111 lines
4.9 KiB
Markdown
111 lines
4.9 KiB
Markdown
---
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layout: "docs"
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page_title: "Upgrading Consul"
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sidebar_current: "docs-upgrading-upgrading"
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description: |-
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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.
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---
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# Upgrading Consul
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Consul is meant to be a long-running agent on any nodes participating in a
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Consul cluster. These nodes consistently communicate with each other. As such,
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protocol level compatibility and ease of upgrades is an important thing to
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keep in mind when using Consul.
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This page documents how to upgrade Consul when a new version is released.
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## Standard Upgrades
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For upgrades we strive to ensure backwards compatibility. To support this,
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nodes gossip their protocol version and builds. This enables clients and
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servers to intelligently enable new features when available, or to gracefully
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fallback to a backward compatible mode of operation otherwise.
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For most upgrades, the process is simple. Assuming the current version of
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Consul is A, and version B is released.
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1. Check the [version's upgrade notes](/docs/upgrade-specific.html) to ensure
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there are no compatibility issues that will affect your workload. If there
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are plan accordingly before continuing.
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2. On each server, install version B of Consul.
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3. One server at a time, shut down version A via `consul leave` and restart with version B. Wait until
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the server is healthy and has rejoined the cluster before moving on to the
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next server.
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4. Once all the servers are upgraded, begin a rollout of clients following
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the same process.
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5. Done! You are now running the latest Consul agent. You can verify this
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by running `consul members` to make sure all members have the latest
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build and highest protocol version.
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## Backward Incompatible Upgrades
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In some cases, a backwards incompatible update may be released. This has not
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been an issue yet, but to support upgrades we support setting an explicit
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protocol version. This disables incompatible features and enables a 2-phase upgrade.
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For the steps below, assume you're running version A of Consul, and then
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version B comes out.
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1. On each node, install version B of Consul.
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2. One server at a time, shut down version A via `consul leave` and start version B with the
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`-protocol=PREVIOUS` flag, where "PREVIOUS" is the protocol version of
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version A (which can be discovered by running `consul -v` or `consul
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members`). Wait until the server is healthy and has rejoined the cluster
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before moving on to the next server.
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3. Once all nodes are running version B, go through every node and restart the
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version B agent _without_ the `-protocol` flag, again wait for each server to
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rejoin the cluster before continuing.
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4. Done! You're now running the latest Consul agent speaking the latest protocol.
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You can verify this is the case by running `consul members` to
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make sure all members are speaking the same, latest protocol version.
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The key to making this work is the [protocol compatibility](/docs/compatibility.html)
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of Consul. The protocol version system is discussed below.
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## Protocol Versions
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By default, Consul agents speak the latest protocol they can. However, each
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new version of Consul is also able to speak the previous protocol, if there
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were any protocol changes.
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You can see what protocol versions your version of Consul understands by
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running `consul -v`. You'll see output similar to that below:
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```
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$ consul -v
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Consul v0.7.0
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Protocol 2 spoken by default, understands 2 to 3 (agent will automatically use protocol >2 when speaking to compatible agents)
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```
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This says the version of Consul as well as the protocol versions this
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agent speaks and can understand.
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Sometimes Consul will default to speak a lower protocol version
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than it understands, in order to ease compatibility with older agents. For
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example, Consul agents that understand version 3 claim to speak version 2,
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and only send version 3 messages to agents that understand version 3. This
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allows features to upshift automatically as agents are upgraded, and is the
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strategy used whenever possible. If this is not possible, then you will need
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to do a backward incompatible upgrade using the instructions above, and such
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a requirement will be clearly outlined in the notes for a given release.
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By specifying the `-protocol` flag on `consul agent`, you can tell the
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Consul agent to speak any protocol version that it can understand. This
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only specifies the protocol version to _speak_. Every Consul agent can
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always understand the entire range of protocol versions it claims to
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on `consul -v`.
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~> **By running a previous protocol version**, some features
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of Consul, especially newer features, may not be available. If this is the
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case, Consul will typically warn you. In general, you should always upgrade
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your cluster so that you can run the latest protocol version.
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