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

What this error means

In a Redis replication setup, the primary node (master) handles all write operations while replica nodes sync data and serve read-only requests. When you try to write to a replica, Redis rejects the command with this error. This is a safety mechanism because replicas should only mirror the master's data, not create independent modifications that would cause inconsistency.

Typical symptoms

• Redis SET, DEL, INCR, or other write commands fail with the READONLY error
• Application cache writes stop working intermittently or consistently
• Error appears in logs after cluster failover or DNS changes
• Only read operations (GET, SCAN, etc.) work successfully

Common causes

1. Application is connected to a replica node instead of the master node
2. Failover event changed the master, but application still connects to old node
3. DNS caching in client library routes requests to wrong node
4. Redis configuration has replica-read-only enabled (default behavior)
5. Connection string points to read-only endpoint in managed Redis service
6. Client library retry logic chose a replica after network timeout

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

redis-cli
> INFO REPLICATION

> INFO REPLICATION
master_host:127.0.0.1
master_port:6379

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

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

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

redis-cli
> SLAVEOF NO ONE

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

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

Advanced notes

Frequently asked questions

What does "READONLY You can't write against a read only replica" mean?

In a Redis replication setup, the primary node (master) handles all write operations while replica nodes sync data and serve read-only requests. When you try to write to a replica, Redis rejects the command with this error. This is a safety mechanism because replicas should only mirror the master's data, not create independent modifications that would cause inconsistency.

How do I fix "READONLY You can't write against a read only replica"?

This error occurs when your application attempts a write operation on a Redis replica instead of the primary master node. Redis replicas are read-only by default to prevent accidental data corruption.

What causes "READONLY You can't write against a read only replica"?

Application is connected to a replica node instead of the master node Failover event changed the master, but application still connects to old node DNS caching in client library routes requests to wrong node

Is the "READONLY You can't write against a read only replica" error serious?

Yes. This is a high-severity error that usually blocks work until it is fixed, but the steps above resolve it.

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Official resources & further reading

• 📚Redis Official Documentation