
            "slave": [
                {
                    "host": "192.168.3.24",
                    "port": "3305"
                },
                {
                    "host": "192.168.3.65",
                    "port": "3309"
                }
            ]
        }
    }

    <?php
    /* All of the following connections will be load balanced */
    $mysqli = new mysqli("myapp", "username", "password", "database");
    $pdo = new PDO('mysql:host=myapp;dbname=database', 'username', 'password');
    $mysql = mysql_connect("myapp", "username", "password");

    $mysqli = new mysqli("localhost", "username", "password", "database");
    ?>

Section names are strings. It is valid to use a section name such as `192.168.2.1`, `127.0.0.1` or `localhost`. If, for example, an application connects to `localhost` and a plugin configuration section `localhost` exists, the semantics of the connect operation are changed. The application will no longer only use the MySQL server running on the host `localhost`, but the plugin will start to load balance MySQL queries following the rules from the `localhost` configuration section. This way you can load balance queries from an application without changing the applications source code. Please keep in mind, that such a configuration may not contribute to overall readability of your applications source code. Using section names that can be mixed up with host names should be seen as a last resort.

Each configuration section contains, at a minimum, a list of master servers and a list of slave servers. The master list is configured with the keyword `master`, while the slave list is configured with the `slave` keyword. Failing to provide a slave list will result in a fatal `E_ERROR` level error, although a slave list may be empty. It is possible to allow no slaves. However, this is only recommended with synchronous clusters, please see also supported clusters. The main part of the documentation focusses on the use of asynchronous MySQL replication clusters.

The master and slave server lists can be optionally indexed by symbolic names for the servers they describe. Alternatively, an array of descriptions for slave and master servers may be used.

### List of anonymous slaves

    "slave": [
        {
            "host": "192.168.3.24",
            "port": "3305"
        },
        {
            "host": "192.168.3.65",
            "port": "3309"
        }
    ]

An anonymous server list is encoded by the `JSON array` type. Optionally, symbolic names may be used for indexing the slave or master servers of a server list, and done so using the `JSON object` type.

### Master list using symbolic names

    "master": {
        "master_0": {
            "host": "localhost"
        }
    }

It is recommended to index the server lists with symbolic server names. The alias names will be shown in error messages.

The order of servers is preserved and taken into account by mysqlnd_ms. If, for example, you configure round robin load balancing strategy, the first `SELECT` statement will be executed on the slave that appears first in the slave server list.

A configured server can be described with the `host`, `port`, `socket`, `db`, `user`, `password` and `connect_flags`. It is mandatory to set the database server host using the `host` keyword. All other settings are optional.

### Keywords to configure a server

    {
        "myapp": {
            "master": {
                "master_0": {
                    "host": "db_server_host",
                    "port": "db_server_port",
                    "socket": "db_server_socket",
                    "db": "database_resp_schema",
                    "user": "user",
                    "password": "password",
                    "connect_flags": 0
                }
            },
            "slave": {
                "slave_0": {
                    "host": "db_server_host",
                    "port": "db_server_port",
                    "socket": "db_server_socket"
                }
            }
        }
    }

If a setting is omitted, the plugin will use the value provided by the user API call used to open a connection. Please, see the using section names example above.

The configuration file format has been changed in version 1.1.0-beta to allow for chained filters. Filters are responsible for filtering the configured list of servers to identify a server for execution of a given statement. Filters are configured with the `filter` keyword. Filters are executed by mysqlnd_ms in the order of their appearance. Defining filters is optional. A configuration section in the plugins configuration file does not need to have a `filters` entry.

Filters replace the `pick[]` setting from prior versions. The new `random` and `roundrobin` provide the same functionality.

### New `roundrobin` filter, old functionality

       {
        "myapp": {
            "master": {
                "master_0": {
                    "host": "localhost"
                }
            },
            "slave": {
                "slave_0": {
                    "host": "192.168.78.136",
                    "port": "3306"
                },
                "slave_1": {
                    "host": "192.168.78.137",
                    "port": "3306"
                }
            },
            "filters": {
                "roundrobin": [

                ]
            }
        }
    }

The function `mysqlnd_ms_set_user_pick_server` has been removed. Setting a callback is now done with the `user` filter. Some filters accept parameters. The `user` filter requires and accepts a mandatory `callback` parameter to set the callback previously set through the function `mysqlnd_ms_set_user_pick_server`.

### The `user` filter replaces `mysqlnd_ms_set_user_pick_server`

    "filters": {
        "user": {
            "callback": "pick_server"
        }
    }

The validity of the configuration file is checked both when reading the configuration file and later when establishing a connection. The configuration file is read during PHP request startup. At this early stage a PHP extension may not display error messages properly. In the worst case, no error is shown and a connection attempt fails without an adequate error message. This problem has been cured in version 1.5.0.

### Common error message in case of configuration file issues (upto version 1.5.0)

    <?php
    $mysqli = new mysqli("myapp", "username", "password", "database");
    ?>

&example.outputs;

    Warning: mysqli::mysqli(): (mysqlnd_ms) (mysqlnd_ms) Failed to parse config file [s1.json]. Please, verify the JSON in Command line code

    Warning: mysqli::mysqli(): (HY000/2002): php_network_getaddresses: getaddrinfo failed: Name or service not known in Command line code on line 1

    Warning: mysqli::query(): Couldn't fetch mysqli in Command line code on line 1

    Fatal error: Call to a member function fetch_assoc() on a non-object in Command line code on line 1

Since version 1.5.0 startup errors are additionally buffered and emitted when a connection attempt is made. Use the configuration directive `mysqlnd_ms.force_config_usage` to set the error type used to display buffered errors. By default an error of type `E_WARNING` will be emitted.

### Improved configuration file validation since 1.5.0

    <?php
    $mysqli = new mysqli("myapp", "username", "password", "database");
    ?>

&example.outputs;

    Warning: mysqli::mysqli(): (mysqlnd_ms) (mysqlnd_ms) Failed to parse config file [s1.json]. Please, verify the JSON in Command line code on line 1

It can be useful to set `mysqlnd_ms.force_config_usage = 1` when debugging potential configuration file errors. This will not only turn the type of buffered startup errors into `E_RECOVERABLE_ERROR` but also help detecting misspelled section names.

### Possibly more precise error due to `mysqlnd_ms.force_config_usage=1`

    mysqlnd_ms.force_config_usage=1

    <?php
    $mysqli = new mysqli("invalid_section", "username", "password", "database");
    ?>

&example.outputs;

    Warning: mysqli::mysqli(): (mysqlnd_ms) Exclusive usage of configuration enforced but did not find the correct INI file section (invalid_section) in Command line code on line 1 line 1

Here is a short explanation of the configuration directives that can be used.

`master` `array or object`

- List of MySQL replication master servers. The list of either of the `JSON type array` to declare an anonymous list of servers or of the `JSON type object`. Please, see above for examples.

  Setting at least one master server is mandatory. The plugin will issue an error of type `E_ERROR` if the user has failed to provide a master server list for a configuration section. The fatal error may read `(mysqlnd_ms) Section [master] doesn't exist for host [name_of_a_config_section] in %s on line %d`.

  A server is described with the `host`, `port`, `socket`, `db`, `user`, `password` and `connect_flags`. It is mandatory to provide at a value for `host`. If any of the other values is not given, it will be taken from the user API connect call, please, see also: using section names example.

  Table of server configuration keywords.

  Keyword | Description | Version
  `host` | Database server host. This is a mandatory setting. Failing to provide, will cause an error of type `E_RECOVERABLE_ERROR` when the plugin tries to connect to the server. The error message may read `(mysqlnd_ms) Cannot find [host] in [%s] section in config in %s on line %d`. | Since 1.1.0.
  `port` | Database server TCP/IP port. | Since 1.1.0.
  `socket` | Database server Unix domain socket. | Since 1.1.0.
  `db` | Database (schemata). | Since 1.1.0.
  `user` | MySQL database user. | Since 1.1.0.
  `password` | MySQL database user password. | Since 1.1.0.
  `connect_flags` | Connection flags. | Since 1.1.0.

  The plugin supports using only one master server. An experimental setting exists to enable multi-master support. The details are not documented. The setting is meant for development only.

`slave` `array or object`

- List of one or more MySQL replication slave servers. The syntax is identical to setting master servers, please, see `master` above for details.

  The plugin supports using one or more slave servers.

  Setting a list of slave servers is mandatory. The plugin will report an error of the type `E_ERROR` if `slave` is not given for a configuration section. The fatal error message may read `(mysqlnd_ms) Section [slave] doesn't exist for host [%s] in %s on line %d`. Note, that it is valid to use an empty slave server list. The error has been introduced to prevent accidently setting no slaves by forgetting about the `slave` setting. A master-only setup is still possible using an empty slave server list.

  If an empty slave list is configured and an attempt is made to execute a statement on a slave the plugin may emit a warning like `mysqlnd_ms) Couldn't find the appropriate slave connection. 0 slaves to choose from.` upon statement execution. It is possible that another warning follows such as `(mysqlnd_ms) No connection selected by the last filter`.

`global_transaction_id_injection` `array or object`

- Global transaction identifier configuration related to both the use of the server built-in global transaction ID feature and the client-side emulation.

  Keyword | Description | Version
  `fetch_last_gtid` | SQL statement for accessing the latest global transaction identifier. The SQL statement is run if the plugin needs to know the most recent global transaction identifier. This can be the case, for example, when checking MySQL Replication slave status. Also used with `mysqlnd_ms_get_last_gtid`. | Since 1.2.0.
  `check_for_gtid` | SQL statement for checking if a replica has replicated all transactions up to and including ones searched for. The SQL statement is run when searching for replicas which can offer a higher level of consistency than eventual consistency. The statement must contain a placeholder `#GTID` which is to be replaced with the global transaction identifier searched for by the plugin. Please, check the quickstart for examples. | Since 1.2.0.
  `report_errors` | Whether to emit an error of type warning if an issue occurs while executing any of the configured SQL statements. | Since 1.2.0.
  `on_commit` | Client-side global transaction ID emulation only. SQL statement to run when a transaction finished to update the global transaction identifier sequence number on the master. Please, see the quickstart for examples. | Since 1.2.0.
  `wait_for_gtid_timeout` | Instructs the plugin to wait up to `wait_for_gtid_timeout` seconds for a slave to catch up when searching for slaves that can deliver session consistency. The setting limits the time spend for polling the slave status. If polling the status takes very long, the total clock time spend waiting may exceed `wait_for_gtid_timeout`. The plugin calls `sleep(1)` to sleep one second between each two polls. The setting can be used both with the plugins client-side emulation and the server-side global transaction identifier feature of MySQL 5.6. Waiting for a slave to replicate a certain GTID needed for session consistency also means throttling the client. By throttling the client the write load on the master is reduced indirectly. A primary copy based replication system, such as MySQL Replication, is given more time to reach a consistent state. This can be desired, for example, to increase the number of data copies for high availability considerations or to prevent the master from being overloaded. | Since 1.4.0.

`filters` `object`

- List of filters. A filter is responsible to filter the list of available servers for executing a given statement. Filters can be chained. The `random` and `roundrobin` filter replace the `pick[]` directive used in prior version to select a load balancing policy. The `user` filter replaces the `mysqlnd_ms_set_user_pick_server` function.

  Filters may accept parameters to refine their actions.

  If no load balancing policy is set, the plugin will default to `random_once`. The `random_once` policy picks a random slave server when running the first read-only statement. The slave server will be used for all read-only statements until the PHP script execution ends. No load balancing policy is set and thus, defaulting takes place, if neither the `random` nor the `roundrobin` are part of a configuration section.

  If a filter chain is configured so that a filter which output no more than once server is used as input for a filter which should be given more than one server as input, the plugin may emit a warning upon opening a connection. The warning may read: `(mysqlnd_ms) Error while creating filter '%s' . Non-multi filter '%s' already created. Stopping in %s on line %d`. Furthermore, an error of the error code `2000`, the sql state `HY000` and an error message similar to the warning may be set on the connection handle.

  ### Invalid filter sequence

             {
          "myapp": {
              "master": {
                  "master_0": {
                      "host": "localhost"
                  }
              },
              "slave": {
                  "slave_0": {
                      "host": "192.168.78.136",
                      "port": "3306"
                  }
              },
              "filters": [
                  "roundrobin",
                  "random"
              ]
          }
      }

      <?php
      $link = new mysqli("myapp", "root", "", "test");
      printf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error());
      $link->query("SELECT 1 FROM DUAL");
      ?>

  &example.outputs;

      PHP Warning:  mysqli::mysqli(): (HY000/2000): (mysqlnd_ms) Error while creating filter 'random' . Non-multi filter 'roundrobin' already created. Stopping in filter_warning.php on line 1
      [2000] (mysqlnd_ms) Error while creating filter 'random' . Non-multi filter 'roundrobin' already created. Stopping
      PHP Warning:  mysqli::query(): Couldn't fetch mysqli in filter_warning.php on line 3

Filter: `random` `object`

- The `random` filter features the random and random once load balancing policies, set through the `pick[]` directive in older versions.

  The random policy will pick a random server whenever a read-only statement is to be executed. The random once strategy picks a random slave server once and continues using the slave for the rest of the PHP web request. Random once is a default, if load balancing is not configured through a filter.

  If the `random` filter is not given any arguments, it stands for random load balancing policy.

  ### Random load balancing with `random` filter

      {
          "myapp": {
              "master": {
                  "master_0": {
                      "host": "localhost"
                  }
              },
              "slave": {
                  "slave_0": {
                      "host": "192.168.78.136",
                      "port": "3306"
                  },
                  "slave_1": {
                      "host": "192.168.78.137",
                      "port": "3306"
                  }
              },
              "filters": [
                  "random"
              ]
          }
      }

  Optionally, the `sticky` argument can be passed to the filter. If the parameter `sticky` is set to the string `1`, the filter follows the random once load balancing strategy.

  ### Random once load balancing with `random` filter

      {
          "filters": {
              "random": {
                  "sticky": "1"
              }
          }
      }

  Both the `random` and `roundrobin` filters support setting a priority, a weight for a server, since PECL/mysqlnd_ms 1.4.0. If the `weight` argument is passed to the filter, it must assign a weight for all servers. Servers must be given an alias name in the `slave` respectively `master` server lists. The alias must be used to reference servers for assigning a priority with `weight`.

  ### Referencing error

      [E_RECOVERABLE_ERROR] mysqli_real_connect(): (mysqlnd_ms) Unknown server 'slave3' in 'random' filter configuration. Stopping in %s on line %d

  Using a wrong alias name with `weight` may result in an error similar to the shown above.

  If `weight` is omitted, the default weight of all servers is one.

  ### Assigning a `weight` for load balancing

      {
         "myapp": {
             "master": {
                 "master1":{
                     "host":"localhost",
                     "socket":"\/var\/run\/mysql\/mysql.sock"
                 }
             },
             "slave": {
                 "slave1": {
                     "host":"192.168.2.28",
                     "port":3306
                 },
                 "slave2": {
                     "host":"192.168.2.29",
                     "port":3306
                 },
                 "slave3": {
                     "host":"192.0.43.10",
                     "port":3306
                 },
             },
             "filters": {
                 "random": {
                     "weights": {
                         "slave1":8,
                         "slave2":4,
                         "slave3":1,
                         "master1":1
                     }
                 }
             }
         }
      }

  At the average a server assigned a weight of two will be selected twice as often as a server assigned a weight of one. Different weights can be assigned to reflect differently sized machines, to prefer co-located slaves which have a low network latency or, to configure a standby failover server. In the latter case, you may want to assign the standby server a very low weight in relation to the other servers. For example, given the configuration above `slave3` will get only some eight percent of the requests in the average. As long as `slave1` and `slave2` are running, it will be used sparsely, similar to a standby failover server. Upon failure of `slave1` and `slave2`, the usage of `slave3` increases. Please, check the notes on failover before using `weight` this way.

  Valid weight values range from 1 to 65535.

  Unknown arguments are ignored by the filter. No warning or error is given.

  The filter expects one or more servers as input. Outputs one server. A filter sequence such as `random`, `roundrobin` may cause a warning and an error message to be set on the connection handle when executing a statement.

  List of filter arguments.

  Keyword | Description | Version
  `sticky` | Enables or disabled random once load balancing policy. See above. | Since 1.2.0.
  `weight` | Assigns a load balancing weight/priority to a server. Please, see above for a description. | Since 1.4.0.

Filter: `roundrobin` `object`

- If using the `roundrobin` filter, the plugin iterates over the list of configured slave servers to pick a server for statement execution. If the plugin reaches the end of the list, it wraps around to the beginning of the list and picks the first configured slave server.

  ### `roundrobin` filter

             {
          "myapp": {
              "master": {
                  "master_0": {
                      "host": "localhost"
                  }
              },
              "slave": {
                  "slave_0": {
                      "host": "192.168.78.136",
                      "port": "3306"
                  }
              },
              "filters": [
                  "roundrobin"
              ]
          }
      }

  Expects one or more servers as input. Outputs one server. A filter sequence such as `roundrobin`, `random` may cause a warning and an error message to be set on the connection handle when executing a statement.

  List of filter arguments.

  Keyword | Description | Version
  `weight` | Assigns a load balancing weight/priority to a server. Please, find a description above. | Since 1.4.0.

Filter: `user` `object`

- The `user` replaces `mysqlnd_ms_set_user_pick_server` function, which was removed in 1.1.0-beta. The filter sets a callback for user-defined read/write splitting and server selection.

  The plugins built-in read/write query split mechanism decisions can be overwritten in two ways. The easiest way is to prepend a query string with the SQL hints `MYSQLND_MS_MASTER_SWITCH`, `MYSQLND_MS_SLAVE_SWITCH` or `MYSQLND_MS_LAST_USED_SWITCH`. Using SQL hints one can control, for example, whether a query shall be send to the MySQL replication master server or one of the slave servers. By help of SQL hints it is not possible to pick a certain slave server for query execution.

  Full control on server selection can be gained using a callback function. Use of a callback is recommended to expert users only because the callback has to cover all cases otherwise handled by the plugin.

  The plugin will invoke the callback function for selecting a server from the lists of configured master and slave servers. The callback function inspects the query to run and picks a server for query execution by returning the hosts URI, as found in the master and slave list.

  If the lazy connections are enabled and the callback chooses a slave server for which no connection has been established so far and establishing the connection to the slave fails, the plugin will return an error upon the next action on the failed connection, for example, when running a query. It is the responsibility of the application developer to handle the error. For example, the application can re-run the query to trigger a new server selection and callback invocation. If so, the callback must make sure to select a different slave, or check slave availability, before returning to the plugin to prevent an endless loop.

  ### Setting a callback

      {
          "myapp": {
              "master": {
                  "master_0": {
                      "host": "localhost"
                  }
              },
              "slave": {
                  "slave_0": {
                      "host": "192.168.78.136",
                      "port": "3306"
                  }
              },
              "filters": {
                  "user": {
                      "callback": "pick_server"
                  }
              }
          }
      }

  The callback is supposed to return a host to run the query on. The host URI is to be taken from the master and slave connection lists passed to the callback function. If callback returns a value neither found in the master nor in the slave connection lists the plugin will emit an error of the type `E_RECOVERABLE_ERROR` The error may read like `(mysqlnd_ms) User filter callback has returned an unknown server. The server 'server that is not in master or slave list' can neither be found in the master list nor in the slave list`. If the application catches the error to ignore it, follow up errors may be set on the connection handle, for example, `(mysqlnd_ms) No connection selected by the last filter` with the error code `2000` and the sqlstate `HY000`. Furthermore a warning may be emitted.

  Referencing a non-existing function as a callback will result in any error of the type `E_RECOVERABLE_ERROR` whenever the plugin tries to callback function. The error message may reads like: `(mysqlnd_ms) Specified callback (pick_server) is not a valid callback`. If the application catches the error to ignore it, follow up errors may be set on the connection handle, for example, `(mysqlnd_ms) Specified callback (pick_server) is not a valid callback` with the error code `2000` and the sqlstate `HY000`. Furthermore a warning may be emitted.

  The following parameters are passed from the plugin to the callback.

  Parameter | Description | Version
  `connected_host` | URI of the currently connected database server. | Since 1.1.0.
  `query` | Query string of the statement for which a server needs to be picked. | Since 1.1.0.
  `masters` | List of master servers to choose from. Note, that the list of master servers may not be identical to the list of configured master servers if the filter is not the first in the filter chain. Previously run filters may have reduced the master list already. | Since 1.1.0.
  `slaves` | List of slave servers to choose from. Note, that the list of master servers may not be identical to the list of configured master servers if the filter is not the first in the filter chain. Previously run filters may have reduced the master list already. | Since 1.1.0.
  `last_used_connection` | URI of the server of the connection used to execute the previous statement on. | Since 1.1.0.
  `in_transaction` | Boolean flag indicating whether the statement is part of an open transaction. If autocommit mode is turned off, this will be set to &true;. Otherwise it is set to &false;. Transaction detection is based on monitoring the mysqlnd library call `set_autocommit`. Monitoring is not possible before PHP 5.4.0. Please, see connection pooling and switching concepts discussion for further details. | Since 1.1.0.

  ### Using a callback

      {
          "myapp": {
              "master": {
                  "master_0": {
                      "host": "localhost"
                  }
              },
              "slave": {
                  "slave_0": {
                      "host": "192.168.2.27",
                      "port": "3306"
                  },
                  "slave_1": {
                      "host": "192.168.78.136",
                      "port": "3306"
                  }
              },
              "filters": {
                  "user": {
                      "callback": "pick_server"
                  }
              }
          }
      }

      <?php
      function pick_server($connected, $query, $masters, $slaves, $last_used_connection, $in_transaction)
      {
       static $slave_idx = 0;
       static $num_slaves = NULL;
       if (is_null($num_slaves))
        $num_slaves = count($slaves);

       /* default: fallback to the plugins build-in logic */
       $ret = NULL;

       printf("User has connected to '%s'...\n", $connected);
       printf("... deciding where to run '%s'\n", $query);

       $where = mysqlnd_ms_query_is_select($query);
       switch ($where)
       {
        case MYSQLND_MS_QUERY_USE_MASTER:
         printf("... using master\n");
         $ret = $masters[0];
         break;
        case MYSQLND_MS_QUERY_USE_SLAVE:
         /* SELECT or SQL hint for using slave */
         if (stristr($query, "FROM table_on_slave_a_only"))
         {
          /* a table which is only on the first configured slave  */
          printf("... access to table available only on slave A detected\n");
          $ret = $slaves[0];
         }
         else
         {
          /* round robin */
          printf("... some read-only query for a slave\n");
          $ret = $slaves[$slave_idx++ % $num_slaves];
         }
         break;
        case MYSQLND_MS_QUERY_LAST_USED:
         printf("... using last used server\n");
         $ret = $last_used_connection;
         break;
       }

       printf("... ret = '%s'\n", $ret);
       return $ret;
      }

      $mysqli = new mysqli("myapp", "root", "", "test");

      if (!($res = $mysqli->query("SELECT 1 FROM DUAL")))
       printf("[%d] %s\n", $mysqli->errno, $mysqli->error);
      else
       $res->close();

      if (!($res = $mysqli->query("SELECT 2 FROM DUAL")))
       printf("[%d] %s\n", $mysqli->errno, $mysqli->error);
      else
       $res->close();

      if (!($res = $mysqli->query("SELECT * FROM table_on_slave_a_only")))
       printf("[%d] %s\n", $mysqli->errno, $mysqli->error);
      else
       $res->close();

      $mysqli->close();
      ?>

  &example.outputs;

      User has connected to 'myapp'...
      ... deciding where to run 'SELECT 1 FROM DUAL'
      ... some read-only query for a slave
      ... ret = 'tcp://192.168.2.27:3306'
      User has connected to 'myapp'...
      ... deciding where to run 'SELECT 2 FROM DUAL'
      ... some read-only query for a slave
      ... ret = 'tcp://192.168.78.136:3306'
      User has connected to 'myapp'...
      ... deciding where to run 'SELECT * FROM table_on_slave_a_only'
      ... access to table available only on slave A detected
      ... ret = 'tcp://192.168.2.27:3306'

Filter: `user_multi` `object`

- The `user_multi` differs from the `user` only in one aspect. Otherwise, their syntax is identical. The `user` filter must pick and return exactly one node for statement execution. A filter chain usually ends with a filter that emits only one node. The filter chain shall reduce the list of candidates for statement execution down to one. This, only one node left, is the case after the `user` filter has been run.

  The `user_multi` filter is a multi filter. It returns a list of slave and a list of master servers. This list needs further filtering to identify exactly one node for statement execution. A multi filter is typically placed at the top of the filter chain. The `quality_of_service` filter is another example of a multi filter.

  The return value of the callback set for `user_multi` must be an an array with two elements. The first element holds a list of selected master servers. The second element contains a list of selected slave servers. The lists shall contain the keys of the slave and master servers as found in the slave and master lists passed to the callback. The below example returns random master and slave lists extracted from the functions input.

  ### Returning random masters and slaves

      <?php
      function pick_server($connected, $query, $masters, $slaves, $last_used_connection, $in_transaction)
      {
        $picked_masters = array()
        foreach ($masters as $key => $value) {
          if (mt_rand(0, 2) > 1)
            $picked_masters[] = $key;
        }
        $picked_slaves = array()
        foreach ($slaves as $key => $value) {
          if (mt_rand(0, 2) > 1)
            $picked_slaves[] = $key;
        }
        return array($picked_masters, $picked_slaves);
      }
      ?>

  The plugin will issue an error of type `E_RECOVERABLE` if the callback fails to return a server list. The error may read `(mysqlnd_ms) User multi filter callback has not returned a list of servers to use. The callback must return an array in %s on line %d`. In case the server list is not empty but has invalid servers key/ids in it, an error of type `E_RECOVERABLE` will the thrown with an error message like `(mysqlnd_ms) User multi filter callback has returned an invalid list of servers to use. Server id is negative in %s on line %d`, or similar.

  Whether an error is emitted in case of an empty slave or master list depends on the configuration. If an empty master list is returned for a write operation, it is likely that the plugin will emit a warning that may read `(mysqlnd_ms) Couldn't find the appropriate master connection. 0 masters to choose from. Something is wrong in %s on line %d`. Typically a follow up error of type `E_ERROR` will happen. In case of a read operation and an empty slave list the behavior depends on the fail over configuration. If fail over to master is enabled, no error should appear. If fail over to master is deactivated the plugin will emit a warning that may read `(mysqlnd_ms) Couldn't find the appropriate slave connection. 0 slaves to choose from. Something is wrong in %s on line %d`.

Filter: `node_groups` `object`

- The `node_groups` filter lets you group cluster nodes and query selected groups, for example, to support data partitioning. Data partitioning can be required for manual sharding, primary copy based clusters running multiple masters, or to avoid hot spots in update everywhere clusters that have no built-in partitioning. The filter is a multi filter which returns zero, one or multiple of its input servers. Thus, it must be followed by other filters to reduce the number of candidates down to one for statement execution.

  Keyword | Description | Version
  `user defined node group name` | One or more node groups must be defined. A node group can have an arbitrary user defined name. The name is used in combination with a SQL hint to restrict query execution to the nodes listed for the node group. To run a query on any of the servers of a node group, the query must begin with the SQL hint `/*user defined node group name*/`. Please note, no white space is allowed around `user defined node group name`. Because `user defined node group name` is used as-is as part of a SQL hint, you should choose the name that is compliant with the SQL language. Each node group entry must contain a list of `master` servers. Additional `slave` servers are allowed. Failing to provide a list of `master` for a node group `name_of_group` may cause an error of type `E_RECOVERABLE_ERROR` like `(mysqlnd_ms) No masters configured in node group 'name_of_group' for 'node_groups' filter`. The list of master and slave servers must reference corresponding entries in the global master respectively slave server list. Referencing an unknown server in either of the both server lists may cause an `E_RECOVERABLE_ERROR` error like `(mysqlnd_ms) Unknown master 'server_alias_name' (section 'name_of_group') in 'node_groups' filter configuration`. ### Manual partitioning
       {
        "myapp": {
             "master": {
                  "master_0": {
                      "host": "localhost",
                      "socket": "\/tmp\/mysql.sock"
                  }
              },
              "slave": {
                  "slave_0": {
                      "host": "192.168.2.28",
                      "port": 3306
                  },
                  "slave_1": {
                      "host": "127.0.0.1",
                      "port": 3311
                  }
              },
              "filters": {
                  "node_groups": {
                      "Partition_A" : {
                          "master": ["master_0"],
                          "slave": ["slave_0"]
                      }
                  },
                 "roundrobin": []
              }
          }
      }
  Please note, if a filter chain generates an empty slave list and the PHP configuration directive `mysqlnd_ms.multi_master=0` is used, the plugin may emit a warning. | Since 1.5.0.

Filter: `quality_of_service` `object`

- The `quality_of_service` identifies cluster nodes capable of delivering a certain quality of service. It is a multi filter which returns zero, one or multiple of its input servers. Thus, it must be followed by other filters to reduce the number of candidates down to one for statement execution.

  The `quality_of_service` filter has been introduced in 1.2.0-alpha. In the 1.2 series the filters focus is on the consistency aspect of service quality. Different types of clusters offer different default data consistencies. For example, an asynchronous MySQL replication slave offers eventual consistency. The slave may not be able to deliver requested data because it has not replicated the write, it may serve stale database because its lagging behind or it may serve current information. Often, this is acceptable. In some cases higher consistency levels are needed for the application to work correct. In those cases, the `quality_of_service` can filter out cluster nodes which cannot deliver the necessary quality of service.

  The `quality_of_service` filter can be replaced or created at runtime. A successful call to `mysqlnd_ms_set_qos` removes all existing `qos` filter entries from the filter list and installs a new one at the very beginning. All settings that can be made through `mysqlnd_ms_set_qos` can also be in the plugins configuration file. However, use of the function is by far the most common use case. Instead of setting session consistency and strong consistency service levels in the plugins configuration file it is recommended to define only masters and no slaves. Both service levels will force the use of masters only. Using an empty slave list shortens the configuration file, thus improving readability. The only service level for which there is a case of defining in the plugins configuration file is the combination of eventual consistency and maximum slave lag.

  Keyword | Description | Version
  `eventual_consistency` | Request eventual consistency. Allows the use of all master and slave servers. Data returned may or may not be current. Eventual consistency accepts an optional `age` parameter. If `age` is given the plugin considers only slaves for reading for which MySQL replication reports a slave lag less or equal to `age`. The replication lag is measure using `SHOW SLAVE STATUS`. If the plugin fails to fetch the replication lag, the slave tested is skipped. Implementation details and tips are given in the quality of service concepts section. Please note, if a filter chain generates an empty slave list and the PHP configuration directive `mysqlnd_ms.multi_master=0` is used, the plugin may emit a warning. ### Global limit on slave lag
      {
          "myapp": {
              "master": {
                  "master_0": {
                      "host": "localhost"
                  }
              },
              "slave": {
                  "slave_0": {
                      "host": "192.168.2.27",
                      "port": "3306"
                  },
                  "slave_1": {
                      "host": "192.168.78.136",
                      "port": "3306"
                  }
              },
              "filters": {
                  "quality_of_service": {
                      "eventual_consistency": {
                          "age":123
                      }
                  }
              }
          }
      }
  | Since 1.2.0.
  `session_consistency` | Request session consistency (read your writes). Allows use of all masters and all slaves which are in sync with the master. If no further parameters are given slaves are filtered out as there is no reliable way to test if a slave has caught up to the master or is lagging behind. Please note, if a filter chain generates an empty slave list and the PHP configuration directive `mysqlnd_ms.multi_master=0` is used, the plugin may emit a warning. Session consistency temporarily requested using `mysqlnd_ms_set_qos` is a valuable alternative to using `master_on_write`. `master_on_write` is likely to send more statements to the master than needed. The application may be able to continue operation at a lower consistency level after it has done some critical reads. | Since 1.1.0.
  `strong_consistency` | Request strong consistency. Only masters will be used. | Since 1.2.0.

`failover` Up to and including 1.3.x: `string`. Since 1.4.0: `object`.

- Failover policy. Supported policies: `disabled` (default), `master`, `loop_before_master` (Since 1.4.0).

  If no failover policy is set, the plugin will not do any automatic failover (`failover=disabled`). Whenever the plugin fails to connect a server it will emit a warning and set the connections error code and message. Thereafter it is up to the application to handle the error and, for example, resent the last statement to trigger the selection of another server.

  Please note, the automatic failover logic is applied when opening connections only. Once a connection has been opened no automatic attempts are made to reopen it in case of an error. If, for example, the server a connection is connected to is shut down and the user attempts to run a statement on the connection, no automatic failover will be tried. Instead, an error will be reported.

  If using `failover=master` the plugin will implicitly failover to a master, if available. Please check the concepts documentation to learn about potential pitfalls and risks of using `failover=master`.

  ### Optional master failover when failing to connect to slave (PECL/mysqlnd_ms < 1.4.0)

      {
          "myapp": {
              "master": {
                  "master_0": {
                      "host": "localhost"
                  }
              },
              "slave": {
                  "slave_0": {
                      "host": "192.168.78.136",
                      "port": "3306"
                  }
              },
              "failover": "master"
          }
      }

  Since PECL/mysqlnd_ms 1.4.0 the failover configuration keyword refers to an object.

  ### New syntax since 1.4.0

      {
          "myapp": {
              "master": {
                  "master_0": {
                      "host": "localhost"
                  }
              },
              "slave": {
                  "slave_0": {
                      "host": "192.168.78.136",
                      "port": "3306"
                  }
              },
              "failover": {"strategy": "master" }
          }
      }

  Keyword | Description | Version
  `strategy` | Failover policy. Possible values: `disabled` (default), `master`, `loop_before_master` A value of `disabled` disables automatic failover. Setting `master` instructs the plugin to try to connect to a master in case of a slave connection error. If the master connection attempt fails, the plugin exists the failover loop and returns an error to the user. If using `loop_before_master` and a slave request is made, the plugin tries to connect to other slaves before failing over to a master. If multiple master are given and multi master is enabled, the plugin also loops over the list of masters and attempts to connect before returning an error to the user. | Since 1.4.0.
  `remember_failed` | Remember failures for the duration of a web request. Default: `false`. If set to `true` the plugin will remember failed hosts and skip the hosts in all future load balancing made for the duration of the current web request. | Since 1.4.0. The feature is only available together with the `random` and `roundrobin` load balancing filter. Use of the setting is recommended.
  `max_retries` | Maximum number of connection attempts before skipping host. Default: `0` (no limit). The setting is used to prevent hosts from being dropped of the host list upon the first failure. If set to `n > 0`, the plugin will keep the node in the node list even after a failed connection attempt. The node will not be removed immediately from the slave respectively master lists after the first connection failure but instead be tried to connect to up to `n` times in future load balancing rounds before being removed. | Since 1.4.0. The feature is only available together with the `random` and `roundrobin` load balancing filter.

  Setting `failover` to any other value but `disabled`, `master` or `loop_before_master` will not emit any warning or error.

`lazy_connections` `bool`

- Controls the use of lazy connections. Lazy connections are connections which are not opened before the client sends the first connection. Lazy connections are a default.

  It is strongly recommended to use lazy connections. Lazy connections help to keep the number of open connections low. If you disable lazy connections and, for example, configure one MySQL replication master server and two MySQL replication slaves, the plugin will open three connections upon the first call to a connect function although the application might use the master connection only.

  Lazy connections bare a risk if you make heavy use of actions which change the state of a connection. The plugin does not dispatch all state changing actions to all connections from the connection pool. The few dispatched actions are applied to already opened connections only. Lazy connections opened in the future are not affected. Only some settings are "remembered" and applied when lazy connections are opened.

  ### Disabling lazy connection

      {
          "myapp": {
              "master": {
                  "master_0": {
                      "host": "localhost"
                  }
              },
              "slave": {
                  "slave_0": {
                      "host": "192.168.78.136",
                      "port": "3306"
                  }
              },
              "lazy_connections": 0
          }
      }

  Please, see also `server_charset` to overcome potential problems with string escaping and servers using different default charsets.

`server_charset` `string`

- The setting has been introduced in 1.4.0. It is recommended to set it if using lazy connections.

  The `server_charset` setting serves two purposes. It acts as a fallback charset to be used for string escaping done before a connection has been established and it helps to avoid escaping pitfalls in heterogeneous environments which servers using different default charsets.

  String escaping takes a connections charset into account. String escaping is not possible before a connection has been opened and the connections charset is known. The use of lazy connections delays the actual opening of connections until a statement is send.

  An application using lazy connections may attempt to escape a string before sending a statement. In fact, this should be a common case as the statement string may contain the string that is to be escaped. However, due to the lazy connection feature no connection has been opened yet and escaping fails. The plugin may report an error of the type `E_WARNING` and a message like `(mysqlnd_ms) string escaping doesn't work without established connection. Possible solution is to add server_charset to your configuration` to inform you of the pitfall.

  Setting `server_charset` makes the plugin use the given charset for string escaping done on lazy connection handles before establishing a network connection to MySQL. Furthermore, the plugin will enforce the use of the charset when the connection is established.

  Enforcing the use of the configured charset used for escaping is done to prevent tapping into the pitfall of using a different charset for escaping than used later for the connection. This has the additional benefit of removing the need to align the charset configuration of all servers used. No matter what the default charset on any of the servers is, the plugin will set the configured one as a default.

  The plugin does not stop the user from changing the charset at any time using the `set_charset` call or corresponding SQL statements. Please, note that the use of SQL is not recommended as it cannot be monitored by the plugin. The user can, for example, change the charset on a lazy connection handle after escaping a string and before the actual connection is opened. The charset set by the user will be used for any subsequent escaping before the connection is established. The connection will be established using the configured charset, no matter what the server charset is or what the user has set before. Once a connection has been opened, `set_charset` is of no meaning anymore.

  ### String escaping on a lazy connection handle

      {
          "myapp": {
              "master": {
                  "master_0": {
                      "host": "localhost"
                  }
              },
              "slave": {
                  "slave_0": {
                      "host": "192.168.78.136",
                      "port": "3306"
                  }
              },
              "lazy_connections": 1,
              "server_charset" : "utf8"
          }
      }

      <?php
      $mysqli = new mysqli("myapp", "username", "password", "database");
      $mysqli->real_escape("this will be escaped using the server_charset setting - utf8");
      $mysqli->set_charset("latin1");
      $mysqli->real_escape("this will be escaped using latin1");
      /* server_charset implicitly set - utf8 connection */
      $mysqli->query("SELECT 'This connection will be set to server_charset upon establishing' AS _msg FROM DUAL");
      /* latin1 used from now on */
      $mysqli->set_charset("latin1");
      ?>

`master_on_write` `bool`

- If set, the plugin will use the master server only after the first statement has been executed on the master. Applications can still send statements to the slaves using SQL hints to overrule the automatic decision.

  The setting may help with replication lag. If an application runs an `INSERT` the plugin will, by default, use the master to execute all following statements, including `SELECT` statements. This helps to avoid problems with reads from slaves which have not replicated the `INSERT` yet.

  ### Master on write for consistent reads

      {
          "myapp": {
              "master": {
                  "master_0": {
                      "host": "localhost"
                  }
              },
              "slave": {
                  "slave_0": {
                      "host": "192.168.78.136",
                      "port": "3306"
                  }
              },
              "master_on_write": 1
          }
      }

  Please, note the `quality_of_service` filter introduced in version 1.2.0-alpha. It gives finer control, for example, for achieving read-your-writes and, it offers additional functionality introducing service levels.

  All transaction stickiness settings, including `trx_stickiness=on`, are overruled by `master_on_write=1`.

`trx_stickiness` `string`

- Transaction stickiness policy. Supported policies: `disabled` (default), `master`.

  The setting requires 5.4.0 or newer. If used with PHP older than 5.4.0, the plugin will emit a warning like `(mysqlnd_ms) trx_stickiness strategy is not supported before PHP 5.3.99`.

  If no transaction stickiness policy is set or, if setting `trx_stickiness=disabled`, the plugin is not transaction aware. Thus, the plugin may load balance connections and switch connections in the middle of a transaction. The plugin is not transaction safe. SQL hints must be used avoid connection switches during a transaction.

  As of PHP 5.4.0 the mysqlnd library allows the plugin to monitor the `autocommit` mode set by calls to the libraries `set_autocommit()` function. If setting `set_stickiness=master` and `autocommit` gets disabled by a PHP MySQL extension invoking the `mysqlnd` library internal function call `set_autocommit()`, the plugin is made aware of the begin of a transaction. Then, the plugin stops load balancing and directs all statements to the master server until `autocommit` is enabled. Thus, no SQL hints are required.

  An example of a PHP MySQL API function calling the `mysqlnd` library internal function call `set_autocommit()` is `mysqli_autocommit`.

  Although setting `trx_stickiness=master`, the plugin cannot be made aware of `autocommit` mode changes caused by SQL statements such as `SET AUTOCOMMIT=0` or `BEGIN`.

  As of PHP 5.5.0, the mysqlnd library features additional C API calls to control transactions. The level of control matches the one offered by SQL statements. The `mysqli` API has been modified to use these calls. Since version 1.5.0, PECL/mysqlnd_ms can monitor not only `mysqli_autocommit`, but also `mysqli_begin`, `mysqli_commit` and `mysqli_rollback` to detect transaction boundaries and stop load balancing for the duration of a transaction.

  ### Using master to execute transactions

      {
          "myapp": {
              "master": {
                  "master_0": {
                      "host": "localhost"
                  }
              },
              "slave": {
                  "slave_0": {
                      "host": "192.168.78.136",
                      "port": "3306"
                  }
              },
              "trx_stickiness": "master"
          }
      }

  Since version 1.5.0 automatic and silent failover is disabled for the duration of a transaction. If the boundaries of a transaction have been properly detected, transaction stickiness is enabled and a server fails, the plugin will not attempt to fail over to the next server, if any, regardless of the failover policy configured. The user must handle the error manually. Depending on the configuration, the plugin may emit an error of type `E_WARNING` reading like `(mysqlnd_ms) Automatic failover is not permitted in the middle of a transaction`. This error may then be overwritten by follow up errors such as `(mysqlnd_ms) No connection selected by the last filter`. Those errors will be generated by the failing query function.

  ### No automatic failover, error handling pitfall

      <?php
      /* assumption: automatic failover configured */
      $mysqli = new mysqli("myapp", "username", "password", "database");

      /* sets plugin internal state in_trx = 1 */
      $mysqli->autocommit(false);

      /* assumption: server fails */
      if (!($res = $mysqli->query("SELECT 'Assume this query fails' AS _msg FROM DUAL"))) {
       /* handle failure of transaction, plugin internal state is still in_trx = 1 */
       printf("[%d] %s", $mysqli->errno, $mysqli->error);
       /*
        If using autocommit() based transaction detection it is a
        MUST to call autocommit(true). Otherwise the plugin assumes
        the current transaction continues and connection
        changes remain forbidden.
       */
       $mysqli->autocommit(true);
       /* Likewise, you'll want to start a new transaction */
       $mysqli->autocommit(false);
      }
      /* latin1 used from now on */
      $mysqli->set_charset("latin1");
      ?>

  If a server fails in the middle of a transaction the plugin continues to refuse to switch connections until the current transaction has been finished. Recall that the plugin monitors API calls to detect transaction boundaries. Thus, you have to, for example, enable auto commit mode to end the current transaction before the plugin continues load balancing and switches the server. Likewise, you will want to start a new transaction immediately thereafter and disable auto commit mode again.

  Not handling failed queries and not ending a failed transaction using API calls may cause all following commands emit errors such as `Commands out of sync; you can't run this command now`. Thus, it is important to handle all errors.

`transient_error` `object`

- The setting has been introduced in 1.6.0.

  A database cluster node may reply a transient error to a client. The client can then repeat the operation on the same node, fail over to a different node or abort the operation. Per definition is it safe for a client to retry the same operation on the same node before giving up.

  `PECL/mysqlnd_ms` can perform the retry loop on behalf of the application. By configuring `transient_error` the plugin can be instructed to repeat operations failing with a certain error code for a certain maximum number of times with a pause between the retries. If the transient error disappears during loop execution, it is hidden from the application. Otherwise, the error is forwarded to the application by the end of the loop.

  ### Retry loop for transient errors

      {
          "myapp": {
              "master": {
                  "master_0": {
                      "host": "localhost"
                  }
              },
              "slave": {
                  "slave_0": {
                      "host": "192.168.78.136",
                      "port": "3306"
                  }
             },
             "transient_error": {
                "mysql_error_codes": [
                  1297
                ],
                "max_retries": 2,
                "usleep_retry": 100
             }
          }
      }

  Keyword | Description | Version
  `mysql_error_codes` | List of transient error codes. You may add any MySQL error code to the list. It is possible to consider any error as transient not only `1297` (`HY000 (ER_GET_TEMPORARY_ERRMSG), Message: Got temporary error %d '%s' from %s`). Before adding other codes but `1297` to the list, make sure your cluster supports a new attempt without impacting the state of your application. | Since 1.6.0.
  `max_retries` | How often to retry an operation which fails with a transient error before forwarding the failure to the user. Default: `1` | Since 1.6.0.
  `usleep_retry` | Milliseconds to sleep between transient error retries. The value is passed to the C function `usleep`, hence the name. Default: `100` | Since 1.6.0.

## Plugin configuration file (<= 1.0.x)

The below description applies to PECL/mysqlnd_ms < 1.1.0-beta. It is not valid for later versions.

The plugin is using its own configuration file. The configuration file holds information on the MySQL replication master server, the MySQL replication slave servers, the server pick (load balancing) policy, the failover strategy and the use of lazy connections.

The PHP configuration directive `mysqlnd_ms.ini_file` is used to set the plugins configuration file.

The configuration file mimics standard the `php.ini` format. It consists of one or more sections. Every section defines its own unit of settings. There is no global section for setting defaults.

Applications reference sections by their name. Applications use section names as the host (server) parameter to the various connect methods of the mysqli, mysql and PDO_MYSQL extensions. Upon connect the mysqlnd plugin compares the hostname with all section names from the plugin configuration file. If hostname and section name match, the plugin will load the sections settings.

### Using section names example

    [myapp]
    master[] = localhost
    slave[] = 192.168.2.27
    slave[] = 192.168.2.28:3306
    [localhost]
    master[] = localhost:/tmp/mysql/mysql.sock
    slave[] = 192.168.3.24:3305
    slave[] = 192.168.3.65:3309

    <?php
    /* All of the following connections will be load balanced */
    $mysqli = new mysqli("myapp", "username", "password", "database");
    $pdo = new PDO('mysql:host=myapp;dbname=database', 'username', 'password');
    $mysql = mysql_connect("myapp", "username", "password");

    $mysqli = new mysqli("localhost", "username", "password", "database");
    ?>

Section names are strings. It is valid to use a section name such as `192.168.2.1`, `127.0.0.1` or `localhost`. If, for example, an application connects to `localhost` and a plugin configuration section `[localhost]` exists, the semantics of the connect operation are changed. The application will no longer only use the MySQL server running on the host `localhost` but the plugin will start to load balance MySQL queries following the rules from the `[localhost]` configuration section. This way you can load balance queries from an application without changing the applications source code.

The `master[]`, `slave[]` and `pick[]` configuration directives use a list-like syntax. Configuration directives supporting list-like syntax may appear multiple times in a configuration section. The plugin maintains the order in which entries appear when interpreting them. For example, the below example shows two `slave[]` configuration directives in the configuration section `[myapp]`. If doing round-robin load balancing for read-only queries, the plugin will send the first read-only query to the MySQL server `mysql_slave_1` because it is the first in the list. The second read-only query will be send to the MySQL server `mysql_slave_2` because it is the second in the list. Configuration directives supporting list-like syntax result are ordered from top to bottom in accordance to their appearance within a configuration section.

### List-like syntax

    [myapp]
    master[] = mysql_master_server
    slave[] = mysql_slave_1
    slave[] = mysql_slave_2

Here is a short explanation of the configuration directives that can be used.

`master[]` `string`

- URI of a MySQL replication master server. The URI follows the syntax `hostname[:port|unix_domain_socket]`.

  The plugin supports using only one master server.

  Setting a master server is mandatory. The plugin will report a warning upon connect if the user has failed to provide a master server for a configuration section. The warning may read `(mysqlnd_ms) Cannot find master section in config`. Furthermore the plugin may set an error code for the connection handle such as `HY000/2000 (CR_UNKNOWN_ERROR)`. The corresponding error message depends on your language settings.

`slave[]` `string`

- URI of one or more MySQL replication slave servers. The URI follows the syntax `hostname[:port|unix_domain_socket]`.

  The plugin supports using one or more slave servers.

  Setting a slave server is mandatory. The plugin will report a warning upon connect if the user has failed to provide at least one slave server for a configuration section. The warning may read `(mysqlnd_ms) Cannot find slaves section in config`. Furthermore the plugin may set an error code for the connection handle such as `HY000/2000 (CR_UNKNOWN_ERROR)`. The corresponding error message depends on your language settings.

`pick[]` `string`

- Load balancing (server picking) policy. Supported policies: `random`, `random_once` (default), `roundrobin`, `user`.

  If no load balancing policy is set, the plugin will default to `random_once`. The `random_once` policy picks a random slave server when running the first read-only statement. The slave server will be used for all read-only statements until the PHP script execution ends.

  The `random` policy will pick a random server whenever a read-only statement is to be executed.

  If using `roundrobin` the plugin iterates over the list of configured slave servers to pick a server for statement execution. If the plugin reaches the end of the list, it wraps around to the beginning of the list and picks the first configured slave server.

  Setting more than one load balancing policy for a configuration section makes only sense in conjunction with `user` and `mysqlnd_ms_set_user_pick_server`. If the user defined callback fails to pick a server, the plugin falls back to the second configured load balancing policy.

`failover` `string`

- Failover policy. Supported policies: `disabled` (default), `master`.

  If no failover policy is set, the plugin will not do any automatic failover (`failover=disabled`). Whenever the plugin fails to connect a server it will emit a warning and set the connections error code and message. Thereafter it is up to the application to handle the error and, for example, resent the last statement to trigger the selection of another server.

  If using `failover=master` the plugin will implicitly failover to a slave, if available. Please check the concepts documentation to learn about potential pitfalls and risks of using `failover=master`.

`lazy_connections` `bool`

- Controls the use of lazy connections. Lazy connections are connections which are not opened before the client sends the first connection.

  It is strongly recommended to use lazy connections. Lazy connections help to keep the number of open connections low. If you disable lazy connections and, for example, configure one MySQL replication master server and two MySQL replication slaves, the plugin will open three connections upon the first call to a connect function although the application might use the master connection only.

  Lazy connections bare a risk if you make heavy use of actions which change the state of a connection. The plugin does not dispatch all state changing actions to all connections from the connection pool. The few dispatched actions are applied to already opened connections only. Lazy connections opened in the future are not affected. If, for example, the connection character set is changed using a PHP MySQL API call, the plugin will change the character set of all currently opened connection. It will not remember the character set change to apply it on lazy connections opened in the future. As a result the internal connection pool would hold connections using different character sets. This is not desired. Remember that character sets are taken into account for escaping.

`master_on_write` `bool`

- If set, the plugin will use the master server only after the first statement has been executed on the master. Applications can still send statements to the slaves using SQL hints to overrule the automatic decision.

  The setting may help with replication lag. If an application runs an `INSERT` the plugin will, by default, use the master to execute all following statements, including `SELECT` statements. This helps to avoid problems with reads from slaves which have not replicated the `INSERT` yet.

`trx_stickiness` `string`

- Transaction stickiness policy. Supported policies: `disabled` (default), `master`.

  Experimental feature.

  The setting requires 5.4.0 or newer. If used with PHP older than 5.4.0, the plugin will emit a warning like `(mysqlnd_ms) trx_stickiness strategy is not supported before PHP 5.3.99`.

  If no transaction stickiness policy is set or, if setting `trx_stickiness=disabled`, the plugin is not transaction aware. Thus, the plugin may load balance connections and switch connections in the middle of a transaction. The plugin is not transaction safe. SQL hints must be used avoid connection switches during a transaction.

  As of PHP 5.4.0 the mysqlnd library allows the plugin to monitor the `autocommit` mode set by calls to the libraries `trx_autocommit()` function. If setting `trx_stickiness=master` and `autocommit` gets disabled by a PHP MySQL extension invoking the `mysqlnd` library internal function call `trx_autocommit()`, the plugin is made aware of the begin of a transaction. Then, the plugin stops load balancing and directs all statements to the master server until `autocommit` is enabled. Thus, no SQL hints are required.

  An example of a PHP MySQL API function calling the `mysqlnd` library internal function call `trx_autocommit()` is `mysqli_autocommit`.

  Although setting `trx_stickiness=master`, the plugin cannot be made aware of `autocommit` mode changes caused by SQL statements such as `SET AUTOCOMMIT=0`.

## Testing

The section applies to mysqlnd_ms 1.1.0 or newer, not the 1.0 series.

The PECL/mysqlnd_ms test suite is in the `tests/` directory of the source distribution. The test suite consists of standard phpt tests, which are described on the PHP Quality Assurance Teams website.

Running the tests requires setting up one to four MySQL servers. Some tests don't connect to MySQL at all. Others require one server for testing. Some require two distinct servers. In some cases two servers are used to emulate a replication setup. In other cases a master and a slave of an existing MySQL replication setup are required for testing. The tests will try to detect how many servers and what kind of servers are given. If the required servers are not found, the test will be skipped automatically.

Before running the tests, edit `tests/config.inc` to configure the MySQL servers to be used for testing.

The most basic configuration is as follows.

     putenv("MYSQL_TEST_HOST=localhost");
     putenv("MYSQL_TEST_PORT=3306");
     putenv("MYSQL_TEST_USER=root");
     putenv("MYSQL_TEST_PASSWD=");
     putenv("MYSQL_TEST_DB=test");
     putenv("MYSQL_TEST_ENGINE=MyISAM");
     putenv("MYSQL_TEST_SOCKET=");

     putenv("MYSQL_TEST_SKIP_CONNECT_FAILURE=1");
     putenv("MYSQL_TEST_CONNECT_FLAGS=0");
     putenv("MYSQL_TEST_EXPERIMENTAL=0");

     /* replication cluster emulation */
     putenv("MYSQL_TEST_EMULATED_MASTER_HOST=". getenv("MYSQL_TEST_HOST"));
     putenv("MYSQL_TEST_EMULATED_SLAVE_HOST=". getenv("MYSQL_TEST_HOST"));

     /* real replication cluster */
     putenv("MYSQL_TEST_MASTER_HOST=". getenv("MYSQL_TEST_EMULATED_MASTER_HOST"));
     putenv("MYSQL_TEST_SLAVE_HOST=". getenv("MYSQL_TEST_EMULATED_SLAVE_HOST"));

`MYSQL_TEST_HOST`, `MYSQL_TEST_PORT` and `MYSQL_TEST_SOCKET` define the hostname, TCP/IP port and Unix domain socket of the default database server. `MYSQL_TEST_USER` and `MYSQL_TEST_PASSWD` contain the user and password needed to connect to the database/schema configured with `MYSQL_TEST_DB`. All configured servers must have the same database user configured to give access to the test database.

Using `host`, `host:port` or `host:/path/to/socket` syntax one can set an alternate host, host and port or host and socket for any of the servers.

    putenv("MYSQL_TEST_SLAVE_HOST=192.168.78.136:3307"));
    putenv("MYSQL_TEST_MASTER_HOST=myserver_hostname:/path/to/socket"));

## Debugging and Tracing

The mysqlnd debug log can be used to debug and trace the actitivities of PECL/mysqlnd_ms. As a mysqlnd PECL/mysqlnd_ms adds trace information to the mysqlnd library debug file. Please, see the `mysqlnd.debug` PHP configuration directive documentation for a detailed description on how to configure the debug log.

Configuration setting example to activate the debug log:

    mysqlnd.debug=d:t:x:O,/tmp/mysqlnd.trace

This feature is only available with a debug build of PHP. Works on Microsoft Windows if using a debug build of PHP and PHP was built using Microsoft Visual C version 9 and above.

The debug log shows mysqlnd library and PECL/mysqlnd_ms plugin function calls, similar to a trace log. Mysqlnd library calls are usually prefixed with `mysqlnd_`. PECL/mysqlnd internal calls begin with `mysqlnd_ms`.

Example excerpt from the debug log (connect):

    [...]
    >mysqlnd_connect
    | info : host=myapp user=root db=test port=3306 flags=131072
    | >mysqlnd_ms::connect
    | | >mysqlnd_ms_config_json_section_exists
    | | | info : section=[myapp] len=[5]
    | | | >mysqlnd_ms_config_json_sub_section_exists
    | | | | info : section=[myapp] len=[5]
    | | | | info : ret=1
    | | | <mysqlnd_ms_config_json_sub_section_exists
    | | | info : ret=1
    | | <mysqlnd_ms_config_json_section_exists
    [...]

The debug log is not only useful for plugin developers but also to find the cause of user errors. For example, if your application does not do proper error handling and fails to record error messages, checking the debug and trace log may help finding the cause. Use of the debug log to debug application issues should be considered only if no other option is available. Writing the debug log to disk is a slow operation and may have negative impact on the application performance.

Example excerpt from the debug log (connection failure):

    [...]
    | | | | | | | info : adding error [Access denied for user 'root'@'localhost' (using password: YES)] to the list
    | | | | | | | info : PACKET_FREE(0)
    | | | | | | | info : PACKET_FREE(0x7f3ef6323f50)
    | | | | | | | info : PACKET_FREE(0x7f3ef6324080)
    | | | | | | <mysqlnd_auth_handshake
    | | | | | | info : switch_to_auth_protocol=n/a
    | | | | | | info : conn->error_info.error_no = 1045
    | | | | | <mysqlnd_connect_run_authentication
    | | | | | info : PACKET_FREE(0x7f3ef63236d8)
    | | | | | >mysqlnd_conn::free_contents
    | | | | | | >mysqlnd_net::free_contents
    | | | | | | <mysqlnd_net::free_contents
    | | | | | | info : Freeing memory of members
    | | | | | | info : scheme=unix:///tmp/mysql.sock
    | | | | | | >mysqlnd_error_list_pdtor
    | | | | | | <mysqlnd_error_list_pdtor
    | | | | | <mysqlnd_conn::free_contents
    | | | | <mysqlnd_conn::connect
    [...]

The trace log can also be used to verify correct behaviour of PECL/mysqlnd_ms itself, for example, to check which server has been selected for query execution and why.

Example excerpt from the debug log (plugin decision):

    [...]
    >mysqlnd_ms::query
    | info : query=DROP TABLE IF EXISTS test
    | >_mysqlnd_plugin_get_plugin_connection_data
    | | info : plugin_id=5
    | <_mysqlnd_plugin_get_plugin_connection_data
    | >mysqlnd_ms_pick_server_ex
    | | info : conn_data=0x7fb6a7d3e5a0 *conn_data=0x7fb6a7d410d0
    | | >mysqlnd_ms_select_servers_all
    | | <mysqlnd_ms_select_servers_all
    | | >mysqlnd_ms_choose_connection_rr
    | | | >mysqlnd_ms_query_is_select
    [...]
    | | | <mysqlnd_ms_query_is_select
    [...]
    | | | info : Init the master context
    | | | info : list(0x7fb6a7d3f598) has 1
    | | | info : Using master connection
    | | | >mysqlnd_ms_advanced_connect
    | | | | >mysqlnd_conn::connect
    | | | | | info : host=localhost user=root db=test port=3306 flags=131072 persistent=0 state=0

In this case the statement `DROP TABLE IF EXISTS test` has been executed. Note that the statement string is shown in the log file. You may want to take measures to restrict access to the log for security considerations.

The statement has been load balanced using round robin policy, as you can easily guess from the functions name `>mysqlnd_ms_choose_connection_rr`. It has been sent to a master server running on `host=localhost user=root db=test port=3306 flags=131072 persistent=0 state=0`.

## Monitoring

Plugin activity can be monitored using the mysqlnd trace log, mysqlnd statistics, mysqlnd_ms plugin statistics and external PHP debugging tools. Use of the trace log should be limited to debugging. It is recommended to use the plugins statistics for monitoring.

Writing a trace log is a slow operation. If using an external PHP debugging tool, please refer to the vendors manual about its performance impact and the type of information collected. In many cases, external debugging tools will provide call stacks. Often, a call stack or a trace log is more difficult to interpret than the statistics provided by the plugin.

Plugin statistics tell how often which kind of cluster node has been used (slave or master), why the node was used, if lazy connections have been used and if global transaction ID injection has been performed. The monitoring information provided enables user to verify plugin decisions and to plan their cluster resources based on usage pattern. The function `mysqlnd_ms_get_stats` is used to access the statistics. Please, see the functions description for a list of available statistics.

Statistics are collected on a per PHP process basis. Their scope is a PHP process. Depending on the PHP deployment model a process may serve one or multiple web requests. If using CGI model, a PHP process serves one web request. If using FastCGI or pre-fork web server models, a PHP process usually serves multiple web requests. The same is the case with a threaded web server. Please, note that threads running in parallel can update the statistics in parallel. Thus, if using a threaded PHP deployment model, statistics can be changed by more than one script at a time. A script cannot rely on the fact that it sees only its own changes to statistics.

### Verify plugin activity in a non-threaded deployment model

    mysqlnd_ms.enable=1
    mysqlnd_ms.collect_statistics=1

    <?php
    /* Load balanced following "myapp" section rules from the plugins config file (not shown) */
    $mysqli = new mysqli("myapp", "username", "password", "database");
    if (mysqli_connect_errno())
      /* Of course, your error handling is nicer... */
      die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

    $stats_before = mysqlnd_ms_get_stats();
    if ($res = $mysqli->query("SELECT 'Read request' FROM DUAL")) {
      var_dump($res->fetch_all());
    }
    $stats_after = mysqlnd_ms_get_stats();
    if ($stats_after['use_slave'] <= $stats_before['use_slave']) {
      echo "According to the statistics the read request has not been run on a slave!";
    }
    ?>

Statistics are aggregated for all plugin activities and all connections handled by the plugin. It is not possible to tell how much a certain connection handle has contributed to the overall statistics.

Utilizing PHPs `register_shutdown_function` function or the `auto_append_file` PHP configuration directive it is easily possible to dump statistics into, for example, a log file when a script finishes. Instead of using a log file it is also possible to send the statistics to an external monitoring tool for recording and display.

### Recording statistics during shutdown

    mysqlnd_ms.enable=1
    mysqlnd_ms.collect_statistics=1
    error_log=/tmp/php_errors.log

    <?php
    function check_stats() {
      $msg = str_repeat("-", 80) . "\n";
      $msg .= var_export(mysqlnd_ms_get_stats(), true) . "\n";
      $msg .= str_repeat("-", 80) . "\n";
      error_log($msg);
    }
    register_shutdown_function("check_stats");
    ?>