How to create global variables in Erlang

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 sta开发者_开发知识库rt() and put it in a global variable, so that fetch_translation can access it?

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.

It may sound as an overkill but you may consider implementing a very basic gen_server. It contains a state that is available to its callbacks and the data can be kept there. For your case you can write a module similar to this one:

-module(your_module_name).

-behaviour(gen_server).
-export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2, code_change/3]).

-export([start/2, filter_loop/1]).

start(Host, Opt) ->
  %% start the named gen server
  gen_server:start({local, ?MODULE}, ?MODULE, Host, []).

filter_packet({From, To, XML}) ->
  %% do your thing
  gen_server:call(?MODULE, {fetch_translation, XmlData}).

%% this will be called by gen_server:start - just pass the Host
init(Host) ->
  {ok, Host}.

handle_call({fetch_translation, XmlData}, _From, Host) ->
  %% do your thing
  {reply, ok, Host}.

%% you can ignore the rest - they are needed to be present
handle_cast(_Msg, State) ->
  {noreply, State}.
handle_info(_Info, State) ->
  {noreply, State}.
code_change(_OldVsn, State, _Extra) ->
  {ok, State}.

You define your global variable on your module top...like below

-define (Your Variable, "your host name here").

eg.

-define (RelayHost, "smtp.gmail.com").

and you can use this Global variable in all your method in your module.

io:fwrite("Global Value ~p", [?RelayHost]).

-AjAy

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.

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

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).

Try use persistent_term:

1> persistent_term:put(hello, <<"world">>).
ok
2> persistent_term:get(hello).       
<<"world">>
3> persistent_term:erase(hello).
true
4> persistent_term:get(hello).  
** exception error: bad argument
     in function  persistent_term:get/1
        called as persistent_term:get(hello)

You cannot create global variable but you can define a record outside your functions and create an instance of that record with properties then pass it down to the methods you call. Therefore, you can only share one record via method parameter.