Dispatch

A distributed service registry built on top of phoenix_pubsub.

Requests are dispatched to one or more services based on hashed keys.

Installation

1. Add dispatch and hash_ring to your list of dependencies in mix.exs:

        def deps do
          [
           {:hash_ring, github: "voicelayer/hash-ring", manager: :rebar},
           {:dispatch, "~> 0.1.2"}
          ]
        end

1. Ensure dispatch is started before your application:

        def application do
          [applications: [:dispatch]]
        end

Usage

Configuration

Configure the registry:

config :dispatch,
  pubsub: [name: Dispatch.PubSub,
           adapter: Phoenix.PubSub.PG2,
           opts: [pool_size: 1]],
  registry: [log_level: :debug,
             broadcast_period: 25,
             max_silent_periods: 3]

When the application is started, a supervisor with be started supervising a pubsub adapter with the name and options specified. One can also customize the tracker options for the registry.

Register a service

iex> {:ok, service_pid} = Agent.start_link(fn _ -> 1 end) # Use your real service here
iex> Dispatch.Registry.add_service(:uploader, service_pid)
{:ok, "g20AAAAI9+IQ28ngDfM="}

In this example, :uploader is the type of the service.

Retrieve all services for a service type

iex> Dispatch.Registry.get_services(:uploader)
[{#PID<0.166.0>,
  %{node: :"slave2@127.0.0.1", phx_ref: "g20AAAAIHAHuxydO084=",
  phx_ref_prev: "g20AAAAI4oU3ICYcsoQ=", state: :online}}]

This retrieves all of the services info.

Finding a service for a key

iex> Dispatch.Registry.find_service(:uploader, "file.png")
{:ok, :"slave1@127.0.0.1", #PID<0.153.0>}

Using find_service/2 returns a tuple in the form {:ok, node, pid} where node is the node that owns the service pid. If no service can be found then {:error, reason} is returned.

Finding a list of count service instances for a particular key

iex> Dispatch.Registry.find_multi_service(2, :uploader, "file.png")
[{:ok, :"slave1@127.0.0.1", #PID<0.153.0>}, {:ok, :"slave2@127.0.0.1", #PID<0.145.0>}]

Convenience API

The Service module can be used to automatically handle registration of a service based on a GenServer.

Call Service.init within your GenServer's init function.

def init(_) do
  :ok = Dispatch.Service.init(type: :uploader)
  {:ok, %{}}
end

This will use the type provided to attach a service to the configured registry pid.

Use Dispatch.Service.cast and Dispatch.Service.call to route the GenServer cast or call to the appropriate service based on the key provided.

Use Dispatch.Service.multi_cast to send cast messages to several service instances at once.

Dispatch.Service.multi_call calls several service instances and waits for all the responses before returning.

# File is a map with the format %{name: "file.png", contents: "test file"} 
def upload(file)
  Dispatch.Service.cast(:uploader, file.name, {:upload, file})
end

def download(file)
  Dispatch.Service.call(:uploader, file.name, {:download, file})
end

def handle_cast({:upload, file}, state) do
  Logger.info("Uploading #{file.name}")
  {:noreply, Map.put(state, file.name, file.contents)}
end

def handle_call({:download, %{name: name}}, from, state) do
  Logger.info("Downloading #{name}")
  {:reply, {:ok, Map.get(state, name}}, state}
end