# 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 v1.54.0 automatically converted the legacy structured logging configuration, which was deprecated in v1.23.0, to the standard library logging configuration.

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
            stream: 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.