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If you are using the Platform.sh-provided Drupal template, most of this work is already done for you. Redis is already configured and will be enabled after the installatoin is complete.

If you are working from an older repository or migrating a pre-built site to Platform.sh, see the instructions below.

Requirements 

Add a Redis service 

In your .platform/services.yaml:

cacheredis:
    type: redis:6.0

Now add a relationship in your .platform.app.yaml file:

relationships:
    rediscache: "cacheredis:redis"

Add the Redis PHP extension 

You will need to enable the PHP Memcached extension. In your .platform.app.yaml file, add the following right after the type block:

runtime:
    extensions:
        - memcached

Add the Drupal module 

You will need to add the Redis module to your project. If you are using Composer to manage your Drupal site (which we recommend), run:

composer require drupal/redis

Then commit the resulting changes to your composer.json and composer.lock files.

Note that the Redis module does not need to be enabled in Drupal except for diagnostic purposes. The configuration below is sufficient to leverage its functionality.

Configuration 

Place the following at the end of settings.platformsh.php. Note the inline comments, as you may wish to customize it further. Also review the README.txt file that comes with the redis module, as it has a great deal more information on possible configuration options. For instance, you may wish to not use Redis for the persistent lock if you have a custom module that needs locks to persist for more than a few seconds.

The example below is intended as a “most common case”. (Note: This example assumes Drupal 8.8/Drupal 9.0 and later.)

<?php

// Set redis configuration.
if ($platformsh->hasRelationship('redis') && !\Drupal\Core\Installer\InstallerKernel::installationAttempted() && extension_loaded('redis')) {
  $redis = $platformsh->credentials('redis');

  // Set Redis as the default backend for any cache bin not otherwise specified.
  $settings['cache']['default'] = 'cache.backend.redis';
  $settings['redis.connection']['host'] = $redis['host'];
  $settings['redis.connection']['port'] = $redis['port'];

  // Apply changes to the container configuration to better leverage Redis.
  // This includes using Redis for the lock and flood control systems, as well
  // as the cache tag checksum. Alternatively, copy the contents of that file
  // to your project-specific services.yml file, modify as appropriate, and
  // remove this line.
  $settings['container_yamls'][] = 'modules/contrib/redis/example.services.yml';

  // Allow the services to work before the Redis module itself is enabled.
  $settings['container_yamls'][] = 'modules/contrib/redis/redis.services.yml';

  // Manually add the classloader path, this is required for the container cache bin definition below
  // and allows to use it without the redis module being enabled.
  $class_loader->addPsr4('Drupal\\redis\\', 'modules/contrib/redis/src');

  // Use redis for container cache.
  // The container cache is used to load the container definition itself, and
  // thus any configuration stored in the container itself is not available
  // yet. These lines force the container cache to use Redis rather than the
  // default SQL cache.
  $settings['bootstrap_container_definition'] = [
    'parameters' => [],
    'services' => [
      'redis.factory' => [
        'class' => 'Drupal\redis\ClientFactory',
      ],
      'cache.backend.redis' => [
        'class' => 'Drupal\redis\Cache\CacheBackendFactory',
        'arguments' => ['@redis.factory', '@cache_tags_provider.container', '@serialization.phpserialize'],
      ],
      'cache.container' => [
        'class' => '\Drupal\redis\Cache\PhpRedis',
        'factory' => ['@cache.backend.redis', 'get'],
        'arguments' => ['container'],
      ],
      'cache_tags_provider.container' => [
        'class' => 'Drupal\redis\Cache\RedisCacheTagsChecksum',
        'arguments' => ['@redis.factory'],
      ],
      'serialization.phpserialize' => [
        'class' => 'Drupal\Component\Serialization\PhpSerialize',
      ],
    ],
  ];
}

The example.services.yml file noted above will also use Redis for the lock and flood control systems.

The redis module is able to use Redis as a queue backend, however, that should not be done on an ephemeral Redis instance as that could result in lost items when the Redis service instance is restarted or fills up. If you wish to use Redis for the queue we recommend using a separate persistent Redis instance. See the Redis documentation page for more information.

Verifying Redis is running 

Run this command in a SSH session in your environment redis-cli -h redis.internal info. You should run it before you push all this new code to your repository.

This should give you a baseline of activity on your Redis installation. There should be very little memory allocated to the Redis cache.

After you push this code, you should run the command and notice that allocated memory will start jumping.

Clear SQL cache tables 

Once you’ve confirmed that your site is using Redis for caching, you can and should purge any remaining cache data in the MySQL database as it is now just taking up space. TRUNCATE any table that begins with cache except for cache_form. Despite its name cache_form is not part of the cache system proper and thus should not be moved out of SQL.