In MySQL 8.0, error logging uses the MySQL component architecture described at Section 5.5, “MySQL 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 5.4.2.4, “Types of Error Log Filtering”. For instructions specific to the JSON and system log sinks, see Section 5.4.2.7, “Error Logging in JSON Format”, and Section 5.4.2.8, “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:

mysql> SELECT @@GLOBAL.log_error_services;
+----------------------------------------+
| @@GLOBAL.log_error_services            |
+----------------------------------------+
| log_filter_internal; log_sink_internal |
+----------------------------------------+

That value indicates that log events first pass through the log_filter_internal filter component, then through the log_sink_internal sink component, both of which are built in. 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.

mysql> SET GLOBAL log_error_services = 'log_filter_internal';
ERROR 1231 (42000): Variable 'log_error_services' can't be set to the value
of 'log_filter_internal'

mysql> SET GLOBAL log_error_services = 'log_filter_internal; log_sink_internal';
Query OK, 0 rows affected (0.00 sec)

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, perform any component-specific configuration, and modify the log_error_services value. Adding or removing log components is subject to these constraints:

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

INSTALL COMPONENT 'file://component_log_sink_syseventlog';
SET GLOBAL log_error_services = 'log_filter_internal; log_sink_syseventlog';

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

SET GLOBAL log_error_services = 'log_filter_internal; log_sink_internal; log_sink_syseventlog';

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

SET GLOBAL log_error_services = 'log_filter_internal; log_sink_internal;
UNINSTALL COMPONENT 'file://component_log_sink_syseventlog';

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 sink (log_sink_json) in addition to the built-in log filter and sink (log_filter_internal, log_sink_internal). First load the JSON sink if it is not loaded:

INSTALL COMPONENT 'file://component_log_sink_json';

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:

SET PERSIST log_error_services = 'log_filter_internal; log_sink_internal; log_sink_json';

The order of components named in log_error_services is significant, particularly with respect to the relative order of filters and sinks. 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 sink, then to the second sink. Both sinks 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 sink, then to the built-in filter, then to the second sink. The first sink receives unfiltered events. The second sink 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.