5.4.2.1 Error Log Component Configuration

In MySQL 8.0, error logging uses the MySQL component architecture described at Section 5.5, “MySQL Server Components”. The error log subsystem consists of components that perform log event filtering and writing, as well as a system variable that configures which components to enable to achieve the desired logging result.

This section discusses how to select components for error logging. For instructions specific to log filters, see Section 5.4.2.3, “Types of Error Log Filtering”. For instructions specific to the JSON and system log writers, see Section 5.4.2.6, “Error Logging in JSON Format”, and Section 5.4.2.7, “Error Logging to the System Log”. For additional details about all available log components, see Section 5.5.3, “Error Log Components”.

Component-based error logging offers these features:

The log_error_services system variable controls which log components to enable for error logging. The variable may contain a list with 0, 1, or many elements. In the latter case, elements may be delimited by semicolon or (as of MySQL 8.0.12) comma, optionally followed by space. A given setting cannot use both semicolon and comma separators. Component order is significant because the server executes components in the order listed.

By default, log_error_services has this value:

That value indicates that log events first pass through the built-in filter component, log_filter_internal, then through the built-in log writer component, log_sink_internal. A filter modifies log events seen by components named later in the log_error_services value. A sink is a destination for log events. Typically, a sink processes log events into log messages that have a particular format and writes these messages to its associated output, such as a file or the system log.

The combination of log_filter_internal and log_sink_internal implements the default error log filtering and output behavior. The action of these components is affected by other server options and system variables:

To change the set of log components used for error logging, load components as necessary and modify the log_error_services value. Adding or removing log components is subject to these constraints:

For example, to use the system log writer (log_sink_syseventlog) instead of the default writer (log_sink_internal), first load the writer component, then modify the log_error_services value:

It is possible to configure multiple log writers, which enables sending output to multiple destinations. To enable the system log writer in addition to (rather than instead of) the default writer, set the log_error_services value like this:

To revert to using only the default writer and unload the system log writer, execute these statements:

To configure a log component to be enabled at each server startup, use this procedure:

Suppose that you want to configure, for every server startup, use of the JSON log writer (log_sink_json) in addition to the built-in log filter and writer (log_filter_internal, log_sink_internal). First load the JSON writer if it is not loaded:

Then set log_error_services to take effect at server startup. You can set it in my.cnf:

[mysqld]
log_error_services='log_filter_internal; log_sink_internal; log_sink_json'

Or you can set it using SET PERSIST:

The order of components named in log_error_services is significant, particularly with respect to the relative order of filters and writers. Consider this log_error_services value:

log_filter_internal; log_sink_1; log_sink_2

In this case, log events pass to the built-in filter, then to the first writer, then to the second writer. Both writers receive the filtered log events.

Compare that to this log_error_services value:

log_sink_1; log_filter_internal; log_sink_2

In this case, log events pass to the first writer, then to the built-in filter, then to the second writer. The first writer receives unfiltered events. The second writer receives filtered events. You might configure error logging this way if you want one log that contains messages for all log events, and another log that contains messages only for a subset of log events.