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How to fix “READONLY You Can’t Write Against a Read-Only Replica”

If you have trouble with the error message “READONLY You can’t write against a read only replica”, you have come to the right place.

This error means we are trying to perform a write operation (such as setting a value) on a Redis instance configured as a read-only replica. A read-only replica is a copy of the primary Redis database that allows only read operations and does not permit modifications.

Today, we will look at some of the causes behind this error and how to avoid them.

Causes of the Error

• The Redis instance we are connected to is a replica (secondary node) in a master-slave or primary-replica setup, which only allows read operations.
• The primary database failed or became temporarily unavailable, leaving only the read-only replica active.
• A security breach or misconfiguration might have inadvertently changed the Redis instance to read-only mode.

Let’s take a look at some solutions.

1. Read-only Mode

The first step is to verify if the Redis instance is in read-only mode. We can use the following command:

redis-cli > CONFIG GET replica-read-only

If the result is `yes`, our instance is configured as read-only.

If we are running Redis in a Docker container, we can disable the read-only setting by modifying your docker-compose.yml file. Add the following command to the Redis container configuration:

command: redis-server --appendonly yes --replica-read-only no

Once applied, restart the Redis container and verify the setting again:

redis-cli > CONFIG GET replica-read-only

If the result is `no`, the read-only restriction has been successfully disabled.

2. Ensure We Are Connecting to the Primary Instance

• Double-check the Django settings (or your application’s configuration) to ensure the connection string points to the primary Redis instance, not a read-only replica.
• If we use a managed Redis service, confirm that the application connects to the primary endpoint.
• If we use Redis Sentinel for high availability, ensure the settings direct connections to the master instance.

3. Secure the Redis Instance

In one case, the error was due to an unsecured Redis instance. The firewall ports were not closed during the initial server setup, allowing unauthorized access to the Redis container. The intruder changed the instance to read-only mode. To prevent this:

• Set a password for Redis, even within a private network.
• Close unnecessary firewall ports to restrict external access.
• Regularly audit your security settings to prevent unauthorized modifications.

4. Ensure Proper Redis Configuration

If we have control over our Redis instance, check the redis.conf file and make sure the following setting is applied:

replica-read-only no

However, if the setup intentionally includes read-only replicas for scaling or high availability, avoid making this change to prevent disrupting the architecture.

5. Change the Redis Image

If we use a Redis container, we may find that switching the Redis image can resolve compatibility issues. One user found success using:

image: redis:alpine

Instead of:

image: redis:latest

If we are experiencing unexpected issues, consider testing different Redis versions to see if one works better with the setup.

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Conclusion

The “READONLY You can’t write against a read-only replica” error usually arises due to misconfigurations, connection issues, or security oversights. By verifying our Redis instance’s role, securing access, and properly configuring your application, we can effectively resolve this issue and ensure smooth Redis operations.

In brief, our Support Experts demonstrated how to fix “READONLY You Can’t Write Against a Read-Only Replica”.