146 lines
5.1 KiB
Markdown
146 lines
5.1 KiB
Markdown
---
|
|
layout: "docs"
|
|
page_title: "Leader Election"
|
|
sidebar_current: "docs-guides-leader"
|
|
description: |-
|
|
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
|
|
|
|
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). Sessions allow us to build a system that
|
|
can gracefully handle failures.
|
|
|
|
-> **Note:** JSON output in this guide has been pretty-printed for easier reading. Actual values returned from the API will not be formatted.
|
|
|
|
## Contending Nodes
|
|
|
|
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 to coordinate. A good pattern is simply:
|
|
|
|
```text
|
|
service/<service name>/leader
|
|
```
|
|
|
|
We'll abbreviate this pattern as simply `<key>` for the rest of this guide.
|
|
|
|
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 containing the session ID:
|
|
|
|
```text
|
|
{
|
|
"ID": "4ca8e74b-6350-7587-addf-a18084928f3c"
|
|
}
|
|
```
|
|
|
|
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>`. 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>
|
|
```
|
|
|
|
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 its lock may be released by an operator
|
|
or automatically released due to a false positive in the failure detector.
|
|
|
|
By default, the session 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
|
|
releasing the lock:
|
|
|
|
```text
|
|
curl -X PUT http://localhost:8500/v1/kv/<key>?release=<session>
|
|
```
|
|
|
|
## Discovering a Leader
|
|
|
|
Another common practice regarding leader election is for nodes to wish to identify the
|
|
leader for a given service.
|
|
|
|
As with 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:
|
|
|
|
```text
|
|
curl http://localhost:8500/v1/kv/<key>
|
|
[
|
|
{
|
|
"Session": "4ca8e74b-6350-7587-addf-a18084928f3c",
|
|
"Value": "Ym9keQ==",
|
|
"Flags": 0,
|
|
"Key": "<key>",
|
|
"LockIndex": 1,
|
|
"ModifyIndex": 29,
|
|
"CreateIndex": 29
|
|
}
|
|
]
|
|
```
|
|
|
|
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
|
|
[
|
|
{
|
|
"LockDelay": 1.5e+10,
|
|
"Checks": [
|
|
"serfHealth"
|
|
],
|
|
"Node": "consul-primary-bjsiobmvdij6-node-lhe5ihreel7y",
|
|
"Name": "dbservice",
|
|
"ID": "4ca8e74b-6350-7587-addf-a18084928f3c",
|
|
"CreateIndex": 28
|
|
}
|
|
]
|
|
```
|
|
|
|
Clients should also watch the key using a blocking query for any
|
|
changes. If the leader 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.
|