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Consul
App Development

Application Leader Election with Sessions

  • 9min

  • Consul

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 tutorial 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.

This tutorial is not related to Consul's leader election. If you are interested in the leader election used internally by Consul, please refer to the consensus protocol documentation instead.

Tip

The content of this tutorial also applies to Consul clusters hosted on HashiCorp Cloud (HCP).

Contending service instances

0 Imagine you have a set of service instances who are attempting to acquire leadership for a given service. All service instances that are participating should agree on a given key to coordinate. A good pattern is simply:

service/<service name>/leader

For this tutorial, our full key could be service/dbservice/leader. However, for clarity, our key will be lead.

Create a session

The first step is to create a session using the Session HTTP API.

$ curl  -X PUT -d '{"Name": "dbservice"}' 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/lead?acquire=ca8e74b-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 key lead. 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 <key>. If they ever notice that the Session of the <key> 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/lead?release=4ca8e74b-6350-7587-addf-a18084928f3c

Discover the leader

Another common practice regarding leader election is for non-leader instances may wish to identify the leader for the given set of service instances.

As with leader election, all instances that are participating should agree on the key being used to coordinate. In this example, the name of the key is lead.

Retrieve the key

Instances have a very simple role, they read the lead 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/lead

[
  {
    "Session": "4ca8e74b-6350-7587-addf-a18084928f3c",
    "Value": "Ym9keQ==",
    "Flags": 0,
    "Key": "dbservice",
    "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": 1.5e10,
    "Checks": ["serfHealth"],
    "Node": "consul-primary-bjsiobmvdij6-node-lhe5ihreel7y",
    "Name": "dbservice",
    "ID": "4ca8e74b-6350-7587-addf-a18084928f3c",
    "CreateIndex": 28
  }
]

Next steps

In this tutorial 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.

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