libcluster v3.0.2 Cluster.Strategy.Gossip

This clustering strategy uses multicast UDP to gossip node names to other nodes on the network. These packets are listened for on each node as well, and a connection will be established between the two nodes if they are reachable on the network, and share the same magic cookie. In this way, a cluster of nodes may be formed dynamically.

The gossip protocol is extremely simple, with a prelude followed by the node name which sent the packet. The node name is parsed from the packet, and a connection attempt is made. It will fail if the two nodes do not share a cookie.

By default, the gossip occurs on port 45892, using the multicast address 230.1.1.251

The gossip protocol is not encrypted by default, but can be by providing a secret in the configuration of the strategy (as shown below). This can also be used to run multiple clusters with the same multicast configuration, as nodes not sharing the same encryption key will not be connected.

You may configure the multicast address, the interface address to bind to, the port, the TTL of the packets and the optional secret using the following settings:

config :libcluster,
  topologies: [
    gossip_example: [
      strategy: Elixir.Cluster.Strategy.Gossip,
      config: [
        port: 45892,
        if_addr: "0.0.0.0",
        multicast_addr: "230.1.1.251",
        multicast_ttl: 1,
        secret: "somepassword"]]]

A TTL of 1 will limit packets to the local network, and is the default TTL.

Debug logging is deactivated by default for this clustering strategy, but it can be easily activated by configuring the application:

use Mix.Config

config :libcluster,
  debug: true

All the checks are done at runtime, so you can flip the debug level without being forced to shutdown your node.

Link to this section Summary

Functions

child_spec(arg)

Returns a specification to start this module under a supervisor

init(list)

Invoked when the server is started. start_link/3 or start/3 will block until it returns

start_link(args)

Callback implementation for Cluster.Strategy.start_link/1

terminate(type, reason, state)

Link to this section Functions

child_spec(arg)

Returns a specification to start this module under a supervisor.

See Supervisor.

init(list)

Invoked when the server is started. start_link/3 or start/3 will block until it returns.

args is the argument term (second argument) passed to start_link/3.

Returning {:ok, state} will cause start_link/3 to return {:ok, pid} and the process to enter its loop.

Returning {:ok, state, timeout} is similar to {:ok, state} except handle_info(:timeout, state) will be called after timeout milliseconds if no messages are received within the timeout.

Returning {:ok, state, :hibernate} is similar to {:ok, state} except the process is hibernated before entering the loop. See c:handle_call/3 for more information on hibernation.

Returning :ignore will cause start_link/3 to return :ignore and the process will exit normally without entering the loop or calling c:terminate/2. If used when part of a supervision tree the parent supervisor will not fail to start nor immediately try to restart the GenServer. The remainder of the supervision tree will be (re)started and so the GenServer should not be required by other processes. It can be started later with Supervisor.restart_child/2 as the child specification is saved in the parent supervisor. The main use cases for this are:

The GenServer is disabled by configuration but might be enabled later.
An error occurred and it will be handled by a different mechanism than the Supervisor. Likely this approach involves calling Supervisor.restart_child/2 after a delay to attempt a restart.

Returning {:stop, reason} will cause start_link/3 to return {:error, reason} and the process to exit with reason reason without entering the loop or calling c:terminate/2.

Callback implementation for GenServer.init/1.

start_link(args)

Callback implementation for Cluster.Strategy.start_link/1.

terminate(type, reason, state)