How to Work with Service Tags

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Read the updated version of this page for Symfony 7.2 (the current stable version).

Service tags are a way to tell Symfony or other third-party bundles that your service should be registered in some special way. Take the following example:

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# config/services.yaml
services:
    App\Twig\AppExtension:
        tags: ['twig.extension']

Services tagged with the twig.extension tag are collected during the initialization of TwigBundle and added to Twig as extensions.

Other tags are used to integrate your services into other systems. For a list of all the tags available in the core Symfony Framework, check out Built-in Symfony Service Tags. Each of these has a different effect on your service and many tags require additional arguments (beyond the name parameter).

For most users, this is all you need to know. If you want to go further and learn how to create your own custom tags, keep reading.

Autoconfiguring Tags

If you enable autoconfigure, then some tags are automatically applied for you. That's true for the twig.extension tag: the container sees that your class extends AbstractExtension (or more accurately, that it implements ExtensionInterface) and adds the tag for you.

If you want to apply tags automatically for your own services, use the _instanceof option:

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# config/services.yaml
services:
    # this config only applies to the services created by this file
    _instanceof:
        # services whose classes are instances of CustomInterface will be tagged automatically
        App\Security\CustomInterface:
            tags: ['app.custom_tag']
    # ...

For more advanced needs, you can define the automatic tags using the registerForAutoconfiguration() method.

In a Symfony application, call this method in your kernel class:

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// src/Kernel.php
class Kernel extends BaseKernel
{
    // ...

    protected function build(ContainerBuilder $container): void
    {
        $container->registerForAutoconfiguration(CustomInterface::class)
            ->addTag('app.custom_tag')
        ;
    }
}

In a Symfony bundle, call this method in the load() method of the bundle extension class:

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// src/DependencyInjection/MyBundleExtension.php
class MyBundleExtension extends Extension
{
    // ...

    public function load(array $configs, ContainerBuilder $container): void
    {
        $container->registerForAutoconfiguration(CustomInterface::class)
            ->addTag('app.custom_tag')
        ;
    }
}

Creating custom Tags

Tags on their own don't actually alter the functionality of your services in any way. But if you choose to, you can ask a container builder for a list of all services that were tagged with some specific tag. This is useful in compiler passes where you can find these services and use or modify them in some specific way.

For example, if you are using Swift Mailer you might imagine that you want to implement a "transport chain", which is a collection of classes implementing \Swift_Transport. Using the chain, you'll want Swift Mailer to try several ways of transporting the message until one succeeds.

To begin with, define the TransportChain class:

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// src/Mail/TransportChain.php
namespace App\Mail;

class TransportChain
{
    private $transports;

    public function __construct()
    {
        $this->transports = [];
    }

    public function addTransport(\Swift_Transport $transport): void
    {
        $this->transports[] = $transport;
    }
}

Then, define the chain as a service:

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# config/services.yaml
services:
    App\Mail\TransportChain: ~

Define Services with a Custom Tag

Now you might want several of the \Swift_Transport classes to be instantiated and added to the chain automatically using the addTransport() method. For example, you may add the following transports as services:

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# config/services.yaml
services:
    Swift_SmtpTransport:
        arguments: ['%mailer_host%']
        tags: ['app.mail_transport']

    Swift_SendmailTransport:
        tags: ['app.mail_transport']

Notice that each service was given a tag named app.mail_transport. This is the custom tag that you'll use in your compiler pass. The compiler pass is what makes this tag "mean" something.

Create a Compiler Pass

You can now use a compiler pass to ask the container for any services with the app.mail_transport tag:

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// src/DependencyInjection/Compiler/MailTransportPass.php
namespace App\DependencyInjection\Compiler;

use App\Mail\TransportChain;
use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Reference;

class MailTransportPass implements CompilerPassInterface
{
    public function process(ContainerBuilder $container): void
    {
        // always first check if the primary service is defined
        if (!$container->has(TransportChain::class)) {
            return;
        }

        $definition = $container->findDefinition(TransportChain::class);

        // find all service IDs with the app.mail_transport tag
        $taggedServices = $container->findTaggedServiceIds('app.mail_transport');

        foreach ($taggedServices as $id => $tags) {
            // add the transport service to the TransportChain service
            $definition->addMethodCall('addTransport', [new Reference($id)]);
        }
    }
}

Register the Pass with the Container

In order to run the compiler pass when the container is compiled, you have to add the compiler pass to the container in a bundle extension or from your kernel:

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// src/Kernel.php
namespace App;

use App\DependencyInjection\Compiler\MailTransportPass;
use Symfony\Component\HttpKernel\Kernel as BaseKernel;
// ...

class Kernel extends BaseKernel
{
    // ...

    protected function build(ContainerBuilder $container): void
    {
        $container->addCompilerPass(new MailTransportPass());
    }
}

Tip

When implementing the CompilerPassInterface in a service extension, you do not need to register it. See the components documentation for more information.

Adding Additional Attributes on Tags

Sometimes you need additional information about each service that's tagged with your tag. For example, you might want to add an alias to each member of the transport chain.

To begin with, change the TransportChain class:

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class TransportChain
{
    private $transports;

    public function __construct()
    {
        $this->transports = [];
    }

    public function addTransport(\Swift_Transport $transport, $alias): void
    {
        $this->transports[$alias] = $transport;
    }

    public function getTransport($alias): ?\Swift_Transport
    {
        if (array_key_exists($alias, $this->transports)) {
            return $this->transports[$alias];
        }

        return null;
    }
}

As you can see, when addTransport() is called, it takes not only a Swift_Transport object, but also a string alias for that transport. So, how can you allow each tagged transport service to also supply an alias?

To answer this, change the service declaration:

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# config/services.yaml
services:
    Swift_SmtpTransport:
        arguments: ['%mailer_host%']
        tags:
            - { name: 'app.mail_transport', alias: 'smtp' }

    Swift_SendmailTransport:
        tags:
            - { name: 'app.mail_transport', alias: 'sendmail' }

Tip

In YAML format, you may provide the tag as a simple string as long as you don't need to specify additional attributes. The following definitions are equivalent.

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# config/services.yaml
services:
    # Compact syntax
    Swift_SendmailTransport:
        class: \Swift_SendmailTransport
        tags: ['app.mail_transport']

    # Verbose syntax
    Swift_SendmailTransport:
        class: \Swift_SendmailTransport
        tags:
            - { name: 'app.mail_transport' }

Notice that you've added a generic alias key to the tag. To actually use this, update the compiler:

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use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Reference;

class TransportCompilerPass implements CompilerPassInterface
{
    public function process(ContainerBuilder $container): void
    {
        // ...

        foreach ($taggedServices as $id => $tags) {

            // a service could have the same tag twice
            foreach ($tags as $attributes) {
                $definition->addMethodCall('addTransport', [
                    new Reference($id),
                    $attributes['alias'],
                ]);
            }
        }
    }
}

The double loop may be confusing. This is because a service can have more than one tag. You tag a service twice or more with the app.mail_transport tag. The second foreach loop iterates over the app.mail_transport tags set for the current service and gives you the attributes.

Reference Tagged Services

Symfony provides a shortcut to inject all services tagged with a specific tag, which is a common need in some applications, so you don't have to write a compiler pass just for that.

In the following example, all services tagged with app.handler are passed as first constructor argument to the App\HandlerCollection service:

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# config/services.yaml
services:
    App\Handler\One:
        tags: ['app.handler']

    App\Handler\Two:
        tags: ['app.handler']

    App\HandlerCollection:
        # inject all services tagged with app.handler as first argument
        arguments:
            - !tagged_iterator app.handler

After compilation the HandlerCollection service is able to iterate over your application handlers:

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// src/HandlerCollection.php
namespace App;

class HandlerCollection
{
    public function __construct(iterable $handlers)
    {
    }
}

See also

See also tagged locator services

Tagged Services with Priority

4.4

The ability to prioritize tagged services was introduced in Symfony 4.4.

The tagged services can be prioritized using the priority attribute. The priority is a positive or negative integer. The higher the number, the earlier the tagged service will be located in the collection:

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# config/services.yaml
services:
    App\Handler\One:
        tags:
            - { name: 'app.handler', priority: 20 }

Another option, which is particularly useful when using autoconfiguring tags, is to implement the static getDefaultPriority() method on the service itself:

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// src/Handler/One.php
namespace App\Handler;

class One
{
    public static function getDefaultPriority(): int
    {
        return 3;
    }
}

If you want to have another method defining the priority (e.g. getPriority() rather than getDefaultPriority()), you can define it in the configuration of the collecting service:

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# config/services.yaml
services:
    App\HandlerCollection:
        # inject all services tagged with app.handler as first argument
        arguments:
            - !tagged_iterator { tag: app.handler, default_priority_method: getPriority }

Tagged Services with Index

If you want to retrieve a specific service within the injected collection you can use the index_by and default_index_method options of the argument in combination with !tagged_iterator.

Using the previous example, this service configuration creates a collection indexed by the key attribute:

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# config/services.yaml
services:
    App\Handler\One:
        tags:
            - { name: 'app.handler', key: 'handler_one' }

    App\Handler\Two:
        tags:
            - { name: 'app.handler', key: 'handler_two' }

    App\HandlerCollection:
        arguments: [!tagged_iterator { tag: 'app.handler', index_by: 'key' }]

After compilation the HandlerCollection is able to iterate over your application handlers. To retrieve a specific service from the iterator, call the iterator_to_array() function and then use the key attribute to get the array element. For example, to retrieve the handler_two handler:

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// src/Handler/HandlerCollection.php
namespace App\Handler;

class HandlerCollection
{
    public function __construct(iterable $handlers)
    {
        $handlers = $handlers instanceof \Traversable ? iterator_to_array($handlers) : $handlers;

        $handlerTwo = $handlers['handler_two'];
    }
}

Tip

Just like the priority, you can also implement a static getDefaultIndexName() method in the handlers and omit the index attribute (key):

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// src/Handler/One.php
namespace App\Handler;

class One
{
    // ...
    public static function getDefaultIndexName(): string
    {
        return 'handler_one';
    }
}

You also can define the name of the static method to implement on each service with the default_index_method attribute on the tagged argument:

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# config/services.yaml
services:
    # ...

    App\HandlerCollection:
        # use getIndex() instead of getDefaultIndexName()
        arguments: [!tagged_iterator { tag: 'app.handler', default_index_method: 'getIndex' }]
This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
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