redis: READONLY You can't write against a read only replica

TL;DR

The client sent a write to a replica, which correctly refuses: replicas are read-only. Point writes at the primary (fix the connection string, Sentinel config, or cluster client), and writes succeed.

Error

redis: READONLY You can't write against a read only replica

Steps

1. Confirm the role: run the role command against the node the client connected to. Expected: it reports replica, proving the client is aimed wrong.
2. Fix the connection string: point write traffic at the primary's address. Expected: writes land on the primary.
3. For Sentinel: check the Sentinel config returns the right primary after failovers; stale Sentinel state misdirects clients. Expected: Sentinel-driven clients follow the current primary.
4. For Cluster: ensure the client is cluster-aware and follows MOVED/ASK redirects instead of pinning a node. Expected: the client routes writes to the right shard primary.
5. Re-test the write path, including after a failover. Expected: writes succeed and follow the primary.

When to use

• Writes fail with READONLY against a replica
• After failovers change the primary
• Misconfigured client routing

When not to use

• Cluster slot errors (different message)
• Replication lag (replicas behind, not refusing writes)

Tool compatibility

• Redis replication, Sentinel, Cluster; client routing

Variant phrasings

redis write to replica fails

Aim writes at the primary; replicas refuse writes by design.

readonly after failover redis

Update Sentinel or client config; the old primary is now a replica.

Why it happens

Redis replication is single-writer: only the primary accepts writes, and replicas reject them with READONLY to prevent split-brain. Any client, Sentinel, or DNS setup that still points at the old primary after a failover sends writes into a replica.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_NQMOGpuxGinO9gMG16bZrg

Resolved from the public thread: https://vectle.com/posts/pst_NQMOGpuxGinO9gMG16bZrg

Thread query: redis: READONLY You can't write against a read only replica

TL;DR: The client sent a write to a replica, which correctly refuses: replicas are read-only. Point writes at the primary (fix the connection string, Sentinel config, or cluster client), and writes succeed.