mirror of https://github.com/hashicorp/consul
166 lines
5.6 KiB
Markdown
166 lines
5.6 KiB
Markdown
---
|
|
layout: docs
|
|
page_title: Application Leader Election with Sessions
|
|
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.
|
|
---
|
|
|
|
# Application Leader Election with Sessions
|
|
|
|
For some applications, like HDFS, it is necessary to set one instance as
|
|
a leader. This ensures the application data is current and stable.
|
|
|
|
This guide describes how to build client-side leader elections for service
|
|
instances, using Consul. Consul's support for
|
|
[sessions](/docs/internals/sessions) allows you to build a system that can gracefully handle failures.
|
|
|
|
If you
|
|
are interested in the leader election used internally by Consul, please refer to the
|
|
[consensus protocol](/docs/internals/consensus) documentation instead.
|
|
|
|
## Contending Service Instances
|
|
|
|
Imagine you have a set of MySQL service instances who are attempting to acquire leadership. All service instances that are participating should agree on a given
|
|
key to coordinate. A good pattern is simply:
|
|
|
|
```text
|
|
service/<service name>/leader
|
|
```
|
|
|
|
This key will be used for all requests to the Consul KV API.
|
|
|
|
We will use the same, simple pattern for the MySQL services for the remainder of the guide.
|
|
|
|
```text
|
|
service/mysql/leader
|
|
```
|
|
|
|
### Create a Session
|
|
|
|
The first step is to create a session using the
|
|
[Session HTTP API](/api/session#session_create).
|
|
|
|
```shell
|
|
$ curl -X PUT -d '{"Name": "mysql-session"}' http://localhost:8500/v1/session/create
|
|
```
|
|
|
|
This will return a JSON object containing the session ID:
|
|
|
|
```json
|
|
{
|
|
"ID": "4ca8e74b-6350-7587-addf-a18084928f3c"
|
|
}
|
|
```
|
|
|
|
### Acquire a Session
|
|
|
|
The next step is to acquire a session for a given key from this instance
|
|
using the PUT method on a [KV entry](/api/kv) with the
|
|
`?acquire=<session>` query parameter.
|
|
|
|
The `<body>` of the PUT should be a
|
|
JSON object representing the local instance. 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).
|
|
|
|
```shell
|
|
$ curl -X PUT -d <body> http://localhost:8500/v1/kv/service/mysql/leader?acquire=4ca8e74b-6350-7587-addf-a18084928f3c
|
|
```
|
|
|
|
This will either return `true` or `false`. If `true`, the lock has been acquired and
|
|
the local service instance is now the leader. If `false` is returned, some other node has acquired
|
|
the lock.
|
|
|
|
### Watch the Session
|
|
|
|
All instances now remain in an idle waiting state. In this state, they watch for changes
|
|
on the key `service/mysql/leader`. This is because the lock may be released or the instance could 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 the key. If they ever
|
|
notice that the `Session` field in the response is blank, there is no leader, and then 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).
|
|
|
|
### Release the Session
|
|
|
|
If the leader ever wishes to step down voluntarily, this should be done by simply
|
|
releasing the lock:
|
|
|
|
```shell
|
|
$ curl -X PUT http://localhost:8500/v1/kv/service/mysql/leader?release=4ca8e74b-6350-7587-addf-a18084928f3c
|
|
```
|
|
|
|
## Discover the Leader
|
|
|
|
It is possible to identify the leader of a set of service instances participating in the election process.
|
|
|
|
As with leader election, all instances that are participating should agree on the key being used to coordinate.
|
|
|
|
### Retrieve the Key
|
|
|
|
Instances have a very simple role, they simply read the Consul KV key to discover the current leader. If the key has an associated `Session`, then there is a leader.
|
|
|
|
```shell
|
|
$ curl -X GET http://localhost:8500/v1/kv/service/mysql/leader
|
|
[
|
|
{
|
|
"Session": "4ca8e74b-6350-7587-addf-a18084928f3c",
|
|
"Value": "Ym9keQ==",
|
|
"Flags": 0,
|
|
"Key": "service/mysql/leader",
|
|
"LockIndex": 1,
|
|
"ModifyIndex": 29,
|
|
"CreateIndex": 29
|
|
}
|
|
]
|
|
```
|
|
|
|
If there is a leader then the value of the key will provide all the
|
|
application-dependent information required as a Base64 encoded blob in
|
|
the `Value` field.
|
|
|
|
### Retrieve Session Information
|
|
|
|
You can query the
|
|
[`/v1/session/info`](/api/session#session_info)
|
|
endpoint to get details about the session
|
|
|
|
```shell
|
|
$ curl -X GET http://localhost:8500/v1/session/info/4ca8e74b-6350-7587-addf-a18084928f3c
|
|
[
|
|
{
|
|
"LockDelay": 1.5e+10,
|
|
"Checks": [
|
|
"serfHealth"
|
|
],
|
|
"Node": "consul-primary-bjsiobmvdij6-node-lhe5ihreel7y",
|
|
"Name": "mysql-session",
|
|
"ID": "4ca8e74b-6350-7587-addf-a18084928f3c",
|
|
"CreateIndex": 28
|
|
}
|
|
]
|
|
```
|
|
|
|
## Summary
|
|
|
|
In this guide you used a session to initiate manual leader election for a
|
|
set of service instances. To fully benefit from this process, instances 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.
|
|
|
|
Using the `acquire` parameter is optional. This means
|
|
that if you use leader election to update a key, you must not update the key
|
|
without the acquire parameter.
|