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Using Monolog

Installation

Monolog is available on Packagist (monolog/monolog) and as such installable via Composer.

composer require monolog/monolog

Core Concepts

Every Logger instance has a channel (name) and a stack of handlers. Whenever you add a record to the logger, it traverses the handler stack. Each handler decides whether it fully handled the record, and if so, the propagation of the record ends there.

This allows for flexible logging setups, for example having a StreamHandler at the bottom of the stack that will log anything to disk, and on top of that add a MailHandler that will send emails only when an error message is logged. Handlers also have a $bubble property which defines whether they block the record or not if they handled it. In this example, setting the MailHandler's $bubble argument to false means that records handled by the MailHandler will not propagate to the StreamHandler anymore.

You can create many Loggers, each defining a channel (e.g.: db, request, router, ..) and each of them combining various handlers, which can be shared or not. The channel is reflected in the logs and allows you to easily see or filter records.

Each Handler also has a Formatter, a default one with settings that make sense will be created if you don't set one. The formatters normalize and format incoming records so that they can be used by the handlers to output useful information.

Custom severity levels are not available. Only the eight RFC 5424 levels (debug, info, notice, warning, error, critical, alert, emergency) are present for basic filtering purposes, but for sorting and other use cases that would require flexibility, you should add Processors to the Logger that can add extra information (tags, user ip, ..) to the records before they are handled.

Log Levels

Monolog supports the logging levels described by RFC 5424.

  • DEBUG (100): Detailed debug information.

  • INFO (200): Interesting events. Examples: User logs in, SQL logs.

  • NOTICE (250): Normal but significant events.

  • WARNING (300): Exceptional occurrences that are not errors. Examples: Use of deprecated APIs, poor use of an API, undesirable things that are not necessarily wrong.

  • ERROR (400): Runtime errors that do not require immediate action but should typically be logged and monitored.

  • CRITICAL (500): Critical conditions. Example: Application component unavailable, unexpected exception.

  • ALERT (550): Action must be taken immediately. Example: Entire website down, database unavailable, etc. This should trigger the SMS alerts and wake you up.

  • EMERGENCY (600): Emergency: system is unusable.

Configuring a logger

Here is a basic setup to log to a file and to firephp on the DEBUG level:

<?php

use Monolog\Level;
use Monolog\Logger;
use Monolog\Handler\StreamHandler;
use Monolog\Handler\FirePHPHandler;

// Create the logger
$logger = new Logger('my_logger');
// Now add some handlers
$logger->pushHandler(new StreamHandler(__DIR__.'/my_app.log', Level::Debug));
$logger->pushHandler(new FirePHPHandler());

// You can now use your logger
$logger->info('My logger is now ready');

Let's explain it. The first step is to create the logger instance which will be used in your code. The argument is a channel name, which is useful when you use several loggers (see below for more details about it).

The logger itself does not know how to handle a record. It delegates it to some handlers. The code above registers two handlers in the stack to allow handling records in two different ways.

Note that the FirePHPHandler is called first as it is added on top of the stack. This allows you to temporarily add a logger with bubbling disabled if you want to override other configured loggers.

Adding extra data in the records

Monolog provides two different ways to add extra information along the simple textual message.

Using the logging context

The first way is the context, allowing to pass an array of data along the record:

<?php

$logger->info('Adding a new user', ['username' => 'Seldaek']);

Simple handlers (like the StreamHandler for instance) will simply format the array to a string but richer handlers can take advantage of the context (FirePHP is able to display arrays in a pretty way for instance).

Using processors

The second way is to add extra data for all records by using a processor. Processors can be any callable. They will get the record as parameter and must return it after having eventually changed the extra part of it. Let's write a processor adding some dummy data in the record:

<?php

$logger->pushProcessor(function ($record) {
    $record->extra['dummy'] = 'Hello world!';

    return $record;
});

Monolog provides some built-in processors that can be used in your project. Look at the dedicated chapter for the list.

Tip: processors can also be registered on a specific handler instead of the logger to apply only for this handler.

Leveraging channels

Channels are a great way to identify to which part of the application a record is related. This is useful in big applications (and is leveraged by MonologBundle in Symfony).

Picture two loggers sharing a handler that writes to a single log file. Channels would allow you to identify the logger that issued every record. You can easily grep through the log files filtering this or that channel.

<?php

use Monolog\Level;
use Monolog\Logger;
use Monolog\Handler\StreamHandler;
use Monolog\Handler\FirePHPHandler;

// Create some handlers
$stream = new StreamHandler(__DIR__.'/my_app.log', Level::Debug);
$firephp = new FirePHPHandler();

// Create the main logger of the app
$logger = new Logger('my_logger');
$logger->pushHandler($stream);
$logger->pushHandler($firephp);

// Create a logger for the security-related stuff with a different channel
$securityLogger = new Logger('security');
$securityLogger->pushHandler($stream);
$securityLogger->pushHandler($firephp);

// Or clone the first one to only change the channel
$securityLogger = $logger->withName('security');

Customizing the log format

In Monolog it's easy to customize the format of the logs written into files, sockets, mails, databases and other handlers; by the use of "Formatters".

As mentioned before, a Formatter is attached to a Handler, and as a general convention, most of the handlers use the

$record->formatted

property in the log record to store its formatted value.

You can choose between predefined formatter classes or write your own (e.g. a multiline text file for human-readable output).

Note:

A very useful formatter to look at, is the LineFormatter.

This formatter, as its name might indicate, is able to return a lineal string representation of the log record provided.

It is also capable to interpolate values from the log record, into the output format template used by the formatter to generate the final result, and in order to do it, you need to provide the log record values you are interested in, in the output template string using the form %value%, e.g: "'%context.foo% => %extra.foo%'" , in this example the values $record->context["foo"] and $record->extra["foo"] will be rendered as part of the final result.

In the following example, we demonstrate how to:

  1. Create a LineFormatter instance and set a custom output format template.
  2. Create a new Handler.
  3. Attach the Formatter to the Handler.
  4. Create a new Logger object.
  5. Attach the Handler to the Logger object.
<?php

// the default date format is "Y-m-d\TH:i:sP"
$dateFormat = "Y n j, g:i a";

// the default output format is "[%datetime%] %channel%.%level_name%: %message% %context% %extra%\n"
// we now change the default output format according to our needs.
$output = "%datetime% > %level_name% > %message% %context% %extra%\n";

// finally, create a formatter
$formatter = new LineFormatter($output, $dateFormat);

// Create a handler
$stream = new StreamHandler(__DIR__.'/my_app.log', Level::Debug);
$stream->setFormatter($formatter);

// bind it to a logger object
$securityLogger = new Logger('security');
$securityLogger->pushHandler($stream);

You may also reuse the same formatter between multiple handlers and share those handlers between multiple loggers.

Long running processes and avoiding memory leaks

When logging lots of data or especially when running background workers which are long-lived processes and do lots of logging over long periods of time, the memory usage of buffered handlers like FingersCrossedHandler or BufferHandler can rise quickly.

Monolog provides the ResettableInterface for this use case, allowing you to end a log cycle and get things back to their initial state.

Calling $logger->reset(); means flushing/cleaning all buffers, resetting internal state, and getting it back to a state in which it can receive log records again.

This is the conceptual equivalent of ending a web request, and can be done between every background job you process, or whenever appropriate. It reduces memory usage and also helps keep logs focused on the task at hand, avoiding log leaks between different jobs.

Handlers, Formatters and Processors →