How to do advanced filtering of Monolog messages in Symfony?

I'm not sure why using a HandlerWrapper is the wrong way to do it.

I had the same issue and figured a way how to wrap a handler in order to filter certain records.

In this answer I describe two ways to solve this, a more complex and an easy one.

(More or less) complex way

First thing I did, was to create a new class wich extends the HandlerWrapper and added some logic where I can filter records:

use Monolog\Handler\HandlerWrapper;class CustomHandler extends HandlerWrapper{    public function isHandling(array $record)    {        if ($this->shouldFilter($record)) {            return false;        }        return $this->handler->isHandling($record);    }    public function handle(array $record)    {        if (!$this->isHandling($record)) {            return false;        }        return $this->handler->handle($record);    }    public function handleBatch(array $records)    {        foreach ($records as $record) {            $this->handle($record);        }    }    private function shouldFilter(array $record)    {        return mt_rand(0, 1) === 1;; // add logic here    }}

Then I created a service definition and a CompilerPass where I can wrap the GroupHandler

services.yml

CustomHandler:    class: CustomHandler    abstract: true    arguments: ['']

use Monolog\Handler\GroupHandler;use Symfony\Component\DependencyInjection\ChildDefinition;use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;use Symfony\Component\DependencyInjection\ContainerBuilder;use Symfony\Component\DependencyInjection\Definition;use Symfony\Component\DependencyInjection\Reference;class CustomMonologHandlerPass implements CompilerPassInterface{    public function process(ContainerBuilder $container)    {        if (!$container->hasDefinition(CustomHandler::class)) {            return;        }        $definitions = $container->getDefinitions();        foreach ($definitions as $serviceId => $definition) {            if (!$this->isValidDefinition($definition)) {                continue;            }            $cacheId = $serviceId . '.wrapper';            $container                ->setDefinition($cacheId, new ChildDefinition(CustomHandler::class))                ->replaceArgument(0, new Reference($cacheId . '.inner'))                ->setDecoratedService($serviceId);        }    }    private function isValidDefinition(Definition $definition): bool    {        return GroupHandler::class === $definition->getClass();    }}

As you can see I go over all definitions here and find the ones which have the GroupHandler set as their class. If this is the case, I add a new definition to the container which decorates the original handler with my CustomHandler.

Side note: At first I tried to wrap all handlers (except the CustomHandler of course :)) but due to some handlers implementing other interfaces (like the ConsoleHandler using the EventSubscriberInterface) this did not work and lead to issues I didn't want to solve in some hacky way.

Don't forget to add this compiler pass to the container in your AppBundle class

class AppBundle extends Bundle{    public function build(ContainerBuilder $container)    {        $container->addCompilerPass(new CustomMonologHandlerPass());    }}

Now that everything is in place you have to group your handlers in order to make this work:

app/config(_prod|_dev).yml

monolog:    handlers:        my_group:            type: group            members: [ 'graylog' ]        graylog:            type: gelf            publisher:                id: my.publisher            level: debug            formatter: my.formatter

Easy way

We use the same CustomHandler as we did in the complex way, then we define our handlers in the config:

app/config(_prod|_dev).yml

monolog:    handlers:        graylog:            type: gelf            publisher:                id: my.publisher            level: debug            formatter: my.formatter

Decorate the handler in your services.yml with your own CustomHandler

services.yml

CustomHandler:    class: CustomHandler    decorates: monolog.handler.graylog    arguments: ['@CustomHandler.inner']

For the decorates property you have to use the format monolog.handler.$NAME_SPECIFIED_AS_KEY_IN_CONFIG, in this case it was graylog.

... and thats it

Summary

While both ways work, I used the first one, as we have several symfony projects where I need this and decorating all handlersmanually is just not what I wanted.

I hope this helps (even though I'm quite late for an answer :))