Redis: OOM command not allowed when used memory > 'maxmemory'

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Root Cause

Redis is an in-memory datastore. This error occurs when you attempt a write operation (like `SET`, `HSET`, `LPUSH`) but the server has reached its configured `maxmemory` limit. Redis protects itself from crashing the host operating system by refusing new data. If an eviction policy isn't configured (or is set to `noeviction`), Redis will continue to serve read requests but will reject all writes until memory is freed manually.

Fix / Solution

You have several options depending on your architecture. You can increase the `maxmemory` limit in `redis.conf` if the server has physical RAM available. Alternatively, you can configure an eviction policy (like `allkeys-lru` or `volatile-lru`) so Redis automatically deletes older or less important keys to make room for new ones. Finally, you can manually delete unnecessary keys or set a Time-To-Live (TTL) on your keys using `EXPIRE` so they clean themselves up.

Code Snippet

# ❌ Redis blocks write command
SET session_123 "data"
# (error) OOM command not allowed...

# ✅ Fix 1: Configure eviction policy in redis.conf
maxmemory 2gb
maxmemory-policy allkeys-lru

# ✅ Fix 2: Always set an expiration (TTL) on volatile data
SETEX session_123 3600 "data"