How to Configure Monolog to Email Errors

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Caution

This feature is compatible with the new Symfony mailer starting from Symfony 5.1. All previous versions require using SwiftMailer.

Monolog can be configured to send an email when an error occurs within an application. The configuration for this requires a few nested handlers in order to avoid receiving too many emails. This configuration looks complicated at first but each handler is fairly straightforward when it is broken down.

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# config/packages/prod/monolog.yaml
monolog:
    handlers:
        main:
            type:         fingers_crossed
            # 500 errors are logged at the critical level
            action_level: critical
            # to also log 400 level errors (but not 404's):
            # action_level: error
            # excluded_http_codes: [404]
            handler:      deduplicated
        deduplicated:
            type:    deduplication
            handler: swift
        swift:
            type:       swift_mailer
            from_email: 'error@example.com'
            to_email:   'error@example.com'
            # or list of recipients
            # to_email:   ['dev1@example.com', 'dev2@example.com', ...]
            subject:    'An Error Occurred! %%message%%'
            level:      debug
            formatter:  monolog.formatter.html
            content_type: text/html

The main handler is a fingers_crossed handler which means that it is only triggered when the action level, in this case critical is reached. The critical level is only triggered for 5xx HTTP code errors. If this level is reached once, the fingers_crossed handler will log all messages regardless of their level. The handler setting means that the output is then passed onto the deduplicated handler.

Tip

If you want both 400 level and 500 level errors to trigger an email, set the action_level to error instead of critical. See the code above for an example.

The deduplicated handler keeps all the messages for a request and then passes them onto the nested handler in one go, but only if the records are unique over a given period of time (60 seconds by default). Duplicated records are discarded. Adding this handler reduces the amount of notifications to a manageable level, specially in critical failure scenarios. You can adjust the time period using the time option:

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# config/packages/prod/monolog.yaml
monolog:
    handlers:
        # ...
        deduplicated:
            type: deduplication
            # the time in seconds during which duplicate entries are discarded (default: 60)
            time: 10
            handler: swift

The messages are then passed to the swift handler. This is the handler that actually deals with emailing you the error. The settings for this are straightforward, the to and from addresses, the formatter, the content type and the subject.

You can combine these handlers with other handlers so that the errors still get logged on the server as well as the emails being sent:

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# config/packages/prod/monolog.yaml
monolog:
    handlers:
        main:
            type:         fingers_crossed
            action_level: critical
            handler:      grouped
        grouped:
            type:    group
            members: [streamed, deduplicated]
        streamed:
            type:  stream
            path:  '%kernel.logs_dir%/%kernel.environment%.log'
            level: debug
        deduplicated:
            type:    deduplication
            handler: swift
        swift:
            type:         swift_mailer
            from_email:   'error@example.com'
            to_email:     'error@example.com'
            subject:      'An Error Occurred! %%message%%'
            level:        debug
            formatter:    monolog.formatter.html
            content_type: text/html

This uses the group handler to send the messages to the two group members, the deduplicated and the stream handlers. The messages will now be both written to the log file and emailed.

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
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