7.4.2 The Error Log

7.4.2.1 Error Log Configuration

7.4.2.2 Default Error Log Destination Configuration

7.4.2.3 Error Event Fields

7.4.2.4 Types of Error Log Filtering

7.4.2.5 Priority-Based Error Log Filtering (log_filter_internal)

7.4.2.6 Rule-Based Error Log Filtering (log_filter_dragnet)

7.4.2.7 Error Logging in JSON Format

7.4.2.8 Error Logging to the System Log

7.4.2.9 Error Log Output Format

7.4.2.10 Error Log File Flushing and Renaming

This section discusses how to configure the MySQL server for logging of diagnostic messages to the error log. For information about selecting the error message character set and language, see Section 12.6, “Error Message Character Set”, and Section 12.12, “Setting the Error Message Language”.

The error log contains a record of mysqld startup and shutdown times. It also contains diagnostic messages such as errors, warnings, and notes that occur during server startup and shutdown, and while the server is running. For example, if mysqld notices that a table needs to be automatically checked or repaired, it writes a message to the error log.

Depending on error log configuration, error messages may also populate the Performance Schema error_log table, to provide an SQL interface to the log and enable its contents to be queried. See Section 29.12.22.3, “The error_log Table”.

On some operating systems, the error log contains a stack trace if mysqld exits abnormally. The trace can be used to determine where mysqld exited. See Section 7.9, “Debugging MySQL”.

When the server starts, it can recover an unprocessed stack trace from the previous server run and emit the trace through the configured error logging pipeline. Each line of the trace is emitted as a separate mandatory system event with the ER_STACK_BACKTRACE error symbol. Consequently, every configured log sink writes the trace in its own format. If an enabled sink supports the Performance Schema error_log table, the trace is also available through that table.

Stack-trace information that was previously available only in the traditional error log can therefore be written to other destinations, such as JSON logs, the system log, or the Windows Event Log. It can also be visible to users who have permission to query error_log.

If used to start mysqld, mysqld_safe may write messages to the error log. For example, when mysqld_safe notices abnormal mysqld exits, it restarts mysqld and writes a mysqld restarted message to the error log.

The following sections discuss aspects of configuring error logging.