Mutable logging configuration

There is a strong desire from operators to be able to reconfigure logging without a service restart. For example, to selectively enable DEBUG logging in response to observed issues. Mutable config patches have enabled options to be mutated at runtime, now we need to do the same for logging config.

Problem description

Mutable logging

Previous work on oslo.config and oslo.log has allowed the ‘debug’ flag to be mutable at runtime. However, oslo.log also accepts a ‘log_config_append’ option. If this is provided, most of the options are ignored and a logging config file is read. In this case, mutating the ‘debug’ flag is insufficient to change the log level.

This flag also provides minimal control over logging. It should be possible to change the log level of individual loggers at runtime. To support this, full mutation of the logging config should be supported.

Proposed change

Mark ‘log_config_append’ as mutable. When the hook is called, call logging.fileConfig if the value of log_config_append changed or the timestamp of the file it points to changed.


The primary usecase for a logging config file is to output log records as JSON suitable for Logstash. If oslo.log provided a simple way to configure this then full logging configurations would be less common. Other suggestions are for ‘syslog’, ‘fluentd’ and ‘color’ logging.

There are three reasons to proceed with the proposed change. Firstly, making one change does not preclude the other. We can “make the simple easy and the complex possible”. Secondly, the plurality of “simple” configurations implies that another could be needed at any moment. Until this can be provided, users would have to do without or use a full logging config.

Lastly, although configuring an additional handler is the most common reason to use a full logging config, it is also common to reconfigure some log levels. If we expose options which can turn on Logstash and/or Fluentd and/or syslog handlers, plus set arbitrary logger levels - have we not reinvented the logging config system? And with a plethora of switches and options rather than a well-defined DSL. This feels like a retrograde step.

Impact on Existing APIs


Security impact


Performance Impact


Configuration Impact


Developer Impact


Testing Impact

Although the oslo.log change is simple, the ‘logging’ module is doing a lot for us. Unit tests will demonstrate the behaviour in cases such as when a handler initially configured is no longer mentioned when the config is reloaded.



Primary assignee:


Other contributors:


Work Items

  • Mark ‘log_config_append’ as mutable.

  • Call logging.fileConfig from the hook.

Documentation Impact



This work relates to the mutate_config_files work in oslo.config, which has already merged. adds a mutate hook to oslo.log. This spec will extend the hook by making log-config-append mutable.


The ‘logging’ module was creating a Lock before eventlet could monkey-patch it. Eventlet now patches that lock so this is no longer a problem. Gevent already has code to do this.


This work is licensed under a Creative Commons Attribution 3.0 Unported License.