How to create global variables in Erlang

Question

Hi,

I am writing an ejabberd module to filter packets. I need to get the hostname to pull some configs using gen_mod:get_module_opt() .

I have 4 important functions :

1. start(Host, _Opt) : This is an ejabberd function to load my module. I get the 'Host' atom here
2. filter_packet({From, To, XML}) : This is my packet filter hook. I cannot pass custom params to this function, as it is a hook in ejabberd.
3. get_translation(XmlData) : 'filter_packet()' calls 'get_translation()' in a loop
4. fetch_translation( XmlData ) : called recursively from 'get_translation()'. This is where i am calling 'gen_mod:get_module_opt()', and hence need the 'Host'.

My question is, how can i take 'Host' from 'start()' and put it in a global variable, so that 'fetch_translation' can access it.

PS: I'm a newbie at erlang

Thanks, Adil

Answer 1

The "easiest way" is to create a named ets table, and put it in there.

start(Host, _Opt) ->
  ets:new(my_table, [named_table, protected, set, {keypos, 1}]),
  ets:insert(my_table, {host, Host}),
  ...

fetch_translation(XmlData) ->
  [{_, Host}] = ets:lookup(my_table, host),
  ...

Note that this is a "general" solution. Ejabberd might provide facilities for what you want, but I cannot help you with that.

Answer 2

You could start a new message filtering process and register it using erlang:register/2, then route all filter_packet/1 requests through it (a potential bottleneck).

-define(?SERVER, msg_filter).

start(Host, Opt) ->
   {ok, Pid} = spawn(?MODULE, filter_loop, [Host, Opt]),
   register(?SERVER, Pid).

filter_loop(Host, Opt) ->
   receive
      {Pid, filter_packet, {_From, _To, XML}} ->
           Trans = get_translation(XML, Host),
           Pid ! {?SERVER, translation, Trans}, 
           filter_loop(Host, Opt)
   end.

filter_packet(Pack) ->
   ?SERVER ! {self(), filter_packet, Pack}
   receive 
      {?SERVER, translation, Trans} ->
           % wrap translation
           UpdatedPacket
   end.

Answer 3

Say you are filtering incoming packets, then To#jid.lserver might be your host.

Answer 4

Hi, guessing for your description than you are in a single-domain ejabberd deployment (no virtual hosts),

yo can get the local XMPP domain using the ?MYNAME macro (see ejabberd.hrl for the definition).