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Website: GH-730 and cleanup for docs/guides/leader-election.html.

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Ryan Breen 2015-02-28 00:08:41 -05:00
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@ -3,45 +3,44 @@ layout: "docs"
page_title: "Leader Election"
sidebar_current: "docs-guides-leader"
description: |-
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.
This guide describes how to build client-side leader election using Consul. If you are interested in the leader election used internally to Consul, please refer to the consensus protocol documentation instead.
---
# Leader Election
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](/docs/internals/consensus.html) instead.
This guide describes how to build client-side leader election using Consul. If you
are interested in the leader election used internally by Consul, please refer to the
[consensus protocol](/docs/internals/consensus.html) documentation instead.
There are a number of ways that leader election can be built, so our goal is not to
cover all the possible methods. Instead, we will focus on using Consul's support for
[sessions](/docs/internals/sessions.html), which allow us to build a system that can
gracefully handle failures.
[sessions](/docs/internals/sessions.html). Sessions allow us to build a system that
can gracefully handle failures.
Note that JSON output in this guide has been pretty-printed for easier
reading. Actual values returned from the API will not be formatted.
## Contending Nodes
The first flow we cover is for nodes who are attempting to acquire leadership
Let's imagine we have a set of nodes who are attempting to acquire leadership
for a given service. All nodes that are participating should agree on a given
key being used to coordinate. A good choice is simply:
key to coordinate. A good pattern is simply:
```text
service/<service name>/leader
```
We will refer to this as just `<key>` for simplicity.
We'll abbreviate this pattern as simply `<key>` for the rest of this guide.
The first step is to create a session. This is done using the [/v1/session/create endpoint][session-api]:
[session-api]: http://www.consul.io/docs/agent/http.html#_v1_session_create
The first step is to create a session using the
[Session HTTP API](/docs/agent/http/session.html#session_create):
```text
curl -X PUT -d '{"Name": "dbservice"}' \
http://localhost:8500/v1/session/create
```
This will return a JSON object contain the session ID:
This will return a JSON object containing the session ID:
```text
{
@ -49,35 +48,38 @@ This will return a JSON object contain the session ID:
}
```
The session by default makes use of only the gossip failure detector. Additional checks
can be specified if desired.
Create `<body>` to represent the local node. This value is opaque to
Consul and should contain whatever information clients require to
The next step is to acquire a session for a given key from this node
using the PUT method on a [KV entry](/docs/agent/http/kv.html) with the
"?acquire=\<session\>" query parameter. The `<body>` of the PUT should be a
JSON object representing the local node. This value is opaque to
Consul, but it should contain whatever information clients require to
communicate with your application (e.g., it could be a JSON object
that contains the node's name and the application's port).
Attempt to `acquire` the `<key>` by doing a `PUT`. This is something like:
Attempt to `acquire` the `<key>`. This will look something like (note that
`<session>` is the ID returned by the call to
[`/v1/session/create`](/docs/agent/http/session.html#session_create)):
```text
curl -X PUT -d <body> http://localhost:8500/v1/kv/<key>?acquire=<session>
```
Where `<session>` is the ID returned by the call to
`/v1/session/create`.
This will either return `true` or `false`. If `true` is returned, the lock
has been acquired and the local node is now the leader. If `false` is returned,
some other node has acquired the lock.
This will either return `true` or `false`. If `true`, the lock has been acquired and
the local node is now the leader. If `false` is returned, some other node has acquired
the lock.
All nodes now remain in an idle waiting state. In this state, we watch for changes
on `<key>`. This is because the lock may be released, the node may fail, etc.
The leader must also watch for changes since it's lock may be released by an operator,
The leader must also watch for changes since its lock may be released by an operator
or automatically released due to a false positive in the failure detector.
Watching for changes is done by doing a blocking query against `<key>`. If we ever
notice that the `Session` of the `<key>` is blank, then there is no leader, and we should
retry acquiring the lock. Each attempt to acquire the key should be separated by a timed
Note that the session by default makes use of only the gossip failure detector. That
is, the session is considered held by a node as long as the default Serf health check
has not declared the node unhealthy. Additional checks can be specified if desired.
Watching for changes is done via a blocking query against `<key>`. If we ever
notice that the `Session` of the `<key>` is blank, there is no leader, and we should
retry lock acquisition. Each attempt to acquire the key should be separated by a timed
wait. This is because Consul may be enforcing a [`lock-delay`](/docs/internals/sessions.html).
If the leader ever wishes to step down voluntarily, this should be done by simply
@ -89,10 +91,11 @@ curl -X PUT http://localhost:8500/v1/kv/<key>?release=<session>
## Discovering a Leader
The second flow is for nodes who are attempting to discover the leader
for a given service. All nodes that are participating should agree on the key
being used to coordinate, including the contenders. This key will be referred
to as just `key`.
Another common practice regarding leader election is for nodes to wish to identify the
leader for a given service.
As with leader leader election, all nodes that are participating should agree on the key
being used to coordinate. This key will be referred to as just `key`.
Clients have a very simple role, they simply read `<key>` to discover who the current
leader is:
@ -112,11 +115,12 @@ curl http://localhost:8500/v1/kv/<key>
]
```
If the key has no associated `Session`, then there is no leader.
Otherwise, the value of the key will provide all the
application-dependent information required as a base64 encoded blob in
the `Value` key. You can query the `/v1/session/info` endpoint to get
details about the session:
If the key has no associated `Session`, then there is no leader. Otherwise, the value of
the key will provide all the application-dependent information required as a Base64 encoded
blob in the `Value` field.
You can query the [`/v1/session/info`](/docs/agent/http/session.html#session_info) endpoint
to get details about the session:
```text
curl http://localhost:8500/v1/session/info/4ca8e74b-6350-7587-addf-a18084928f3c
@ -135,5 +139,5 @@ curl http://localhost:8500/v1/session/info/4ca8e74b-6350-7587-addf-a18084928f3c
```
Clients should also watch the key using a blocking query for any changes. If the leader
steps down, or fails, then the `Session` associated with the key will be cleared. When
steps down or fails, the `Session` associated with the key will be cleared. When
a new leader is elected, the key value will also be updated.