This software provides an Erlang client for GOOGLE CLOUD MESSAGING.

Using gcm-erlang you can:

1. start several gen_servers representing different GCM applications defined by different GCM API keys
2. send notification messages to Android mobile devices registered to your specific application and registered to GCM using a specific registration id

So far gcm-erlang does only provide support for JSON messages since GCM does not allow to send multicast messages using plain text.

The first thing you have to do is to compile all the Erlang files using rebar.

$ ./rebar compile

Once all the Erlang files are compiled you can start the application gcm-erlang. The application does use the module httpc so it is mandatory to start also the Erlang application inets.

$ erl -pa ebin
1> application:start(inets).
ok
2> application:start(gcm).
ok

While gcm-erlang is running you can start several supervised gen_servers, one for each GCM application. Every gen_server is defined by an atom used internally for registration and by a GCM API key.

3> gcm:start(foo, "myapikey").
{ok,<0.60.0>}
4> gcm:start(bar, "myotherapikey").
{ok,<0.65.0>}
5> gcm:start(baz, "mylastapikey").
{ok,<0.79.0>}

You can stop a gen_server representing a GCM Application using:

6> gcm:stop(foo).

At any time you can send a GCM message to one or more mobile devices by calling:

7> gcm:push(RegisteredName, RegIds, Message).

Where RegistereName is the atom used during registration, RegIds is a list (max 1000 elements) of Registration Ids specified as Erlang binaries and Message is an Erlang binary representing the actual message.

The JSON message is built using mochijson2.erl in the module gcm.erl and in the end will have the following form:

{
  "registration_ids" : ["APA91bHun4MxP5egoKMwt2KZFBaFUH-1RYqx..."],
  "data" : {
    "message" : "the message you passed as argument"
  },
  "time_to_live" : 3600,
  "collapse_key" : "your_update"
}

By looking at the code in gcm.erl you can easily change the JSON in order to satisfy your needs.

gcm-erlang will push the message for you to Google Cloud Messaging servers and will parse the JSON provided as result.

In case of errors or if some of the Registration Ids must be updated gcm-erlang will print a message on the standard output. I will add some better logging in the future (problably using Lager but other suggestions are more than accepted).

Some stuff I would like to add to gcm-erlang:

1. better logging
2. improved and adaptive construction of the JSON messages
3. resending of all the message not sent due to GCM timeout using exponential backoff

In some cases when you receive a success/error message we should update/remove in our database the Registration Ids. I did not implement this because it depends on your needs. Problably I will store in different ets tables some information so that you will be able to inspect them and update your database. Feel free to edit in gcm.erl the functions parse_results/2 and handle_error/2 if you want to handle these issues in you own way.

Some of the concepts I used for building this Erlang application are based on this blog post and on this Erlang application for APN.