Logging with Monolog

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Logging with Monolog

Symfony comes with an outside library - called Monolog - that allows you to create logs that can be stored in a variety of different places.

Logging a Message

To log a message, fetch the logger service from the container in your controller:

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public function indexAction()
{
    $logger = $this->get('logger');
    $logger->info('I just got the logger');
    $logger->error('An error occurred');

    $logger->critical('I left the oven on!', array(
        // include extra "context" info in your logs
        'cause' => 'in_hurry',
    ));

    // ...
}

The logger service has different methods for different logging levels/priorities. You can configure the logger to do different things based on the level of a message (e.g. send an email when an error occurs).

See LoggerInterface for a list of all of the methods on the logger.

Where Logs are Stored

The configuration for where logs are stored lives in the specific environment configuration files: config_dev.yml and config_prod.yml.

By default, log entries are written to the app/logs/dev.log file when you're in the dev environment. In the prod environment, logs are written to app/logs/prod.log, but only during a request where an error or high-priority log entry was made (i.e. error() , critical(), alert() or emergency()).

To control this, you'll configure different handlers that handle log entries, sometimes modify them, and ultimately store them.

Handlers: Writing Logs to different Locations

The logger has a stack of handlers, and each can be used to write the log entries to different locations (e.g. files, database, Slack, etc).

Tip

You can also configure logging "channels", which are like categories. Each channel can have its own handlers, which means you can store different log messages in different places. See How to Log Messages to different Files.

Symfony pre-configures some basic handlers in the config_dev.yml and config_prod.yml files. Check these out for some real-world examples.

This example uses two handlers: stream (to write to a file) and syslog to write logs using the syslog function:

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# app/config/config.yml
monolog:
    handlers:
        # this "file_log" key could be anything
        file_log:
            type: stream
            # log to app/logs/(environment).log
            path: "%kernel.logs_dir%/%kernel.environment%.log"
            # log *all* messages (debug is lowest level)
            level: debug

        syslog_handler:
            type: syslog
            # log error-level messages and higher
            level: error

This defines a stack of handlers and each handler is called in the order that it's defined.

Handlers that Modify Log Entries

Instead of writing log files somewhere, some handlers are used to filter or modify log entries before sending them to other handlers. One powerful, built-in handler called fingers_crossed is used in the prod environment by default. It stores all log messages during a request but only passes them to a second handler if one of the messages reaches an action_level. Take this example:

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# app/config/config.yml
monolog:
    handlers:
        filter_for_errors:
            type: fingers_crossed
            # if *one* log is error or higher, pass *all* to file_log
            action_level: error
            handler: file_log

        # now passed *all* logs, but only if one log is error or higher
        file_log:
            type: stream
            path: "%kernel.logs_dir%/%kernel.environment%.log"

        # still passed *all* logs, and still only logs error or higher
        syslog_handler:
            type: syslog
            level: error

Now, if even one log entry has an error level or higher, then all log entries for that request are saved to a file via the file_log handler. That means that your log file will contain all the details about the problematic request - making debugging much easier!

Tip

The handler named "file_log" will not be included in the stack itself as it is used as a nested handler of the fingers_crossed handler.

Note

If you want to override the monolog configuration via another config file, you will need to redefine the entire handlers stack. The configuration from the two files cannot be merged because the order matters and a merge does not allow to control the order.

All Built-in Handlers

Monolog comes with many built-in handlers for emailing logs, sending them to Loggly, or notifying you in Slack. These are documented inside of MonologBundle itself. For a full list, see Monolog Configuration.

How to Rotate your Log Files

Over time, log files can grow to be huge, both while developing and on production. One best-practice solution is to use a tool like the logrotate Linux command to rotate log files before they become too large.

Another option is to have Monolog rotate the files for you by using the rotating_file handler. This handler creates a new log file every day and can also remove old files automatically. To use it, just set the type option of your handler to rotating_file:

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# app/config/config_dev.yml
monolog:
    handlers:
        main:
            type:  rotating_file
            path:  '%kernel.logs_dir%/%kernel.environment%.log'
            level: debug
            # max number of log files to keep
            # defaults to zero, which means infinite files
            max_files: 10

Using a Logger inside a Service

To use a logger in your own services, add the @logger service as an argument of those services. If you want to use a pre-configured logger which uses a specific channel (app by default), use the monolog.logger tag with the channel property as explained in the Dependency Injection reference.

Adding extra Data to each Log (e.g. a unique request token)

Monolog also supports processors: functions that can dynamically add extra information to your log entries.

See How to Add extra Data to Log Messages via a Processor for details.

Learn more

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