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Upgrading Consul

Consul is meant to be a long-running agent on any nodes participating in a Consul cluster. These nodes consistently communicate with each other. As such, protocol level compatibility and ease of upgrades is an important thing to keep in mind when using Consul.

This page documents how to upgrade Consul when a new version is released.

Upgrading Consul

In short, upgrading Consul is a short series of easy steps. For the steps below, assume you're running version A of Consul, and then version B comes out.

  1. On each node, install version B of Consul.

  2. Shut down version A, and start version B with the -protocol=PREVIOUS flag, where "PREVIOUS" is the protocol version of version A (which can be discovered by running consul -v or consul members).

  3. Once all nodes are running version B, go through every node and restart the version B agent without the -protocol flag.

  4. Done! You're now running the latest Consul agent speaking the latest protocol. You can verify this is the case by running consul members to make sure all members are speaking the same, latest protocol version.

The key to making this work is the protocol compatibility of Consul. The protocol version system is discussed below.

Protocol Versions

By default, Consul agents speak the latest protocol they can. However, each new version of Consul is also able to speak the previous protocol, if there were any protocol changes.

You can see what protocol versions your version of Consul understands by running consul -v. You'll see output similar to that below:

$ consul -v
Consul v0.1.0
Consul Protocol: 1 (Understands back to: 1)

This says the version of Consul as well as the latest protocol version (1, in this case). It also says the earliest protocol version that this Consul agent can understand (0, in this case).

By specifying the -protocol flag on consul agent, you can tell the Consul agent to speak any protocol version that it can understand. This only specifies the protocol version to speak. Every Consul agent can always understand the entire range of protocol versions it claims to on consul -v.

By running a previous protocol version, some features of Consul, especially newer features, may not be available. If this is the case, Consul will typically warn you. In general, you should always upgrade your cluster so that you can run the latest protocol version.