Apple Push Notifications with Erlang

Continuing from the Node.js based example I wrote earlier, here is example how to do the same with Erlang. You can check more details from the previous post, but as reminder the Apple Push Notification interface is simple binary based protocol that you use over SSL authenticated socket.

1. Prerequisites

I assume you have erlang installed, the version I’m using here is Erlang R13B03 (erts-5.7.4).

Check instructions here at Node.js based example how to get the push certificates as .pem files.

Install mochiweb package in your erlang environment.

Check that you’ve all set.

$ ERL_LIBS=. erl
Erlang R13B03 (erts-5.7.4)  [64-bit] [rq:1] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.7.4  (abort with ^G)
2> mochijson:encode("cat").
"\"cat\""
3> application:start(ssl).
ok

Later releases of Erlang may require you to start ‘crypto’ and ‘public_key’ applications before starting ssl.

2. Sending Push Notification

First code to convert hexadecimal strings to binary format. This is mainly for readability for the example. I don’t remember where I snacked that code, but it seems to be found from several sites around the Intertubes.

-module(hex).
-export([bin_to_hexstr/1,hexstr_to_bin/1]).

bin_to_hexstr(Bin) ->
   lists:flatten([io_lib:format("~2.16.0B", [X]) ||
                  X <- binary_to_list(Bin)]).

hexstr_to_bin(S) ->
   hexstr_to_bin(S, []).
hexstr_to_bin([], Acc) ->
   list_to_binary(lists:reverse(Acc));
hexstr_to_bin([X,Y|T], Acc) ->
   {ok, [V], []} = io_lib:fread("~16u", [X,Y]),
   hexstr_to_bin(T, [V | Acc]).

Then the code to actually connect to APN and send the PDU

-module(ssltest).
-export([sendpush/0]).
-import(hex).

sendpush() ->
  Address = "gateway.sandbox.push.apple.com",
  Port = 2195,
  Cert = "cert.pem",
  Key = "key-noenc.pem",  

  %Options = [{cacertfile, CaCert}, {certfile, Cert}, {keyfile, Key}, {mode, binary}],
  Options = [{certfile, Cert}, {keyfile, Key}, {mode, binary}],
  Timeout = 1000,
  {ok, Socket} = ssl:connect(Address, Port, Options, Timeout),

Open SSL socket to the APN server with application certificate and private key.

  Payload = mochijson:encode({struct, [{"aps", {struct, [{"alert", "This is Message"}]}}]}),
  BPayload = erlang:list_to_binary(Payload),
  PayloadLen = erlang:byte_size(BPayload),

Convert JSON payload to binary

  Token = "7518b1c2c7686d3b5dcac8232313d5d0047cf0dc0ed5d753c017ffb64ad25b60",
  BToken = hex:hexstr_to_bin(Token),
  BTokenLength = erlang:byte_size(BToken),

Convert token from hexadecimal string to binary

  SomeID= 1,
  {MSeconds,Seconds,_} = erlang:now(),
  Expiry = MSeconds * 1000000 + Seconds + 3600*1,

Transaction id (can be always 0) and 1 hour  expiration time

  Packet = <<1:8, SomeID:32/big, Expiry:32/big, BTokenLength:16/big, BToken/binary, PayloadLen:16/big, BPayload/binary>>,

Construct the binary packet.

  ssl:send(Socket, Packet),
  ssl:close(Socket).

Send the PDU and close the socket

3. Listening for Errors

In case something went wrong, Apple will send you back single error packet for the first error and closes the socket. You need to read that one error code. The packet that triggered error is identified by the ID you set when sending it.

See table 5-1 at Apple documentation to interpret error codes.

Example error listener

recv(Parent) ->
   receive
       {ssl, Sock, <<Command, Status, SomeID:32/big>>} ->
           error_logger:error_msg("Received", 
                                  [Command, Status, SomeID]),
           ssl:close(Sock),
           Parent ! {error, SomeID}; % notify parent
      {ssl_closed, _Sock} -> ok  %
   end.

And remember to spawn process and set it as the controlling process after creating the socket

  Pid = self(),
  ssl:controlling_process(Sock, spawn(fun() -> recv(Pid) end)),

Note that you need to implement also poller application to read feedback info from Apple Feedback server. This is very similar to the receiver above as it only needs to connect and wait for packets from Apple server until it closes the socket. See Apple documentation for more in depth explanation.