Magento 2 cart loss – Redis session and eviction policy

Customers were losing their cart contents and getting logged out randomly during shopping. The problem intensified on weekends with high traffic. Fix time: 2 hours.

Symptoms

• Customers randomly logged out during shopping
• Cart resets without reason
• Problem worse during high traffic
• Redis INFO memory: used_memory close to maxmemory

Cause

Redis was configured with maxmemory-policy allkeys-lru – when free memory ran out it evicted the least recently used keys, including active user sessions.

Solution

redis-cli CONFIG SET maxmemory-policy volatile-lru
redis-cli CONFIG SET maxmemory 2gb

<?php
// app/etc/env.php - separate Redis instances for sessions and cache
'session' => [
    'save' => 'redis',
    'redis' => ['host' => '127.0.0.1', 'port' => '6380'],  // sessions
],
'cache' => [
    'frontend' => ['default' => [
        'backend_options' => ['server' => '127.0.0.1', 'port' => '6379'],  // cache
    ]],
],

Takeaways

Using Redis for both cache and sessions simultaneously is asking for trouble. Cache can be evicted – sessions cannot. Separate them into two instances with different eviction policies: allkeys-lru for cache, noeviction or volatile-lru for sessions.