Redis vs Memcached for WordPress Object Caching: Full Comparison

WordPress object caching with an external backend — Redis or Memcached — eliminates redundant database queries by storing the results of wp_cache_get() calls in memory. Choosing between Redis and Memcached depends on your persistence, data-structure, and cluster requirements.

Problem: WordPress object cache hits are volatile — every PHP-FPM worker has its own in-memory cache that is discarded at request end, so the same expensive query runs once per worker per request instead of being shared.

Solution: Install a persistent object cache backend: Redis for sites that need data persistence and WooCommerce session storage (use the wp-redis drop-in), or Memcached for pure read caching without persistence requirements. Drop the object-cache.php file into wp-content/ and configure the connection in wp-config.php.

The comparison below covers installation, WordPress integration, persistence, eviction policies, and the key functional differences that matter for WordPress sites.

# ── REDIS ──
# Install on Ubuntu/Debian
sudo apt install redis-server php-redis
# Enable in php.ini / conf.d:
# extension=redis.so

# Install the WP Redis drop-in (Automattic)
composer require "automattic/wp-redis"
# or via WP-CLI:
wp plugin install redis-cache --activate
wp redis enable   # copies object-cache.php drop-in to wp-content/

# ── MEMCACHED ──
sudo apt install memcached php-memcached
# Install the WP Memcached drop-in:
# Download object-cache.php from wordpress.org/plugins/memcached/
# Copy to wp-content/object-cache.php

Configure each backend in wp-config.php and understand the key differences:

<?php
// ── REDIS wp-config.php ──
define( 'WP_REDIS_HOST', '127.0.0.1' );
define( 'WP_REDIS_PORT', 6379 );
define( 'WP_REDIS_DATABASE', 0 );       // Redis supports 16 logical DBs
define( 'WP_REDIS_PREFIX', 'helloadmin_' ); // namespace for multisite
define( 'WP_REDIS_TIMEOUT', 1 );        // connection timeout in seconds
define( 'WP_REDIS_READ_TIMEOUT', 1 );
define( 'WP_REDIS_MAXTTL', 3600 );      // cap all TTLs at 1 hour

// ── MEMCACHED wp-config.php ──
global $memcached_servers;
$memcached_servers = [ 'default' => [ [ '127.0.0.1', 11211 ] ] ];

/*
 * REDIS vs MEMCACHED — quick reference:
 *
 * Feature               Redis              Memcached
 * ────────────
 * Persistence           Yes (RDB/AOF)      No (RAM only)
 * Data structures       Strings,Hash,      Strings only
 *                       List,Set,ZSet
 * Pub/Sub               Yes                No
 * Cluster/HA            Redis Cluster      Built-in multi-server
 * Atomic operations     Yes (MULTI/EXEC)   CAS only
 * Memory efficiency     Moderate           Slightly better
 * WP multisite support  Excellent          Good
 *
 * Choose Redis if:  you need persistence after restart, you use
 *                   WooCommerce sessions, or you want Pub/Sub.
 * Choose Memcached if: you already run Memcached, or you need
 *                       the absolute simplest memory-only cache.
 */

// Test the object cache from WP-CLI:
// wp cache get <key> <group>
// wp cache flush
// wp redis status

NOTE: For most WordPress sites running WooCommerce, Redis is the better choice because it persists session data across restarts and supports WooCommerce's session handler. For purely static-content sites where cache loss on restart is acceptable, Memcached is slightly simpler to operate.

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