caravan v0.6.0 Caravan.Cluster.DnsStrategy

Implements a libcluster strategy for node distribution based on Consul DNS. By default it uses :inet_res to query the nameservers, though it can be overridden.

Prerequisites

First things first, is that you’ll need to have Consul and Nomad setup. You’ll need to create a service that will return SRV records with the hostname and distribution port. The Consul documentation has directions on what needs to be setup and how to test with dig.

Let’s look at an example:

'likes-service-3434@prod.socialmedia.consul`

Above, likes-service is an app name. It will correspond with the :node_sname config option. The port is the last integer to the left of the ‘@’. You’ll need this because our nodes will be using Caravan.Epmd.Client and Caravan.Epmd.Dist_dist to use the port number of the node name instead of being assigned a port randomly by epmd.

Also note that the hostname of cluster nodes returned by Consul must be the same as that in the nodes -name parameter

Configuration

Here’s a sample configuration

config :libcluster,
  topologies: [
    caravan: [
      # The selected clustering strategy. Required.
      strategy: Caravan.Cluster.DnsStrategy,
      config: [
        #service name that returns the distribution port in a SRV record
        consul_service: "likes-service-dist.service.consul",
        #forms the base of the node name. App name is a good one.
        node_sname: "profile-service",
        #If you need to override the default DNS server. Must be an ip port
        #combo like below. Defaults to [] (dns are inherited from system)
        nameservers: [{"10.0.183.34", "8700"}],
        #The poll interval for the Consul service in milliseconds. Defaults to 5s
        poll_interval: 5_000
        #The module of the DNS client to use.
        dns_client: Caravan.DnsClient
      ],
    ]
  ]

Link to this section Summary

Functions

Returns a specification to start this module under a supervisor

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

Callback implementation for Cluster.Strategy.start_link/1

Link to this section Functions

Link to this function child_spec(arg)

Returns a specification to start this module under a supervisor.

See Supervisor.

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.

Link to this function start_link(opts)

Callback implementation for Cluster.Strategy.start_link/1.