Redis

WARNING: This document is a draft and the guidelines that it contains may change in the future as the Sentinel project evolves.

Guidelines for Redis clients with support for Redis Sentinel

Redis Sentinel is a monitoring solution for Redis instances that handles different aspects of monitoring, including notification of events, automatic failover. Sentinel can also play the role of configuration source for Redis clients. This document is targetted at Redis clients developers that want to support Sentinel in their clients implementation with the following goals:

For details about how Redis Sentinel works, please check the Redis Documentation itself, as this document only contains informations needed for Redis client developers.

Redis service discovery via Sentinel

Redis Sentinel identify every master with a name like "stats" or "cache". However the address of the Redis master that is used for a specific purpose inside a network may change after events like an automatic failover, a manually triggered failover (for instance in order to upgrade a Redis instance), and other reasons.

Normally Redis clients have some kind of hard-coded configuraiton that specifies the address of a Redis master instance within a network as IP address and port number. However if the master address changes, manual intervention in every client is needed.

A Redis client supporting Sentinel can automatically discover the address of a Redis master from the master name using Redis Sentinel. So instead of an hard coded IP address and port, a client supporting Sentinel should optionally be able to take as input:

This is the procedure a client should follow in order to obtain the master address starting from the list of Sentinels and the service name.

Step 1: connecting to the first Sentinel

The client should iterate the list of Sentinel addresses. For every address it should try to connect to the Sentinel, using a short timeout. On errors or timeouts the next Sentinel address should be tried.

If all the Sentinel addresses were tried without success, an error should be returned to the client.

Step 2: ask for master address

Once a connection with a Sentinel is established, the client should retry to execute the following command on the Sentinel:

SENTINEL get-master-addr-by-name master-name

Where master-name should be replaced with the actual service name specified by the user.

The result from this call can be one of the following three replies:

If an ip:port pair is received, this address should be used to connect to the Redis master. Otherwise if a null reply or -IDONTKNOW reply is received, the client should try the next Sentinel in the list.

Step 3: give priority to the replying Sentinel

When a correct ip:port pair is received, the replying Sentinel address should be put at the top of the list of Sentinel addresses, so that the next time we'll try the responding Sentinel before any other.

IMPORTANT: The result of this procedure should not be cached by the Redis client. Every time a new connection should be performed to a master the full resolving procedure should be used instead.

Handling reconnections

Once the service name is resoled into the master address and a connection is established with the Redis master instance, every time a reconnection is needed, the client should resolve again the address using Sentinels. For instance:

In the above cases and any other the client should resolve the master address again.

Connection pools

For clients implementing connection pools, on reconnection of a single connection, the Sentinel should be contacted again, and in case of a master address change all the existing connections should be closed and connected to the new address.

Error reporting

The client should correctly return the information to the user in case of errors. Specifically:

Sentinels list automatic refresh

Optionally once a successful reply to get-master-addr-by-name is received, a client may update its internal list of Sentinel nodes following this procedure:

It is not needed for a client to be able to make the list persistent updating its own configuration. The ability to upgrade the in-memory representation of the list of Sentinels can be already useful to improve reliability.

Additional information

For additional information or to discuss specific aspects of this guidelines, please drop a message to the Redis Google Group.