=================================
 Configuration Management System
=================================

The configuration management system exists to provide every daemon with the
proper configuration information. The configuration can be viewed as a set of
key-value pairs.

How can the configuration be set? Well, there are several sources:
 - the ceph configuration file, usually named ceph.conf
 - command line arguments::
    --debug-ms=1
    --debug-pg=10
    etc.
 - arguments injected at runtime by using injectargs


The Configuration File
======================

Most configuration settings originate in the Ceph configuration file.

How do we find the configuration file? Well, in order, we check:
 - the default locations
 - the environment variable CEPH_CONF
 - the command line argument -c

Each stanza of the configuration file describes the key-value pairs that will be in
effect for a particular subset of the daemons. The "global" stanza applies to
everything. The "mon", "osd", and "mds" stanzas specify settings to take effect
for all monitors, all OSDs, and all mds servers, respectively.  A stanza of the
form mon.$name, osd.$name, or mds.$name gives settings for the monitor, OSD, or
MDS of that name, respectively. Configuration values that appear later in the
file win over earlier ones.

A sample configuration file can be found in src/sample.ceph.conf.


Metavariables
=============

The configuration system allows any configuration value to be
substituted into another value using the ``$varname`` syntax, similar
to how bash shell expansion works.

A few additional special metavariables are also defined:
 - $host: expands to the current hostname
 - $type: expands to one of "mds", "osd", "mon", or "client"
 - $id: expands to the daemon identifier. For ``osd.0``, this would be ``0``; for ``mds.a``, it would be ``a``; for ``client.admin``, it would be ``admin``.
 - $num: same as $id
 - $name: expands to $type.$id


Reading configuration values
====================================================

There are two ways for Ceph code to get configuration values. One way is to
read it directly from a variable named "g_conf," or equivalently,
"g_ceph_ctx->_conf." The other is to register an observer that will be called
every time the relevant configuration values changes. This observer will be
called soon after the initial configuration is read, and every time after that
when one of the relevant values changes. Each observer tracks a set of keys
and is invoked only when one of the relevant keys changes.

The interface to implement is found in common/config_obs.h.

The observer method should be preferred in new code because
 - It is more flexible, allowing the code to do whatever reinitialization needs
   to be done to implement the new configuration value.
 - It is the only way to create a std::string configuration variable that can
   be changed by injectargs.
 - Even for int-valued configuration options, changing the values in one thread
   while another thread is reading them can lead to subtle and
   impossible-to-diagnose bugs.

For these reasons, reading directly from g_conf should be considered deprecated
and not done in new code.  Do not ever alter g_conf.

Changing configuration values
====================================================

Configuration values can be changed by calling g_conf->set_val. After changing
the configuration, you should call g_conf->apply_changes to re-run all the
affected configuration observers. For convenience, you can call
g_conf->set_val_or_die to make a configuration change which you think should
never fail.

Injectargs, parse_argv, and parse_env are three other functions which modify
the configuration. Just like with set_val, you should call apply_changes after
calling these functions to make sure your changes get applied.


Defining config options
=======================

New-style config options are defined in common/options.cc. All new config
options should go here (and not into legacy_config_opts.h).

Levels
------

The Option constructor takes a "level" value:

* *LEVEL_BASIC* is for basic config options that a normal operator is likely to adjust.
* *LEVEL_ADVANCED* is for options that an operator *can* adjust, but should not touch unless they understand what they are doing. Adjusting advanced options poorly can lead to problems (performance or even data loss) if done incorrectly.
* *LEVEL_DEV* is for options in place for use by developers only, either for testing purposes, or to describe constants that no user should adjust but we prefer not to compile into the code.

Description and long description
--------------------------------

Short description of the option. Sentence fragment. e.g.::

  .set_description("Default checksum algorithm to use")

The long description is complete sentences, perhaps even multiple
paragraphs, and may include other detailed information or notes.::

  .set_long_description("crc32c, xxhash32, and xxhash64 are available.  The _16 and _8 variants use only a subset of the bits for more compact (but less reliable) checksumming.")

Default values
--------------

There is a default value for every config option. In some cases, there may
also be a *daemon default* that only applies to code that declares itself
as a daemon (in thise case, the regular default only applies to non-daemons).

Safety
------

If an option can be safely changed at runtime::

  .set_safe()

Service
-------

Service is a component name, like "common", "osd", "rgw", "mds", etc. It may
be a list of components, like::

  .add_service("mon mds osd mgr")

For example, the rocksdb options affect both the osd and mon.

Tags
----

Tags identify options across services that relate in some way. Example include;

  - network -- options affecting network configuration
  - mkfs -- options that only matter at mkfs time

Enums
-----

For options with a defined set of allowed values::

  .set_enum_allowed({"none", "crc32c", "crc32c_16", "crc32c_8", "xxhash32", "xxhash64"})
