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eredis_smart_sub

eredis_smart_sub is a layer on top of eredis to multiplex sub connections among several subscriptor processes in pubsub context

eredis, a more low-level approach to redis pubsub, requires a process to be the controller of every subscription done in a connection. However, if you have multiple processes that want to subscribe to different channels, you are faced with a decision:

  • Open a Redis connection for each process that wants to subscribe to channels. However, this can cause you to have to open several connections to Eedis. There are some issues here:
    • While cheap, redis connections are not free.
    • A linux machine is capped to 65535 ports.
    • This kind of problem is not the type one likes to face in the middle of the night.
    • Opening a connection for every Erlang process does not feel very clean, does it?
  • Write a layer on top of eredis with a process that will receive all the messages and send them back to the processes that are listening to the corresponding channels.

eredis_smart_sub implements this second approach, providing that layer.

Installation

With rebar:

Add this line to your deps in rebar.config:

 {eredis, ".*", {git, "git://github.com/wooga/eredis.git", {tag, "v0.2"}}}

Usage

Startup

Start eredis_sub

{ok, EredisSubClient} = eredis_sub:start_link(),
% Start eredis_smart_sub with the eredis_sub client
% eredis_smart_sub will be assigned as the controller process of the eredis_sub client
{ok, SubClient} = eredis_smart_sub:start_link(EredisSubClient),

Subscription

Now we can subscribe to channels from any process, and messages received in their channels will be sent to these processes

gen_server:cast(SubClient, {subscribe, [<<"channel1">>], self()}),

Now the process can receive messages.

Publish something to channel1 and run

 Message = receive
    {message, M} -> M
 end.

Unsubscription

When the process is done with a channel can unsubscribe from it (or from a list of channels).

gen_server:cast(SubClient, {unsubscribe, [<<"channel1">>], self()}),

Handling of death of subscriptor processes

If the subscriber process dies eredis_smart_sub will send UNSUSCRIBE messages to redis for the channels where the process was the only subscriber. It is a bit more expensive, though, than simply unsubscribing explicitely from the channels we were subscribed to before termination. Of course, one never knows when a process is going to die in a pool of blood, so eredis_smart_sub has this covered.

To build

rebar get-deps
rebar compile

To run tests

rebar compile eunit skip_deps=true