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Structured Logging

A structured logging system can be useful when your logs are destined for a machine to parse and process. By maintaining its machine-readable characteristics, it enables more efficient searching and aggregations when consumed by software such as the "ELK stack".

Synapse's structured logging system is configured via the file that Synapse's log_config config option points to. The file should include a formatter which uses the synapse.logging.TerseJsonFormatter class included with Synapse and a handler which uses the above formatter.

There is also a synapse.logging.JsonFormatter option which does not include a timestamp in the resulting JSON. This is useful if the log ingester adds its own timestamp.

A structured logging configuration looks similar to the following:

version: 1

formatters:
    structured:
        class: synapse.logging.TerseJsonFormatter

handlers:
    file:
        class: logging.handlers.TimedRotatingFileHandler
        formatter: structured
        filename: /path/to/my/logs/homeserver.log
        when: midnight
        backupCount: 3  # Does not include the current log file.
        encoding: utf8

loggers:
    synapse:
        level: INFO
        handlers: [remote]
    synapse.storage.SQL:
        level: WARNING

The above logging config will set Synapse as 'INFO' logging level by default, with the SQL layer at 'WARNING', and will log to a file, stored as JSON.

It is also possible to figure Synapse to log to a remote endpoint by using the synapse.logging.RemoteHandler class included with Synapse. It takes the following arguments:

  • host: Hostname or IP address of the log aggregator.
  • port: Numerical port to contact on the host.
  • maximum_buffer: (Optional, defaults to 1000) The maximum buffer size to allow.

A remote structured logging configuration looks similar to the following:

version: 1

formatters:
    structured:
        class: synapse.logging.TerseJsonFormatter

handlers:
    remote:
        class: synapse.logging.RemoteHandler
        formatter: structured
        host: 10.1.2.3
        port: 9999

loggers:
    synapse:
        level: INFO
        handlers: [remote]
    synapse.storage.SQL:
        level: WARNING

The above logging config will set Synapse as 'INFO' logging level by default, with the SQL layer at 'WARNING', and will log JSON formatted messages to a remote endpoint at 10.1.2.3:9999.

Upgrading from legacy structured logging configuration

Versions of Synapse prior to v1.23.0 included a custom structured logging configuration which is deprecated. It used a structured: true flag and configured drains instead of handlers and formatters.

Synapse currently automatically converts the old configuration to the new configuration, but this will be removed in a future version of Synapse. The following reference can be used to update your configuration. Based on the drain type, we can pick a new handler:

  1. For a type of console, console_json, or console_json_terse: a handler with a class of logging.StreamHandler and a stream of ext://sys.stdout or ext://sys.stderr should be used.
  2. For a type of file or file_json: a handler of logging.FileHandler with a location of the file path should be used.
  3. For a type of network_json_terse: a handler of synapse.logging.RemoteHandler with the host and port should be used.

Then based on the drain type we can pick a new formatter:

  1. For a type of console or file no formatter is necessary.
  2. For a type of console_json or file_json: a formatter of synapse.logging.JsonFormatter should be used.
  3. For a type of console_json_terse or network_json_terse: a formatter of synapse.logging.TerseJsonFormatter should be used.

For each new handler and formatter they should be added to the logging configuration and then assigned to either a logger or the root logger.

An example legacy configuration:

structured: true

loggers:
    synapse:
        level: INFO
    synapse.storage.SQL:
        level: WARNING

drains:
    console:
        type: console
        location: stdout
    file:
        type: file_json
        location: homeserver.log

Would be converted into a new configuration:

version: 1

formatters:
    json:
        class: synapse.logging.JsonFormatter

handlers:
    console:
        class: logging.StreamHandler
        location: ext://sys.stdout
    file:
        class: logging.FileHandler
        formatter: json
        filename: homeserver.log

loggers:
    synapse:
        level: INFO
        handlers: [console, file]
    synapse.storage.SQL:
        level: WARNING

The new logging configuration is a bit more verbose, but significantly more flexible. It allows for configuration that were not previously possible, such as sending plain logs over the network, or using different handlers for different modules.