open-consul/website/source/intro/getting-started/join.html.markdown

layout	page_title	sidebar_current
intro	Join a Cluster	gettingstarted-join

Join a Cluster

In the previous page, we started our first agent. While it showed how easy it is to run Serf, it wasn't very exciting since we simply made a cluster of one member. In this page, we'll create a real cluster with multiple members.

When starting a Serf agent, it begins without knowledge of any other node, and is an isolated cluster of one. To learn about other cluster members, the agent must join an existing cluster. To join an existing cluster, Serf only needs to know about a single existing member. After it joins, the agent will gossip with this member and quickly discover the other members in the cluster.

Starting the Agents

First, let's start two agents. Serf agents must all listen on a unique IP and port pair, so we must bind each agent to a different ports.

The first agent we'll start will listen on 127.0.0.1:7946. We also will specify a node name. The node name must be unique and is how a machine is uniquely identified. By default it is the hostname of the machine, but since we'll be running multiple agents on a single machine, we'll manually override it.

$ serf agent -node=agent-one -bind=127.0.0.1:7946
...

Then, in another terminal, start a second agent. We'll bind this agent to 127.0.0.1:7947. In addition to overriding the node name, we're also going to override the RPC address. The RPC address is the address that Serf binds to for RPC operations. The other serf commands communicate with a running Serf agent over RPC. We left the first agent with the default RPC address so lets select another for this agent.

$ serf agent -node=agent-two -bind=127.0.0.1:7947 -rpc-addr=127.0.0.1:7374
...

At this point, you have two Serf agents running. The two Serf agents still don't know anything about each other, and are each part of their own clusters (of one member). You can verify this by running serf members against each agent and noting that only one member is a part of each.

Joining a Cluster

Now, let's tell the first agent to join the second agent by running the following command in a new terminal:

$ serf join 127.0.0.1:7947
Successfully joined cluster by contacting 1 nodes.

You should see some log output in each of the agent logs. If you read carefully, you'll see that they received join information. If you run serf members against each agent, you'll see that both agents now know about each other:

$ serf members
agent-one    127.0.0.1:7946    alive
agent-two    127.0.0.1:7947    alive

$ serf members -rpc-addr=127.0.0.1:7374
agent-two    127.0.0.1:7947    alive
agent-one    127.0.0.1:4946    alive

Remember: To join a cluster, a Serf agent needs to only learn about one existing member. After joining the cluster, the agents gossip with each other to propagate full membership information.

In addition to using serf join you can use the -join flag on serf agent to join a cluster as part of starting up the agent.

Leaving a Cluster

To leave the cluster, you can either gracefully quit an agent (using Ctrl-C) or force kill one of the agents. Gracefully leaving allows the node to transition into the left state, otherwise other nodes will detect it as having failed. The difference is covered in more detail here.