» Bootstrapping a Datacenter

An agent can run in both client and server mode. Server nodes are responsible for running the consensus protocol and storing the cluster state. The client nodes are mostly stateless and rely heavily on the server nodes.

Before a Consul cluster can begin to service requests, a server node must be elected leader. Thus, the first nodes that are started are generally the server nodes. Bootstrapping is the process of joining these initial server nodes into a cluster.

The recommended way to bootstrap is to use the -bootstrap-expect configuration option. This option informs Consul of the expected number of server nodes and automatically bootstraps when that many servers are available. To prevent inconsistencies and split-brain situations (that is, clusters where multiple servers consider themselves leader), all servers should either specify the same value for -bootstrap-expect or specify no value at all. Only servers that specify a value will attempt to bootstrap the cluster.

We recommend 3 or 5 total servers per datacenter. A single server deployment is highly discouraged as data loss is inevitable in a failure scenario. Please refer to the deployment table for more detail.

Suppose we are starting a 3 server cluster. We can start Node A, Node B, and Node C with each providing the -bootstrap-expect 3 flag. Once the nodes are started, you should see a message like:

[WARN] raft: EnableSingleNode disabled, and no known peers. Aborting election.

This indicates that the nodes are expecting 2 peers but none are known yet. To prevent a split-brain scenario, the servers will not elect themselves leader.

» Creating the cluster

To trigger leader election, we must join these machines together and create a cluster. There are multiple options for joining the machines:

  • Manually specified list of machines with -join and start_join options
  • Manually specified list of machines with -retry-join option
  • Automatic joining by tag for supported cloud environments with the -retry-join option

Choose the method which best suits your environment and specific use case.

» Manually Creating a Cluster

To manually create a cluster, access one of the machines and run the following:

$ consul join <Node A Address> <Node B Address> <Node C Address>
Successfully joined cluster by contacting 3 nodes.

Since a join operation is symmetric, it does not matter which node initiates it. Once the join is successful, one of the nodes will output something like:

[INFO] consul: adding server foo (Addr: (DC: dc1)
[INFO] consul: adding server bar (Addr: (DC: dc1)
[INFO] consul: Attempting bootstrap with nodes: []
[INFO] consul: cluster leadership acquired

» Verifying the Cluster

As a sanity check, the consul info command is a useful tool. It can be used to verify raft.num_peers is now 2, and you can view the latest log index under raft.last_log_index. When running consul info on the followers, you should see raft.last_log_index converge to the same value once the leader begins replication. That value represents the last log entry that has been stored on disk.

Now that the servers are all started and replicating to each other, all the remaining clients can be joined. Clients are much easier as they can join against any existing node. All nodes participate in a gossip protocol to perform basic discovery, so once joined to any member of the cluster, new clients will automatically find the servers and register themselves.

» Manual Bootstrapping

In versions of Consul prior to 0.4, bootstrapping was a more manual process. For details on using the -bootstrap flag directly, see the manual bootstrapping guide.

Manual bootstrapping is not recommended as it is more error-prone than automatic bootstrapping with -bootstrap-expect.