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

What this error means

In a Redis replication architecture, the primary master node handles all write operations while replica nodes synchronize data and serve read-only requests. When a write command (SET, DEL, INCR, HSET, etc.) is issued against a replica, Redis immediately rejects it with this READONLY error. This protection ensures that replicas remain exact copies of the master and prevents split-brain scenarios where replicas diverge from the master's state.

Typical symptoms

• Redis write operations fail with the exact error message 'READONLY You can't write against a read only replica'
• All write commands (SET, DEL, INCR, LPUSH, HSET, etc.) are rejected
• Read-only operations (GET, SCAN, HGET, etc.) work correctly
• Error appears suddenly after cluster failover, node restart, or DNS changes
• Error occurs intermittently if load balancing routes some requests to replicas

Common causes

1. Application connection string points to a replica node instead of the master node
2. Automatic failover promoted a replica to master, but the application still connects to the old master (now a replica)
3. Load balancer or connection pooling is incorrectly routing write requests to replica nodes
4. Redis Cluster misconfiguration where client doesn't understand slot-to-master mapping
5. DNS caching in application or client library stale after topology change
6. Redis client library's retry logic is retrying failed writes on wrong node
7. Managed Redis service (AWS ElastiCache, Google Memorystore) configured with read-only endpoint as primary

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

redis-cli -h <your-redis-host> -p <your-redis-port>
> INFO REPLICATION

redis-cli -h <replica-host> -p <replica-port>
> INFO REPLICATION

master_host:10.0.0.5
master_port:6379

const redis = require('ioredis');
const client = new redis({
  host: 'master-host-ip',  // NOT replica-host-ip
  port: 6379,
  password: 'your-password',
  retryStrategy: (times) => Math.min(times * 50, 2000)
});

const { createClient } = require('redis');
const client = createClient({
  host: 'master-host-ip',
  port: 6379
});

import redis
r = redis.Redis(
    host='master-host-ip',
    port=6379,
    password='your-password',
    decode_responses=True
)
# Test with a write
r.set('test-key', 'test-value')

Jedis jedis = new Jedis("master-host-ip", 6379);
jedis.set("test-key", "test-value");

const redis = require('ioredis');
const client = new redis.Sentinel([
  { host: 'sentinel-1', port: 26379 },
  { host: 'sentinel-2', port: 26379 },
  { host: 'sentinel-3', port: 26379 }
], {
  name: 'mymaster'  // Service name in sentinel config
});

from redis.sentinel import Sentinel
sentinels = [('sentinel-1', 26379), ('sentinel-2', 26379)]
sentinel = Sentinel(sentinels)
master = sentinel.master_for('mymaster', socket_timeout=0.1)
master.set('key', 'value')

const redis = require('ioredis');
const cluster = new redis.Cluster([
  { host: 'node-1', port: 6379 },
  { host: 'node-2', port: 6379 },
  { host: 'node-3', port: 6379 }
]);
cluster.set('key', 'value');  // Client routes to correct slot master

from rediscluster import RedisCluster
rc = RedisCluster(
    startup_nodes=[{'host': 'node-1', 'port': 6379}],
    decode_responses=True
)
rc.set('key', 'value')

Advanced notes

redis-cli -h <replica-host>
> CONFIG SET replica-read-only no

Frequently asked questions

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

In a Redis replication architecture, the primary master node handles all write operations while replica nodes synchronize data and serve read-only requests. ) is issued against a replica, Redis immediately rejects it with this READONLY error. This protection ensures that replicas remain exact copies of the master and prevents split-brain scenarios where replicas diverge from the master's state.

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 design to maintain data consistency across the replication cluster.

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

Application connection string points to a replica node instead of the master node Automatic failover promoted a replica to master, but the application still connects to the old master (now a replica) Load balancer or connection pooling is incorrectly routing write requests to replica nodes

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.

Related errors

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Official resources & further reading

• 📚Redis Official Documentation
• 📖
  Redis Error Handling Guide(Official Docs)
• 📖
  Redis Replication Documentation(Official Docs)
• 📖
  Redis Sentinel Guide(Official Docs)