Chapter 5 MySQL Services for Plugins

MySQL server plugins have access to server “plugin services.” The plugin services interface exposes server functionality that plugins can call. It complements the plugin API and has these characteristics:

• Services enable plugins to access code inside the server using ordinary function calls. Services are also available to loadable functions.

• Services are portable and work on multiple platforms.

• The interface includes a versioning mechanism so that service versions supported by the server can be checked at load time against plugin versions. Versioning protects against incompatibilities between the version of a service that the server provides and the version of the service expected or required by a plugin.

• For information about plugins for testing plugin services, see the Plugins for Testing Plugin Services section of the MySQL Server Doxygen documentation, available at https://dev.mysql.com/doc/index-other.html.

The plugin services interface differs from the plugin API as follows:

• The plugin API enables plugins to be used by the server. The calling initiative lies with the server to invoke plugins. This enables plugins to extend server functionality or register to receive notifications about server processing.

• The plugin services interface enables plugins to call code inside the server. The calling initiative lies with plugins to invoke service functions. This enables functionality already implemented in the server to be used by many plugins; they need not individually implement it themselves.

To determine what services exist and what functions they provide, look in the include/mysql directory of a MySQL source distribution. The relevant files are:

• plugin.h includes services.h, which is the “umbrella” header that includes all available service-specific header files.

• Service-specific headers have names of the form service_xxx.h.

Each service-specific header should contain comments that provide full usage documentation for a given service, including what service functions are available, their calling sequences, and return values.

For developers who wish to modify the server to add a new service, see MySQL Internals: MySQL Services for Plugins.

Available services include the following:

• get_sysvar_source: A service that enables plugins to retrieve the source of system variable settings.

• locking_service: A service that implements locks with three attributes: Lock namespace, lock name, and lock mode. This locking interface is accessible at two levels: 1) At the SQL level, as a set of loadable functions that each map onto calls to the service routines; 2) As a C language interface, callable as a plugin service from server plugins or loadable functions. For more information, see The Locking Service.

• my_plugin_log_service: A service that enables plugins to report errors and specify error messages. The server writes the messages to its error log.

• status_variable_registration. A service for registering status variables.

• my_thd_scheduler: A service for plugins to select a thread scheduler.

• mysql_keyring: A service for keyring storage, accessible at two levels: 1) At the SQL level, as a set of loadable functions that each map onto calls to the service routines; 2) As a C language interface, callable as a plugin service from server plugins or loadable functions. For more information, see The Keyring Service.

• mysql_password_policy: A service for password validation and strength checking.

• plugin_registry_service: MySQL Server includes a component-based infrastructure for improving server extensibility; see MySQL Components. However, MySQL plugins use an interface that predates the component interface. The plugin_registry_service enables plugins to access the component registry and its services.

• security_context: A service that enables plugins to examine or manipulate thread security contexts. This service provides setter and getter routines to access attributes of the server Security_context class, which includes attributes such as operating system user and host, authenticated user and host, and client IP address.

• thd_alloc: A memory-allocation service.

• thd_wait: A service for plugins to report when they are going to sleep or stall.

The remainder of this section describes how a plugin uses server functionality that is available as a service. See also the source for the “daemon” example plugin, which uses the my_snprintf service. Within a MySQL source distribution, that plugin is located in the plugin/daemon_example directory.

To use a service or services from within a plugin, the plugin source file must include the plugin.h header file to access service-related information:

#include <mysql/plugin.h>

This does not represent any additional setup cost. A plugin must include that file anyway because it contains definitions and structures that every plugin needs.

To access a service, a plugin calls service functions like any other function.

To report an error that the server will write to it error log, first choose an error level. mysql/service_my_plugin_log.h defines these levels:

enum plugin_log_level
{
  MY_ERROR_LEVEL,
  MY_WARNING_LEVEL,
  MY_INFORMATION_LEVEL
};

Then invoke my_plugin_log_message():

int my_plugin_log_message(MYSQL_PLUGIN *plugin, enum plugin_log_level level,
                          const char *format, ...);

For example:

my_plugin_log_message(plugin_ptr, MY_ERROR_LEVEL, "Cannot initialize plugin");

Some services for plugins may be provided by plugins and thus are available only if the service-providing plugin is loaded. Any MySQL component that uses such a service should check whether the service is available.

When you build your plugin, use the -lmysqlservices flag at link time to link in the libmysqlservices library. For example, for CMake, put this in the top-level CMakeLists.txt file:

FIND_LIBRARY(MYSQLSERVICES_LIB mysqlservices
 PATHS "${MYSQL_SRCDIR}/libservices" NO_DEFAULT_PATH)

Put this in the CMakeLists.txt file in the directory containing the plugin source:

# the plugin needs the mysql services library for error logging
TARGET_LINK_LIBRARIES (your_plugin_library_name ${MYSQLSERVICES_LIB})