» 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.

» Standard Upgrades

For upgrades we strive to ensure backwards compatibility. To support this, nodes gossip their protocol version and builds. This enables clients and servers to intelligently enable new features when available, or to gracefully fallback to a backward compatible mode of operation otherwise.

For most upgrades, the process is simple. Assuming the current version of Consul is A, and version B is released.

Check the version's upgrade notes to ensure there are no compatibility issues that will affect your workload. If there are plan accordingly before continuing.
On each server, install version B of Consul.
One server at a time, shut down version A via consul leave and restart with version B. Wait until the server is healthy and has rejoined the cluster before moving on to the next server.
Once all the servers are upgraded, begin a rollout of clients following the same process.
Done! You are now running the latest Consul agent. You can verify this by running consul members to make sure all members have the latest build and highest protocol version.

» Backward Incompatible Upgrades

In some cases, a backwards incompatible update may be released. This has not been an issue yet, but to support upgrades we support setting an explicit protocol version. This disables incompatible features and enables a 2-phase upgrade.

For the steps below, assume you're running version A of Consul, and then version B comes out.

On each node, install version B of Consul.
One server at a time, shut down version A via consul leave 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). Wait until the server is healthy and has rejoined the cluster before moving on to the next server.
Once all nodes are running version B, go through every node and restart the version B agent without the -protocol flag, again wait for each server to rejoin the cluster before continuing.
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 v0.7.0
Protocol 2 spoken by default, understands 2 to 3 (agent will automatically use protocol >2 when speaking to compatible agents)

This says the version of Consul as well as the protocol versions this agent speaks and can understand.

Sometimes Consul will default to speak a lower protocol version than it understands, in order to ease compatibility with older agents. For example, Consul agents that understand version 3 claim to speak version 2, and only send version 3 messages to agents that understand version 3. This allows features to upshift automatically as agents are upgraded, and is the strategy used whenever possible. If this is not possible, then you will need to do a backward incompatible upgrade using the instructions above, and such a requirement will be clearly outlined in the notes for a given release.

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.