» Serf Agent

The Serf agent is the core process of Serf. The agent maintains membership information, propagates events, invokes event handlers, detects failures, and more. The agent must run on every node that is part of a Serf cluster.

» Running an Agent

The agent is started with the serf agent command. This command blocks, running forever or until told to quit. The agent command takes a variety of configuration options but the defaults are usually good enough. When running serf agent, you should see output similar to that below:

$ serf agent
==> Starting Serf agent...
==> Starting Serf agent RPC...
==> Serf agent running!
                    Node name: 'mitchellh.local'
                    Bind addr: ''
                     RPC addr: ''
                    Encrypted: false
                     Snapshot: false
                      Profile: lan
  Message Compression Enabled: true

==> Log data will now stream in as it occurs:

2013/10/22 10:35:33 [INFO] Serf agent starting
2013/10/22 10:35:33 [INFO] serf: EventMemberJoin: mitchellh.local
2013/10/22 10:35:33 [INFO] Serf agent started
2013/10/22 10:35:33 [INFO] agent: Received event: member-join

There are six important components that serf agent outputs:

  • Node name: This is a unique name for the agent. By default this is the hostname of the machine, but you may customize it to whatever you'd like using the -node flag.

  • Bind addr: This is the address and port used for communication between Serf agents in a cluster. Every Serf agent in a cluster does not have to use the same port.

  • RPC addr: This is the address and port used for RPC communications for other serf commands. Other Serf commands such as serf members connect to a running agent and use RPC to query and control the agent. By default, this binds only to localhost on the default port. If you change this address, you'll have to specify an -rpc-addr to commands such as serf members so they know how to talk to the agent. This is also the address other applications can use over RPC to control Serf.

  • Encrypted: This shows if Serf is encrypting all traffic that it sends and expects to receive. It is a good sanity check to avoid sending non-encrypted traffic over any public networks. You can read more about encryption here.

  • Snapshot: This shows if Serf snapshotting is enabled. The snapshot file enables Serf to automatically re-join a cluster after failure and prevents replay of events that have already been seen. It requires storing state on disk, and must be configured using a CLI flag or in the configuration directory. If it is not provided, other nodes will still attempt to reconnect on recovery, however the node will take longer to join the cluster and will replay old events.

  • Profile: The profile controls various timing values which should be appropriate to the environment Serf is running in. It defaults to optimizing for a LAN environment, but can also be set for WAN or local-only communication. The profile can be set in the configuration.

» Stopping an Agent

An agent can be stopped in two ways: gracefully or forcefully. To gracefully halt an agent, send the process an interrupt signal, which is usually Ctrl-C from a terminal. When gracefully exiting, the agent first notifies the cluster it intends to leave the cluster. This way, other cluster members notify the cluster that the node has left.

Alternatively, you can force kill the agent by sending it a kill signal. When force killed, the agent ends immediately. The rest of the cluster will eventually (usually within seconds) detect that the node has died and will notify the cluster that the node has failed.

The difference between a node failing and a node leaving may not be important for your use case. For example, for a web server and load balancer setup, both result in the same action: remove the web node from the load balancer pool. But for other situations, you may handle each scenario differently.