» 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 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 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:
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.
service/mysql/leader
» Create a Session
The first step is to create a session using the Session HTTP API.
$ curl -X PUT -d '{"Name": "mysql-session"}' http://localhost:8500/v1/session/create
This will return a JSON object containing the session ID:
{
"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 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).
$ 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.
» Release the Session
If the leader ever wishes to step down voluntarily, this should be done by simply releasing the lock:
$ 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.
$ 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
endpoint to get details about the session
$ 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.