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
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 callingSupervisor.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
.
Callback implementation for Cluster.Strategy.start_link/1
.