Redis in Magento 2 – three roles, env.php configuration, separate instances, monitoring

Redis in Magento 2 is not a single thing – it serves three distinct roles that should ideally run as separate instances with different configurations. Object cache, Full Page Cache, and session storage have very different access patterns and eviction requirements. I show how to configure each, why separate instances matter, and how to monitor what is happening.

Three roles, three configurations

env.php configuration – three separate Redis instances

<?php

// app/etc/env.php
return [
    'cache' => [
        'frontend' => [
            'default' => [
                'id_prefix' => 'abc_',
                'backend' => 'Magento\\Framework\\Cache\\Backend\\Redis',
                'backend_options' => [
                    'server'            => 'redis-cache',   // dedicated instance
                    'port'              => '6379',
                    'database'          => '0',
                    'password'          => '',
                    'compress_data'     => '1',
                    'compress_threshold'=> '2048',
                    'compression_lib'   => 'gzip',
                ],
            ],
            'page_cache' => [
                'id_prefix' => 'abc_',
                'backend' => 'Magento\\Framework\\Cache\\Backend\\Redis',
                'backend_options' => [
                    'server'            => 'redis-fpc',     // separate FPC instance
                    'port'              => '6379',
                    'database'          => '0',
                    'compress_data'     => '1',
                ],
            ],
        ],
    ],
    'session' => [
        'save'    => 'redis',
        'redis'   => [
            'host'              => 'redis-session', // separate session instance
            'port'              => '6379',
            'database'          => '0',
            'password'          => '',
            'timeout'           => '2.5',
            'persistent_identifier' => '',
            'compression_threshold' => '2048',
            'compression_library'   => 'gzip',
            'log_level'             => '1',
            'max_concurrency'       => '6',
            'break_after_frontend'  => '5',
            'break_after_adminhtml' => '30',
            'first_lifetime'        => '600',
            'bot_first_lifetime'    => '60',
            'bot_lifetime'          => '7200',
            'disable_locking'       => '0',
            'min_lifetime'          => '60',
            'max_lifetime'          => '2592000',
        ],
    ],
];

Why separate instances?

Running all three on one Redis instance creates problems:

• Sessions must never be evicted (user gets logged out) but cache can be
• FPC entries are large; object cache entries are small – different memory characteristics
• A spike in FPC writes can evict active sessions if they share memory
• Different maxmemory-policy settings cannot coexist on one instance

# redis-cache: allkeys-lru - evict any key when full
redis-cli -h redis-cache CONFIG SET maxmemory-policy allkeys-lru
redis-cli -h redis-cache CONFIG SET maxmemory 2gb

# redis-fpc: allkeys-lru
redis-cli -h redis-fpc CONFIG SET maxmemory-policy allkeys-lru
redis-cli -h redis-fpc CONFIG SET maxmemory 4gb

# redis-session: volatile-lru - only evict keys with TTL set (not sessions)
redis-cli -h redis-session CONFIG SET maxmemory-policy volatile-lru
redis-cli -h redis-session CONFIG SET maxmemory 512mb

Monitoring Redis from CLI

# Connection stats and memory
redis-cli INFO server | grep redis_version
redis-cli INFO memory | grep -E "used_memory_human|maxmemory_human"
redis-cli INFO stats | grep -E "keyspace_hits|keyspace_misses|evicted_keys"

# Hit rate calculation
# hit_rate = keyspace_hits / (keyspace_hits + keyspace_misses)
# Good: > 90%   Warning: 70-90%   Problem: < 70%

# Real-time commands monitor (use sparingly on production)
redis-cli MONITOR | grep -v PING

# Key count per database
redis-cli INFO keyspace

# Memory usage of a specific key
redis-cli MEMORY USAGE "abc_mage-tags-EAV_ENTITY_TYPES_1"

# Slowlog - find slow commands
redis-cli SLOWLOG GET 10

Redis in DDEV - multi-instance setup

# .ddev/docker-compose.redis.yaml
version: '3.6'
services:
  redis-cache:
    image: redis:7-alpine
    command: redis-server --maxmemory 512mb --maxmemory-policy allkeys-lru
    labels:
      com.ddev.site-name: ${DDEV_SITENAME}

  redis-fpc:
    image: redis:7-alpine
    command: redis-server --maxmemory 1gb --maxmemory-policy allkeys-lru
    labels:
      com.ddev.site-name: ${DDEV_SITENAME}

  redis-session:
    image: redis:7-alpine
    command: redis-server --maxmemory 256mb --maxmemory-policy volatile-lru --appendonly yes
    labels:
      com.ddev.site-name: ${DDEV_SITENAME}

Summary

Redis is the backbone of Magento 2 performance - but only when configured correctly. Separate instances with appropriate eviction policies prevent the worst failure mode: evicting user sessions because FPC filled the shared memory. Monitor hit rates weekly - a drop below 80% on the object cache means something is generating too many unique cache keys, which warrants investigation.