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Cache Backend Configuration

When to Use

When you need to configure cache backends (Redis, Memcache, APCu) in settings.php. Backend configuration determines where cached data is stored and how it's accessed.

Steps

  1. Install backend module and dependencies 

    # Redis
composer require drupal/redis
drush en redis

# Memcache
composer require drupal/memcache
drush en memcache

  2. Configure backend in settings.php 

    // Edit sites/default/settings.php

  3. Set default backend or per-bin backends 

    // All bins use Redis
$settings['cache']['default'] = 'cache.backend.redis';

// Or per-bin configuration
$settings['cache']['bins']['render'] = 'cache.backend.redis';
$settings['cache']['bins']['page'] = 'cache.backend.redis';

  4. Configure connection settings 

    // Redis connection
$settings['redis.connection']['interface'] = 'PhpRedis';
$settings['redis.connection']['host'] = '127.0.0.1';

// Memcache connection
$settings['memcache']['servers'] = ['127.0.0.1:11211' => 'default'];

Decision Points

At this step... If... Then...
Choosing backend High-traffic production site Use Redis or Memcache, not database
Choosing backend Multi-server setup Use shared backend (Redis/Memcache), not APCu
Choosing backend Single-server low-traffic Database backend acceptable
Configuring bins Small stable bins (bootstrap, discovery) Use ChainedFastBackend with APCu
Configuring bins Large high-churn bins (render, page) Use Redis or Memcache

Pattern

Redis configuration (recommended): 

// settings.php
$settings['redis.connection']['interface'] = 'PhpRedis';
$settings['redis.connection']['host'] = '127.0.0.1';
$settings['redis.connection']['port'] = 6379;
$settings['redis.connection']['base'] = 0;
$settings['redis.connection']['password'] = NULL;
$settings['cache']['default'] = 'cache.backend.redis';

// Optional: compression
$settings['redis_compress_length'] = 100;
$settings['redis_compress_level'] = 1;

Memcache configuration: 

// settings.php
$settings['memcache']['servers'] = ['127.0.0.1:11211' => 'default'];
$settings['memcache']['bins'] = ['default' => 'default'];
$settings['memcache']['key_prefix'] = '';
$settings['cache']['default'] = 'cache.backend.memcache';

ChainedFastBackend (APCu + Redis): 

// settings.php
$settings['cache']['bins']['bootstrap'] = 'cache.backend.chainedfast';
$settings['cache']['bins']['discovery'] = 'cache.backend.chainedfast';

// ChainedFast uses APCu (fast) + configured default backend (consistent)

Per-bin configuration: 

// Different backends for different bins
$settings['cache']['default'] = 'cache.backend.redis';
$settings['cache']['bins']['bootstrap'] = 'cache.backend.chainedfast';
$settings['cache']['bins']['page'] = 'cache.backend.redis';
$settings['cache']['bins']['render'] = 'cache.backend.redis';

Reference: /sites/default/default.settings.php (search for "cache")

Common Mistakes

  • Using database backend on high-traffic sites — Slow; use Redis/Memcache for production
  • Not setting Redis eviction policy — Redis fills up; set maxmemory and maxmemory-policy allkeys-lru
  • Using APCu on multi-server — Per-server cache causes inconsistency; use shared backend
  • Mixing Redis and Memcache — Pick one; don't split bins across different shared backends
  • Not monitoring memory usage — Redis/Memcache can run out of memory; monitor and adjust limits

See Also