READONLY error

A command that changes data was sent to a read-only replica.

Redis returns this error when you send a command that changes data to a replica. A replica keeps a copy of a primary's data, and by default it's read-only. Only the primary accepts writes.

READONLY You can't write against a read only replica.

In redis-py, this error is ReadOnlyError: You can't write against a read only replica.

A Lua function, or a script that starts with a #!lua line, gets the same error on a replica before it starts, unless it has the no-writes flag. A script without that line gets the error when it calls a command that changes data, with the script's details added to the end.

Example

> SET mykey "value"
(error) READONLY You can't write against a read only replica.
> GET mykey
"value"

How to fix it

Check which node you're connected to with ROLE. A replica replies with slave as the first item.

• You meant to connect to the primary. Check the host and port in your app's configuration.
• The primary changed. After a failover, a replica becomes the new primary, and the old primary can come back as a replica. An app that connects to a fixed address then ends up on a replica. Use a client that finds the current primary for you, through Redis Sentinel.

Don't make the replica writable to work around this error. The replica-read-only no setting exists for historical reasons, and writes to a replica can make its data differ from the primary's. See Read-only replica.