AppKernel.php

<?php
use AppBundle\SncRedis\ClientsListPass;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\HttpKernel\Kernel;

class AppKernel extends Kernel
{
    public function registerBundles()
    {
        $bundles = array(/* listing bundles used in the app... */);
        return $bundles;
    }
    
    public function build(ContainerBuilder $container)
    {
        $container->addCompilerPass(new ClientsListPass());
    }

    // ... other AppKernel functions
}

ClientsList.php

<?php
namespace AppBundle\SncRedis;

class ClientsList
{
    private static $clients;

    public function __construct()
    {
        self::$clients = array();
    }

    public function addRedisClient($id, $alias = null)
    {
        self::$clients[$alias] = $id;
    }

    public function getClient(string $alias)
    {
        if (array_key_exists($alias, self::$clients)) {
            return self::$clients[$alias];
        }

        return null;
    }

    public function getClients()
    {
        return self::$clients;
    }
}

ClientsListPass.php

<?php
namespace AppBundle\SncRedis;

use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
use Symfony\Component\DependencyInjection\Reference;
use AppBundle\SncRedis\ClientsList;

/**
 * Find the services tagged 'snc_redis.client' and collect all the data
 *
 * The `addRedisClient` method isn't actually called yet. It will be only
 * be run if `AppBundle\SncRedis\ClientsList` is used in the app.
 */
class ClientsListPass implements CompilerPassInterface
{
    public function process(ContainerBuilder $container)
    {
        // Register the service for storage of Redis Clients
        $container->register(ClientsList::class, ClientsList::class);

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

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

        foreach ($taggedServices as $id => $tags) {
            // a service could have the same tag more than once
            foreach ($tags as $attributes) {
                $definition->addMethodCall('addRedisClient', array(
                    new Reference($id),
                    $attributes["alias"]
                ));
            }
        }
    }
}

DisplayRedisClients.php

<?php
// .....
// Show the list of snc_redis clients:
$sncRedisClients = $this->getContainer()->get(ClientsList::class);
$arrayOfSncRedisClients = $sncRedisClients->getClients();

$redisInfo = [];
foreach ($arrayOfSncRedisClients as $alias => $redis) {
    $redisInfo[] = [$redis->getDbNum(), $alias];
}

$table = new Table($output);
$table
    ->setHeaders(array('dbNum', 'Alias'))
    ->setRows($redisInfo);
$table->render();