Erlang Pattern: The Router

Hypothetical Labs - kevin - October 19, 2009

There are three components to this pattern. The client, the router, and the target. The client begins the process by calling the router with a message destined for a target. The router uses the message, and possibly other metadata, to locate a desirable target. Then the router hands off the message to the selected target. The target replies directly to the client and completes the process.

I’ve written this code a number of times in several different languages. What makes the Erlang implementation so nice is it’s brevity and simplicity. Using the gen_server behavior I can implement the core logic in just a few lines:

1. Client calls the router with a message destined for a target:
   gen_server:call(?ROUTER, {invoke, Target, Msg})

2. The router looks up the target and forwards the message on:
   
handle_call({invoke, Target, Msg}, From, State) ->
  %% Target selection code goes here
  gen_server:cast(TargetPidOrName, {From, Msg}),
  {noreply, State};


3. The target replies directly to the waiting client:
   
handle_cast({Originator, Msg}, State) ->
  %% Server logic goes here
  gen_server:reply(Originator, Reply),
  {noreply, State};


The pretty bit is that all this plumbing can be hidden away inside a function. So the original call to the router in the client winds up looking like this: router:invoke_target(Target, Msg). The rest happens behind the scenes and appears as just another gen_server cal.

I’m pretty sure there are no bugs in using this approach. I’ve benchmarked a recent implementation of this pattern at over 6000 requests/sec without a hiccup.

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