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

What this error means

In Redis, replication creates a master-replica topology where the master accepts both read and write operations, while replicas (also called slaves) only accept read operations. Since Redis 2.6, replicas are configured with replica-read-only enabled by default to protect against accidental writes that would cause data inconsistency. When your application sends a write command (SET, DEL, INCR, etc.) to a replica, Redis immediately rejects it with the READONLY error because replicas are designed to be read-only copies of the master data.

Typical symptoms

• Write operations fail with "READONLY You can't write against a read only replica" error
• Application crashes or hangs when attempting SET, LPUSH, HSET, or other write commands
• Error occurs sporadically during or after a failover event
• Connection string seems correct but writes fail silently in some requests
• Works in development but fails in production with Redis replication setup

Common causes

1. Connecting to a replica node endpoint instead of the master primary endpoint
2. Application is inadvertently routing traffic to a read-only replica due to DNS caching or load balancer misconfiguration
3. Redis failover occurred and the old primary node was demoted to a replica, but your application still connects to it
4. Using cluster mode with read replicas and attempting writes without proper READONLY/READWRITE command sequencing
5. Unauthorized access changed the Redis instance to read-only mode via CONFIG SET replica-read-only yes
6. Sentinel or HA setup is not properly directing connections to the new master after failover

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

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

const redis = require("redis");
const client = redis.createClient({
  host: "myredis-primary.abc123.ng.0001.use1.cache.amazonaws.com", // Correct: primary endpoint
  port: 6379,
});

// Wrong: using node endpoint instead of cluster endpoint
const client = redis.createClient({
  host: "myredis-node-001.abc123.ng.0001.use1.cache.amazonaws.com",
  port: 6379,
});

# On Linux, flush systemd DNS cache
sudo resolvectl flush-caches

# Or use dnsmasq
sudo systemctl restart dnsmasq

from redis.sentinel import Sentinel
sentinel = Sentinel([(host, 26379) for host in sentinel_hosts])
redis_conn = sentinel.master_for("mymaster", socket_connect_timeout=0.2)

# Query your infrastructure for the new primary endpoint
# AWS ElastiCache example:
aws elasticache describe-replication-groups --replication-group-id myredis

# Look for "PrimaryEndpoint" in the response

const redis = require("redis");
const client = redis.createClient({
  sentinels: [
    { host: "sentinel1", port: 26379 },
    { host: "sentinel2", port: 26379 },
  ],
  name: "mymaster", // The logical name of the master set
});

redis-cli CONFIG GET replica-read-only
# Returns: "replica-read-only" "yes"

redis-cli CONFIG SET replica-read-only no

replica-read-only no

const redis = require("redis");
const { promisify } = require("util");

const client = redis.createClient();
const infoAsync = promisify(client.info).bind(client);

async function setWithFailover(key: string, value: string, maxRetries = 3) {
  for (let i = 0; i < maxRetries; i++) {
    try {
      // Attempt the write
      return await promisify(client.set).bind(client)(key, value);
    } catch (err) {
      if (err.message.includes("READONLY")) {
        // Reconnect to primary endpoint
        console.warn("Connected to replica, reconnecting to primary...");
        // Reload configuration or reconnect client
        continue;
      }
      throw err;
    }
  }
}

Advanced notes

Frequently asked questions

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

In Redis, replication creates a master-replica topology where the master accepts both read and write operations, while replicas (also called slaves) only accept read operations. 6, replicas are configured with replica-read-only enabled by default to protect against accidental writes that would cause data inconsistency. ) to a replica, Redis immediately rejects it with the READONLY error because replicas are designed to be read-only copies of the master data.

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

This error occurs when your application tries to write to a Redis read-only replica instead of the primary master node. Redis replicas are intentionally set to read-only by default to prevent accidental writes and data inconsistency.

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

Connecting to a replica node endpoint instead of the master primary endpoint Application is inadvertently routing traffic to a read-only replica due to DNS caching or load balancer misconfiguration Redis failover occurred and the old primary node was demoted to a replica, but your application still connects to it

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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• LOADING Redis is loading the dataset in memory

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Official resources & further reading

• 📚Redis Official Documentation
• 📖
  Redis Replication Documentation(Official Docs)
• 📖
  Redis READONLY Command(Official Docs)
• AWS ElastiCache READONLY Troubleshooting
• Comprehensive READONLY Error Guide