# SQLClient documentation

### Authors

Richard Frith-Macdonald (`rfm@gnu.org`)

Version: 1.4

Date: 2004/05/07 08:16:16

Copyright: (C) 2004 Free Software Foundation, Inc.

### Contents -

1. The SQLClient library
2. What is the SQLClient library?
3. What backend bundles are available?
4. Where can you get it? How can you install it?
5. Software documentation for the SQLClient class
6. Software documentation for the SQLRecord class
7. Software documentation for the SQLClient(Convenience) category
8. Software documentation for the SQLClient(Logging) category
9. Software documentation for the SQLClient(Subclass) category
10. SQLClient variables

# The SQLClient library

## What is the SQLClient library?

The SQLClient library is designed to provide a simple interface to SQL databases for GNUstep applications. It does not attempt the sort of abstraction provided by the much more sophisticated GDL2 library, but rather allows applications to directly execute SQL queries and statements.

The major features of the SQLClient library are -

- Simple API for executing queries and statements... a variable length sequence of comma separated strings and other objects (NSNumber, NSDate, NSData) are concatenated into a single SQL statement and executed.
- Support multiple sumultaneous named connections to a database server in a thread-safe manner.
- Support multiple simultaneous connections to different database servers with backend driver bundles loaded for different database engines. Clear, simple subclassing of the abstract base class to enable easy implementation of new backend bundles.
- Configuration for all connections held in one place and referenced by connection name for ease of configuration control. Changes via NSUserDefaults can even allow reconfiguration of client instances within a running application.
- Thread safe operation... The base class supports locking such that a single instance can be shared between multiple threads.

## What backend bundles are available?

Current backend bundles are -

- ECPG - a bundle using the embedded SQL interface for postgres.
  This is based on a similar code which has been in production use for over eighteen months, so it should be reliable.
- Postgres - a bundle using the libpq native interface for postgres.
  This is the preferred backend as it allows 'SELECT FOR UPDATE', which the ECPG backend cannot support due to limitations in the postgres implementation of cursors. The code is however not as well tested as the ECPG interface.
- MySQL - a bundle using the mysqlclient library for *recent* MySQL.
  I don't use MySQL... but the test program ran successfully with a vanilla install of the MySQL packages for recent Debian unstable.
- Oracle - a bundle using embedded SQL for Oracle.
  Completely untested... may even need some work to compile... but this *is* based on code which was working about a year ago.
  No support for BLOBs yet.

## Where can you get it? How can you install it?

The SQLClient library is currently only available via CVS from the GNUstep CVS repository.
See <https://savannah.gnu.org/cvs/?group=gnustep>
You need to check out `gnustep/dev-libs/SQLClient`

To build this library you must have a basic GNUstep environment set up...

- The gnustep-make package must have been built and installed.
- The gnustep-base package must have been built and installed.
- You must hace sourced the GNUstep.sh script (from gnustep-make) to set up environment variables needed for building this.
- If this environment is in place, all you should need to do is run 'make' to configure and build the library, 'make install' to install it.
- Then you can run the test programs.
- Your most likely problems are that the configure script may not detect the database libraries you want... Please figure out how to modify `configure.ac` so that it will detect the required headers and libraries on your system, and supply na patch.
- Once the library is installed, you can include the header file `<SQLClient/SQLClient.h%gt;` and link your programs with the `SQLClient` library to use it.

Bug reports, patches, and contributions (eg a backend bundle for a new database) should be entered on the GNUstep project page <http://savannah.gnu.org/projects/gnustep> and the bug reporting page <http://savannah.gnu.org/bugs/?group=gnustep>

# Software documentation for the SQLClient class

## SQLClient : NSObject

> Declared in:
>   SQLClient.h

The SQLClient class encapsulates dynamic SQL access to relational database systems. A shared instance of the class is used for each database (as identified by the name of the database), and the number of simultanous database connections is managed too.

SQLClient is an abstract base class... when you create an instance of it, you are actually creating an instance of a concrete subclass whose implementation is loaded from a bundle.

## Instance Variables for SQLClient Class

### _client

@private NSString* _client;

Identifier within backend

### _database

@private NSString* _database;

The configured database name/host

### _debugging

@private unsigned int _debugging;

The current debugging level

### _duration

@private NSTimeInterval _duration;

Description forthcoming.

### _inTransaction

@private BOOL _inTransaction;

A flag indicating whether this instance is currently within a transaction. This variable must only be set by the -begin , -commit or -rollback methods.
Are we inside a transaction?

### _lastOperation

@private NSDate* _lastOperation;

Timestamp of last operation.
Maintained by the -simpleExecute: and -simpleQuery: methods.

### _name

@private NSString* _name;

Unique identifier for instance

### _password

@private NSString* _password;

The configured password

### _user

@private NSString* _user;

The configured user

### connected

@private BOOL connected;

A flag indicating whether this instance is currently connected to the backend database server. This variable must only be set by the -backendConnect or -backendDisconnect methods.

### extra

@private void* extra;

For subclass specific data

### lock

@private NSRecursiveLock* lock;

Maintain thread-safety

Method summary

- -queryRecord:,...
- -queryString:,...
- -singletons:
- +debugging
- +durationLogging
- +setDebugging:
- +setDurationLogging:
- -debug:,...
- -debugging
- -durationLogging
- -setDebugging:
- -setDurationLogging:
- -backendConnect
- -backendDisconnect
- -backendExecute:
- -backendQuery:
- -copyEscapedBLOB:into:
- -insertBLOBs:intoStatement:length:withMarker:length:giving:
- -lengthOfEscapedBLOB:
- +allClients
- +clientWithConfiguration:name:
- +existingClient:
- +maxConnections
- +purgeConnections:
- +setMaxConnections:
- -begin
- -clientName
- -commit
- -connect
- -connected
- -database
- -disconnect
- -execute:,...
- -execute:with:
- -initWithConfiguration:
- -initWithConfiguration:name:
- -isInTransaction
- -lastOperation
- -name
- -password
- -query:,...
- -query:with:
- -quote:
- -quoteCString:
- -quoteChar:
- -quoteFloat:
- -quoteInteger:
- -rollback
- -setDatabase:
- -setName:
- -setPassword:
- -setUser:
- -simpleExecute:
- -simpleQuery:
- -user

### allClients

+ (NSArray*) allClients;

Returns an array containing all the SQLClient instances.

### clientWithConfiguration: name:

+ (SQLClient*) clientWithConfiguration: (NSDictionary*)config name: (NSString*)reference;

Return an existing SQLClient instance (using +existingClient:) if possible, or creates one, initialises it using -initWithConfiguration:name: , and returns the new instance (autoreleased).
Returns `nil` on failure.

### existingClient:

+ (SQLClient*) existingClient: (NSString*)reference;

Return an existing SQLClient instance for the specified name if one exists, otherwise returns `nil`.

### maxConnections

+ (unsigned int) maxConnections;

Return the maximum number of simultaneous database connections permitted (set by +setMaxConnections: and defaults to 8)

### purgeConnections:

+ (void) purgeConnections: (NSDate*)since;

Use this method to reduce the number of database connections currently active so that it is less than the limit set by the +setMaxConnections: method. This mechanism is used internally by the class to ensure that, when it is about to open a new connection, the limit is not exceeded.

If since is not `nil`, then any connection which has not been used more recently than that date is disconnected anyway.
You can (and probably should) use this periodically to purge idle connections, but you can also pass a date in the future to close all connections.

### setMaxConnections:

+ (void) setMaxConnections: (unsigned int)c;

Set the maximum number of simultaneous database connections permitted (defaults to 8 and may not be set less than 1).

This value is used by the +purgeConnections: method to determine how many connections should be disconnected when it is called.

### begin

- (void) begin;

Start a transaction for this database client.
You must match this with either a -commit or a -rollback .

Normally, if you execute an SQL statement without using this method first, the autocommit feature is employed, and the statement takes effect immediately. Use of this method permits you to execute several statements in sequence, and only have them take effect (as a single operation) when you call the -commit method.

NB. You must not execute an SQL statement which would start a transaction directly... use only this method.

### clientName

- (NSString*) clientName;

Return the client name for this instance.
Normally this is useful only for debugging/reporting purposes, but if you are using multiple instances of this class in your application, and you are using embedded SQL, you will need to use this method to fetch the client/connection name and store its C-string representation in a variable 'connectionName' declared to the sql preprocessor, so you can then have statements of the form - 'exec sql at :connectionName...'.

### commit

- (void) commit;

Complete a transaction for this database client.
This must match an earlier -begin .

NB. You must not execute an SQL statement which would commit or rollback a transaction directly... use only this method or the -rollback method.

### connect

- (BOOL) connect;

If the connected instance variable is `NO`, this method calls -backendConnect to ensure that there is a connection to the database server established. Returns the result.
Performs any necessary locking for thread safety.

### connected

- (BOOL) connected;

Return a flag to say whether a connection to the database server is currently live. This is mostly useful for debug/reporting, but is used internally to keep track of active connections.

### database

- (NSString*) database;

Return the database name for this instance (or `nil`).

### disconnect

- (void) disconnect;

If the connected instance variable is `YES`, this method calls -backendDisconnect to ensure that the connection to the database server is dropped.
Performs any necessary locking for thread safety.

### execute: ,...

- (void) execute: (NSString*)stmt,...;

Perform arbitrary operation which does not return any value.
This arguments to this method are a `nil` terminated list which are concatenated in the manner of the * -prepare:args: method.
Any string arguments are assumed to have been quoted appropriately already, but non-string arguments are automatically quoted using the -quote: method.

       [db execute: @"UPDATE ", table, @" SET Name = ",
         myName, " WHERE ID = ", myId, nil];

### execute: with:

- (void) execute: (NSString*)stmt with: (NSDictionary*)values;

Takes the statement and substitutes in values from the dictionary where markup of the format {key} is found.
Passes the result to the -execute:,... method.

       [db execute: @"UPDATE {Table} SET Name = {Name} WHERE ID = {ID}"
              with: values];

Any non-string values in the dictionary will be replaced by the results of the -quote: method.
The markup format may also be {key?default} where default is a string to be used if there is no value for the key in the dictionary.

### initWithConfiguration:

- (id) initWithConfiguration: (NSDictionary*)config;

Calls -initWithConfiguration:name: passing a `nil` reference name.

### initWithConfiguration: name:

- (id) initWithConfiguration: (NSDictionary*)config name: (NSString*)reference;

Initialise using the supplied configuration, or if that is `nil`, try to use values from NSUserDefaults (and automatically update when the defaults change).
Uses the reference name to determine configuration information... and if a `nil` name is supplied, defaults to the value of the SQLClientName as a user default string or 'Database' if no other name is provided.
If a SQLClient instance already exists with the name used for this instance, the receiver is deallocated and the existing instance is retained and returned... there may only ever be one instance for a particular reference name.
The config argument (or the SQLClientReferences user default) is a dictionary with names as keys and dictionaries as its values. Configuration entries from the dictionary corresponding to the database client are used if possible, general entries are used otherwise.
Database... is the name of the database to use, if it is missing then 'Database' may be used instead.
User... is the name of the database user to use, if it is missing then 'User' may be used instead.
Password... is the name of the database user password, if it is missing then 'Password' may be used instead.
ServerType... is the name of the backend server to be used... by convention the name of a bundle containing the interface to that backend. If this is missing then 'Postgres' is used.

### isInTransaction

- (BOOL) isInTransaction;

Return the state of the flag indicating whether the library thinks a transaction is in progress. This flag is normally maintained by -begin , -commit , and -rollback .

### lastOperation

- (NSDate*) lastOperation;

Returns the date/time stamp of the last database operation performed by the receiver, or `nil` if no operation has ever been done by it.
Simply connecting to or disconnecting from the databsse does not count as an operation.

### name

- (NSString*) name;

Return the database reference name for this instance (or `nil`).

### password

- (NSString*) password;

Return the database password for this instance (or `nil`).

### query: ,...

- (NSMutableArray*) query: (NSString*)stmt,...;

Perform arbitrary query which returns values.

This method has at least one argument, the string starting the statement to be executed (which must have the prefix 'select ').

Additional arguments are a `nil` terminated list which also be strings, and these are appended to the statement.
Any string arguments are assumed to have been quoted appropriately already, but non-string arguments are automatically quoted using the -quote: method.

       result = [db query: @"SELECT Name FROM ", table, nil];

Upon error, an exception is raised.

The query returns an array of records (each of which is represented by an SQLRecord object).

Each SQLRecord object contains one or more fields, in the order in which they occurred in the query. Fields may also be retrieved by name.

NULL field items are returned as NSNull objects.

Most other field items are returned as NSString objects.

Date and timestamp field items are returned as NSDate objects.

### query: with:

- (NSMutableArray*) query: (NSString*)stmt with: (NSDictionary*)values;

Takes the query statement and substitutes in values from the dictionary where markup of the format {key} is found.
Passes the result to the -query:,... method to execute.

       result = [db query: @"SELECT Name FROM {Table} WHERE ID = {ID}"
                     with: values];

Any non-string values in the dictionary will be replaced by the results of the -quote: method.
The markup format may also be {key?default} where default is a string to be used if there is no value for the key in the dictionary.

### quote:

- (NSString*) quote: (id)obj;

Convert an object to a string suitable for use in an SQL query.
Normally the -execute:,... , and -query:,... methods will call this method automatically for everything apart from string objects.
Strings have to be handled specially, because they are used both for parts of the SQL command, and as values (where they need to be quoted). So where you need to pass a string value which needs quoting, you must call this method explicitly.
Subclasses may override this method to provide appropriate quoting for types of object which need database backend specific quoting conventions. However, the defalt implementation should be OK for most cases.
The base class implementation formats NSDate objects as
YYYY-MM-DD hh:mm:ss.mmm ?ZZZZ
NSData objects are not quoted... they must not appear in queries, and where used for insert/update operations, they need to be passed to the -backendExecute: method unchanged.
For a `nil` or NSNull object, we return NULL.
For a number, we simply convert directly to a string.
For a date, we convert to the text format used by the database, and add leading and trailing quotes.
For a data object, we don't quote... the other parts of the code need to know they have an NSData object and pass it on unchanged to the -backendExecute: method.
For any other type of data, we just produce a quoted string representation.

### quoteCString:

- (NSString*) quoteCString: (const char*)s;

Convert a 'C' string to a string suitable for use in an SQL query.

### quoteChar:

- (NSString*) quoteChar: (char)c;

Convert a single character to a string suitable for use in an SQL query.

### quoteFloat:

- (NSString*) quoteFloat: (float)f;

Convert a float to a string suitable for use in an SQL query.

### quoteInteger:

- (NSString*) quoteInteger: (int)i;

Convert an integer to a string suitable for use in an SQL query.

### rollback

- (void) rollback;

Revert a transaction for this database client.
This must match an earlier -begin .

NB. You must not execute an SQL statement which would commit or rollback a transaction directly... use only this method or the -rollback method.

### setDatabase:

- (void) setDatabase: (NSString*)s;

Set the database host/name for this object.
This is called automatically to configure the connection... you normally shouldn't need to call it yourself.

### setName:

- (void) setName: (NSString*)s;

Set the database reference name for this object. This is used to differentiate between multiple connections to the database.
This is called automatically to configure the connection... you normally shouldn't need to call it yourself.
NB. attempts to change the name of an instance to that of an existing instance are ignored.

### setPassword:

- (void) setPassword: (NSString*)s;

Set the database password for this object.
This is called automatically to configure the connection... you normally shouldn't need to call it yourself.

### setUser:

- (void) setUser: (NSString*)s;

Set the database user for this object.
This is called automatically to configure the connection... you normally shouldn't need to call it yourself.

### simpleExecute:

- (void) simpleExecute: (NSArray*)info;

Calls -backendExecute: in a safe manner.
Handles locking.
Maintains -lastOperation date.

### simpleQuery:

- (NSMutableArray*) simpleQuery: (NSString*)stmt;

Calls -backendQuery: in a safe manner.
Handles locking.
Maintains -lastOperation date.

### user

- (NSString*) user;

Return the database user for this instance (or `nil`).

# Software documentation for the SQLRecord class

## SQLRecord : NSArray

> Declared in:
>   SQLClient.h

An enhanced array to represent a record returned from a query. You should NOT try to create instances of this class except via the +newWithValues:keys:count: method.

## Instance Variables for SQLRecord Class

### count

@private unsigned int count;

Description forthcoming.

Method summary

- +newWithValues:keys:count:
- -allKeys
- -objectForKey:

### newWithValues: keys: count:

+ (id) newWithValues: (id*)v keys: (id*)k count: (unsigned int)c;

Description forthcoming.

### allKeys

- (NSArray*) allKeys;

Returns an array containing the names of all the fields in the record, in the order in which they occur in the record.

### objectForKey:

- (id) objectForKey: (NSString*)key;

Returns the first field in the record whose name matches the specified key. Uses an exact match in preference to a case-insensitive match.

# Software documentation for the SQLClient(Convenience) category

## SQLClient(Convenience)

> Declared in:
>   SQLClient.h

This category contains convenience methods including those for frequently performed database operations... message logging etc.

Method summary

- -queryRecord:,...
- -queryString:,...
- -singletons:

### queryRecord: ,...

- (SQLRecord*) queryRecord: (NSString*)stmt,...;

Executes a query (like the -query:,... method) and checks the result (raising an exception if the query did not contain a single record) and returns the resulting record.

### queryString: ,...

- (NSString*) queryString: (NSString*)stmt,...;

Executes a query (like the -query:,... method) and checks the result.
Raises an exception if the query did not contain a single record, or if the record did not contain a single field.
Returns the resulting field as a string.

### singletons:

- (void) singletons: (NSMutableArray*)records;

Convenience method to deal with the results of a query where each record contains a single field... it converts the array of records returned by the query to an array containing the fields.

# Software documentation for the SQLClient(Logging) category

## SQLClient(Logging)

> Declared in:
>   SQLClient.h

This category porovides basic methods for logging debug information.

Method summary

- +debugging
- +durationLogging
- +setDebugging:
- +setDurationLogging:
- -debug:,...
- -debugging
- -durationLogging
- -setDebugging:
- -setDurationLogging:

### debugging

+ (unsigned int) debugging;

Return the class-wide debugging level, which is inherited by all newly created instances.

### durationLogging

+ (NSTimeInterval) durationLogging;

Return the class-wide duration logging threshold, which is inherited by all newly created instances.

### setDebugging:

+ (void) setDebugging: (unsigned int)level;

Set the debugging level to be inherited by all new instances.

### setDurationLogging:

+ (void) setDurationLogging: (NSTimeInterval)threshold;

Set the duration logging threshold to be inherited by all new instances.

### debug: ,...

- (void) debug: (NSString*)fmt,...;

The default implementation calls NSLogv to log a debug message.
Override this in a category to provide more sophisticated logging.

### debugging

- (unsigned int) debugging;

Return the current debugging level.

### durationLogging

- (NSTimeInterval) durationLogging;

Returns the threshold above which queries and statements taking a long time to execute are logged. A negative value (default) indicates that this logging is disabled. A value of zero means that all statements are logged.

### setDebugging:

- (void) setDebugging: (unsigned int)level;

Set the debugging level of this instance... overrides the default level inherited from the class.

### setDurationLogging:

- (void) setDurationLogging: (NSTimeInterval)threshold;

Set a threshold above which queries and statements taking a long time to execute are logged. A negative value (default) disables this logging. A value of zero logs all statements.

# Software documentation for the SQLClient(Subclass) category

## SQLClient(Subclass)

> Declared in:
>   SQLClient.h

This category contains the methods which a subclass must override to provide a working instance, and helper methods for the backend implementations.
Application programmers should not call the backend methods directly.

When subclassing to produce a backend driver bundle, please be aware that the subclass must NOT introduce additional instance variables. Instead the extra instance variable is provided for use as a pointer to subclass specific data.

Method summary

- -backendConnect
- -backendDisconnect
- -backendExecute:
- -backendQuery:
- -copyEscapedBLOB:into:
- -insertBLOBs:intoStatement:length:withMarker:length:giving:
- -lengthOfEscapedBLOB:

### backendConnect

- (BOOL) backendConnect;
Subclasses should override this method.

Attempts to establish a connection to the database server.
Returns a flag to indicate whether the connection has been established.
If a connection was already established, returns `YES` and does nothing.
You should not need to use this method normally, as it is called for you automatically when necessary.

Subclasses must implement this method to establish a connection to the database server process (and initialise the extra instance variable if necessary), setting the connected instance variable to indicate the state of the object.

This method must call +purgeConnections: to ensure that there is a free slot for the new connection.

Application code must not call this method directly, it is for internal use only. The -connect method calls this method if the connected instance variable is `NO`.

### backendDisconnect

- (void) backendDisconnect;
Subclasses should override this method.

Disconnect from the database unless already disconnected.

This method is called automatically when the receiver is deallocated or reconfigured, and may also be called automatically when there are too many database connections active.

If the receiver is an instance of a subclass which uses the extra instance variable, it must clear that variable in the -backendDisconnect method, because a reconfiguration may cause the class of the receiver to change.

This method must set the connected instance variable to `NO`.

Application code must not call this method directly, it is for internal use only. The -disconnect method calls this method if the connected instance variable is `YES`.

### backendExecute:

- (void) backendExecute: (NSArray*)info;
Subclasses should override this method.

Perform arbitrary operation which does not return any value.
This method has a single argument, an array containing the string representing the statement to be executed as its first object, and an optional sequence of data objects following it.

       [db backendExecute: [NSArray arrayWithObject:
         @"UPDATE MyTable SET Name = 'The name' WHERE ID = 123"]];

The backend implementation is required to perform the SQL statement using the supplied NSData objects at the points in the statement marked by the `'''` sequence. The marker saequences are inserted into the statement at an earlier stage by the -execute:,... and -execute:with: methods.

This method should lock the instance using the lock instance variable for the duration of the operation, and unlock it afterwards.

NB. callers (other than the -begin , -commit , and -rollback methods) should not pass any statement to this method which would cause a transaction to begin or end.

Application code must not call this method directly, it is for internal use only.

### backendQuery:

- (NSMutableArray*) backendQuery: (NSString*)stmt;
Subclasses should override this method.

Perform arbitrary query which returns values.

       result = [db backendQuery: @"SELECT Name FROM Table"];

Upon error, an exception is raised.

The query returns an array of records (each of which is represented by an SQLRecord object).

Each SQLRecord object contains one or more fields, in the order in which they occurred in the query. Fields may also be retrieved by name.

NULL field items are returned as NSNull objects.

This method should lock the instance using the lock instance variable for the duration of the operation, and unlock it afterwards.

Application code must not call this method directly, it is for internal use only.

### copyEscapedBLOB: into:

- (unsigned) copyEscapedBLOB: (NSData*)blob into: (void*)buf;
Subclasses should override this method.

This method is only for the use of the -insertBLOBs:intoStatement:length:withMarker:length:giving: method.
Subclasses which need to insert binary data into a statement must implement this method to copy the escaped data into place and return the number of bytes actually copied.

### insertBLOBs: intoStatement: length: withMarker: length: giving:

- (const void*) insertBLOBs: (NSArray*)blobs intoStatement: (const void*)statement length: (unsigned)sLength withMarker: (const void*)marker length: (unsigned)mLength giving: (unsigned*)result;

This method is a convenience method provided for subclasses which need to insert escaped binary data into an SQL statement before sending the statement to a backend server process. This method makes use of the -copyEscapedBLOB:into: and -lengthOfEscapedBLOB: methods, which must be implemented by the subclass.

The blobs array is an array containing the original SQL statement string (unused by this method) followed by the data items to be inserted.

The statement and sLength arguments specify the datastream to be copied and into which the BLOBs are to be inserted.

The marker and mLength arguments specify the sequence of marker bytes in the statement which indicate a position for insertion of a n escaped BLOB.

The method returns either the original statement or a copy containing the escaped BLOBs. The length of the returned data is stored in result.

### lengthOfEscapedBLOB:

- (unsigned) lengthOfEscapedBLOB: (NSData*)blob;
Subclasses should override this method.

This method is only for the use of the -insertBLOBs:intoStatement:length:withMarker:length:giving: method.
Subclasses which need to insert binary data into a statement must implement this method to return the length of the escaped bytestream which will be inserted.

# SQLClient variables

### SQLConnectionException

NSString* SQLConnectionException;

Exception for when a connection to the server is lost.

### SQLEmptyException

NSString* SQLEmptyException;

Exception for when a query is supposed to return data and doesn't.

### SQLException

NSString* SQLException;

Exception raised when an error with the remote database server occurs.

### SQLUniqueException

NSString* SQLUniqueException;

Exception for when an insert/update would break the uniqueness of a field or index.