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docs Leader Election docs-guides-leader 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.

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 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, which 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 for a given service. All nodes that are participating should agree on a given key being used to coordinate. A good choice is simply:

service/<service name>/leader

We will refer to this as just <key> for simplicity.

The first step is to create a session. This is done using the /v1/session/create endpoint:

curl  -X PUT -d '{"Name": "dbservice"}' \
  http://localhost:8500/v1/session/create

This will return a JSON object contain the session ID:

{
  "ID": "4ca8e74b-6350-7587-addf-a18084928f3c"
}

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 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:

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.

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, 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 wait. This is because Consul may be enforcing a lock-delay.

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/<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.

Clients have a very simple role, they simply read <key> to discover who the current leader is:

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 key. You can query the /v1/session/info endpoint to get details about the session:

curl http://localhost:8500/v1/session/info/4ca8e74b-6350-7587-addf-a18084928f3c
[
  {
    "LockDelay": 1.5e+10,
    "Checks": [
      "serfHealth"
    ],
    "Node": "consul-master-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, then the Session associated with the key will be cleared. When a new leader is elected, the key value will also be updated.