Conditionally tag a service in Symfony

README.md

My use case was a cache warmer that should only be run on the servers but not in other places. The cache warmer connects to a server to pre-fetch data. The warmer is configured with environment variables / DotEnv variables.

This might come in handy if you need your own conditional services.

1. Prevent autoconfigure for this service
2. Have a compiler pass that checks if the environment variable is defined, tag service if it exists

To know if a variable exists, let the ContainerBuilder resolve the env placeholders with format as true:

$container->resolveEnvPlaceholders('%env(API_DOMAIN)%', true);

Note that this evaluates the environment variable in the context where the container is built. If you build the container in a ci environment and then push to services, this would not see environment variables available only on the server. For that scenario, you would need to run a separate command on your servers when initializing the application.

CacheWarmerCompilerPass.php

<?php
// src/DependencyInjection/CacheWarmerCompilerPass.php

namespace App\DependencyInjection;

use App\Cache\MyCacheWarmer;
use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Exception\EnvNotFoundException;

/**
 * Decide whether to warm the cache, depending on whether the api config parameter exists.
 *
 * (Not available on the CI server).
 */
class CacheWarmerCompilerPass implements CompilerPassInterface
{
    public function process(ContainerBuilder $container)
    {
        try {
            $container->resolveEnvPlaceholders('%env(API_DOMAIN)%', true);
        } catch(EnvNotFoundException $e) {
            // do not tag service
            return;
        }
        $warmerDefinition = $container->getDefinition(MyCacheWarmer::class);
        $warmerDefinition->addTag('kernel.cache_warmer');
    }
}

services.yaml

# app/services.yaml
services:
    _defaults:
        autowire: true
        autoconfigure: true

    # Implements Symfony\Component\HttpKernel\CacheWarmer\CacheWarmerInterface
    App\Cache\MyCacheWarmer:
        arguments:
            # API_DOMAIN may or may not exist
            $domain: '%env(API_DOMAIN)'
            $cache: '@api_cache'
        # Disable autoconfigure for this service, otherwise it will always be loaded
        autoconfigure: false