Redis "READONLY You can't write against a read only replica" in CI

The connection landed on a replica, and replicas reject writes by default (replica-read-only yes). Redis returns "READONLY You can't write against a read only replica." for SET and other write commands. In CI this usually means the test connected to a replica node or a Sentinel-managed pool pointed at one.

What this error means

Reads work but any write fails with "READONLY You can't write against a read only replica." during tests that use a replicated Redis or a Sentinel setup.

redis-cli

(error) READONLY You can't write against a read only replica.
# python:
redis.exceptions.ReadOnlyError: You can't write against a read only replica.

Common causes

The client connected to a replica, not the primary

A replicated or Sentinel topology routed the connection to a read-only replica, so writes are rejected while reads succeed.

A single node was misconfigured as a replica

The service was started with replicaof/slaveof, leaving it read-only even though tests expect a writable primary.

How to fix it

Write to the primary

For a simple test, run a standalone Redis with no replica role.
For Sentinel, resolve the master via the client and send writes there.
Verify with a test write against the resolved primary.

.github/workflows/ci.yml

# standalone, writable primary for tests
services:
  redis:
    image: redis:7
    ports: ['6379:6379']   # no replicaof, writes allowed

Route writes through the Sentinel master

Ask Sentinel for the current master and open a write connection to it, rather than a fixed replica address.

conftest.py

from redis.sentinel import Sentinel
s = Sentinel([("localhost", 26379)])
master = s.master_for("mymaster")
master.set("k", "v")

How to prevent it

Use a standalone writable Redis for tests unless replication is under test.
Resolve the master through Sentinel before writing.
Do not start the test node with replicaof/slaveof.