6.3.11.4. Audit Log Plugin Logging Control

This section describes how the audit_log plugin performs logging and the system variables that control how logging occurs. It assumes familiarity with the log file format described in Section 6.3.11.3, "The Audit Log File".

When the audit log plugin opens its log file, it checks whether the XML declaration and opening <AUDIT> root element tag need to be written and writes them if so. When the audit log plugin terminates, it writes a closing </AUDIT> tag to the file.

If the log file exists at open time, the plugin checks whether the file ends with an </AUDIT> tag and truncates it if so before writing any <AUDIT_RECORD> elements. If the log file exists but does not end with </AUDIT> or the </AUDIT> tag cannot be truncated, the plugin considers the file malformed and fails to initialize. This can occur if the server crashes or is killed with the audit log plugin running. No logging occurs until the problem is rectified. Check the error log for diagnostic information:

[ERROR] Plugin 'audit_log' init function returned error.

To deal with this problem, you must either remove or rename the malformed log file and restart the server.

The MySQL server calls the audit log plugin to write an <AUDIT_RECORD> element whenever an auditable event occurs, such as when it completes execution of an SQL statement received from a client. Typically the first <AUDIT_RECORD> element written after server startup has the server description and startup options. Elements following that one represent events such as client connect and disconnect events, executed SQL statements, and so forth. Only top-level statements are logged, not statements within stored programs such as triggers or stored procedures. Contents of files referenced by statements such as LOAD DATA INFILE are not logged.

To permit control over how logging occurs, the audit_log plugin provides several system variables, described following. For more information, see Section 6.3.11.5, "Audit Log Plugin Options and Variables".

audit_log_file: The name of the log file. By default, the name is audit.log in the server data directory. For security reasons, the audit log file should be written to a directory accessible only to the MySQL server and users with a legitimate reason to view the log.
audit_log_policy: What kinds of information the plugin writes. By default, this variable is set to ALL (write all auditable events), but also permits values of LOGINS or QUERIES to log only login or query events, or NONE to disable logging.
audit_log_strategy: The method used for log writes. By default, the strategy value is ASYNCHRONOUS and the plugin logs asynchronously to a buffer, waiting if the buffer is full. It's possible to tell the plugin not to wait (PERFORMANCE) or to log synchronously, either using file system caching (SEMISYNCHRONOUS) or forcing output with a sync() call after each write request (SYNCHRONOUS).

Asynchronous logging strategy has these characteristics:
- Minimal impact on server performance and scalability.
- Blocking of threads that generate audit events for the shortest possible time; that is, time to allocate the buffer plus time to copy the event to the buffer.
- Output goes to the buffer. Writes from the buffer to the log file are handled by a separate thread.
A disadvantage of PERFORMANCE strategy is that it drops events when the buffer is full. For a heavily loaded server, it is more likely that the audit log will be missing events.

With asynchronous logging, the integrity of the log file may be compromised if a problem occurs during a write to the file or if the plugin does not shut down cleanly (for example, in the event that the server host crashes). To reduce this risk, set audit_log_strategy use synchronous logging. Regardless of strategy, logging occurs on a best-effort basis, with no guarantee of consistency.
audit_log_buffer_size: The size of the buffer for asynchronous logging. The plugin uses a single buffer, which it allocates when it initializes and removes when it terminates. The plugin allocates this buffer only if logging is asynchronous.
audit_log_rotate_on_size, audit_log_flush: These variables permit audit log file rotation and flushing. The audit log file has the potential to grow very large and consume a lot of disk space. To manage the space used, either enable automatic log rotation, or manually rename the audit file and flush the log to open a new file. The renamed file can be removed or backed up as desired.

By default, audit_log_rotate_on_size=0 and there is no log rotation. In this case, the audit log plugin closes and reopens the log file when the audit_log_flush value changes from disabled to enabled. Log file renaming must be done externally to the server. Suppose that you want to maintain the three most recent log files, which cycle through the names audit.log.1.xml through audit.log.3.xml. On Unix, perform rotation manually like this:
1. From the command line, rename the current log files:
```
shell> mv audit.log.2.xml
                                audit.log.3.xmlshell> mv
                                audit.log.1.xml audit.log.2.xmlshell> mv audit.log audit.log.1.xml
```
  At this point, the plugin is still writing to the current log file, which has been renamed to audit.log.1.xml.
2. Connect to the server and flush the log file so the plugin closes it and reopens a new audit.log file:
```
mysql> SET GLOBAL
                                audit_log_flush = ON;
```
If audit_log_rotate_on_size is greater than 0, setting audit_log_flush has no effect. In this case, the audit log plugin closes and reopens its log file whenever a write to the file causes its size to exceed the audit_log_rotate_on_size value. The plugin renames the original file to have an extension consisting of a timestamp and .xml suffix. For example, audit.log might be renamed to audit.log.13660396477726520.xml. The last 7 digits of the timestamp are a fractional second part. The first 10 digits are a Unix timestamp value that can be interpreted using the FROM_UNIXTIME() function:
```
mysql> SELECT
                    FROM_UNIXTIME(1366039647);+---------------------------+| FROM_UNIXTIME(1366039647) |+---------------------------+| 2013-04-15 10:27:27       |+---------------------------+
```