mirror of https://github.com/hashicorp/consul
140 lines
4.8 KiB
Markdown
140 lines
4.8 KiB
Markdown
---
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layout: "docs"
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page_title: "Leader Election"
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sidebar_current: "docs-guides-leader"
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description: |-
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The goal of this guide is to cover how to build client-side leader election using Consul. If you are interested in the leader election used internally to Consul, you want to read about the consensus protocol instead.
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---
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# Leader Election
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The goal of this guide is to cover how to build client-side leader election using Consul.
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If you are interested in the leader election used internally to Consul, you want to
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read about the [consensus protocol](/docs/internals/consensus.html) instead.
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There are a number of ways that leader election can be built, so our goal is not to
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cover all the possible methods. Instead, we will focus on using Consul's support for
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[sessions](/docs/internals/sessions.html), which allow us to build a system that can
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gracefully handle failures.
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Note that JSON output in this guide has been pretty-printed for easier
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reading. Actual values returned from the API will not be formatted.
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## Contending Nodes
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The first flow we cover is for nodes who are attempting to acquire leadership
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for a given service. All nodes that are participating should agree on a given
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key being used to coordinate. A good choice is simply:
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```text
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service/<service name>/leader
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```
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We will refer to this as just `<key>` for simplicity.
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The first step is to create a session. This is done using the [/v1/session/create endpoint][session-api]:
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[session-api]: http://www.consul.io/docs/agent/http.html#_v1_session_create
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```text
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curl -X PUT -d '{"Name": "dbservice"}' \
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http://localhost:8500/v1/session/create
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```
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This will return a JSON object contain the session ID:
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```text
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{
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"ID": "4ca8e74b-6350-7587-addf-a18084928f3c"
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}
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```
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The session by default makes use of only the gossip failure detector. Additional checks
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can be specified if desired.
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Create `<body>` to represent the local node. This value is opaque to
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Consul and should contain whatever information clients require to
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communicate with your application (e.g., it could be a JSON object
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that contains the node's name and the application's port).
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Attempt to `acquire` the `<key>` by doing a `PUT`. This is something like:
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```text
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curl -X PUT -d <body> http://localhost:8500/v1/kv/<key>?acquire=<session>
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```
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Where `<session>` is the ID returned by the call to
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`/v1/session/create`.
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This will either return `true` or `false`. If `true` is returned, the lock
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has been acquired and the local node is now the leader. If `false` is returned,
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some other node has acquired the lock.
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All nodes now remain in an idle waiting state. In this state, we watch for changes
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on `<key>`. This is because the lock may be released, the node may fail, etc.
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The leader must also watch for changes since it's lock may be released by an operator,
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or automatically released due to a false positive in the failure detector.
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Watching for changes is done by doing a blocking query against `<key>`. If we ever
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notice that the `Session` of the `<key>` is blank, then there is no leader, and we should
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retry acquiring the lock. Each attempt to acquire the key should be separated by a timed
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wait. This is because Consul may be enforcing a [`lock-delay`](/docs/internals/sessions.html).
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If the leader ever wishes to step down voluntarily, this should be done by simply
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releasing the lock:
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```text
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curl -X PUT http://localhost:8500/v1/kv/<key>?release=<session>
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```
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## Discovering a Leader
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The second flow is for nodes who are attempting to discover the leader
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for a given service. All nodes that are participating should agree on the key
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being used to coordinate, including the contenders. This key will be referred
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to as just `key`.
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Clients have a very simple role, they simply read `<key>` to discover who the current
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leader is:
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```text
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curl http://localhost:8500/v1/kv/<key>
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[
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{
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"Session": "4ca8e74b-6350-7587-addf-a18084928f3c",
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"Value": "Ym9keQ==",
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"Flags": 0,
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"Key": "<key>",
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"LockIndex": 1,
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"ModifyIndex": 29,
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"CreateIndex": 29
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}
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]
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```
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If the key has no associated `Session`, then there is no leader.
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Otherwise, the value of the key will provide all the
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application-dependent information required as a base64 encoded blob in
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the `Value` key. You can query the `/v1/session/info` endpoint to get
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details about the session:
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```text
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curl http://localhost:8500/v1/session/info/4ca8e74b-6350-7587-addf-a18084928f3c
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[
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{
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"LockDelay": 1.5e+10,
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"Checks": [
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"serfHealth"
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],
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"Node": "consul-master-bjsiobmvdij6-node-lhe5ihreel7y",
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"Name": "dbservice",
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"ID": "4ca8e74b-6350-7587-addf-a18084928f3c",
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"CreateIndex": 28
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}
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]
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```
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Clients should also watch the key using a blocking query for any changes. If the leader
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steps down, or fails, then the `Session` associated with the key will be cleared. When
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a new leader is elected, the key value will also be updated.
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