How to fix READONLY You can't write against a read only replica in Redis

Connect to your Redis instance and check its role:

redis-cli
> INFO REPLICATION

Look for the role field. If it shows role:slave, you're on a replica. If it shows role:master, the replica-read-only setting is the issue.

Get the master node's address from the replica:

> INFO REPLICATION
master_host:127.0.0.1
master_port:6379

Note the master's host and port. You should connect your application here for write operations.

Change your Redis client to connect to the master node's address:

Node.js example:

const redis = require('redis');
const client = redis.createClient({
  host: 'master-node-ip',  // Use master, not replica
  port: 6379
});

Python example:

import redis
r = redis.Redis(host='master-node-ip', port=6379, db=0)

Verify with a test write:

r.set('test-key', 'value')

If using AWS ElastiCache, Google Cloud Memorystore, or Azure Cache:

1. Open your Redis service console
2. Find the primary/master endpoint (not read-replica endpoint)
3. Update your application configuration to use this endpoint

Example AWS ElastiCache:
- Use: my-redis-cluster.abc123.ng.0001.use1.cache.amazonaws.com:6379
- NOT: my-redis-replica.abc123.ng.0001.use1.cache.amazonaws.com:6379

Only do this if the original master is permanently down and you understand the implications:

redis-cli
> SLAVEOF NO ONE

This makes the replica a standalone master. Use with caution as you may lose recent writes if the original master was still running.

If using Redis Sentinel or Cluster mode:

For Sentinel:
- Connect to sentinel port (26379) and query for master
- Let the client library handle master discovery
- Ensure your client supports sentinel protocol

For Cluster:
- Use a cluster-aware client library
- Set cluster-enabled yes in redis.conf only on cluster nodes
- Let the client discover all nodes and send writes to slots' owners

Connection pooling tip:

// Bad: Creates new connection per request
const client = redis.createClient({host: 'master'});

// Good: Reuse connection pool
const pool = redis.createPool({host: 'master', max: 10});