Hooks, handlers and accumulators

The hooks and handlers mechanism is one of the core architectural features of MongooseIM. It allows for loose coupling between components of the system by calling only those which are available and configured to be used at runtime.

It can be thought of as a simple eventing mechanism notifying about certain things happening in the server. That results in an extensible system with pluggable extra functionality.

To focus our attention, we'll analyze mod_offline which is responsible for storing messages for delivery to users unavailable at the time of sending. mod_offline is an implementation of XEP-0203.

Running a hook

Basic usage

ejabberd_sm (ejabberd/MongooseIM session manager) is the module discovering whether the recipient of a message is available or not. That's where storing the message for later delivery takes place. It is possible, but not recommended, to save a message in an offline storage by calling mod_offline directly:

mod_offline:store_packet(From, To, Packet)

Note that in this example ejabberd_sm is coupled with mod_offline. I.e. if mod_offline was not available, the code would simply crash; if it was misconfigured or turned off, the behaviour would be undefined. To avoid that coupling and also to enable other (possibly yet to be written) code to carry out some action at this particular moment, ejabberd_sm instead calls:

Acc1 = ejabberd_hooks:run_fold(offline_message_hook,
                               LServer,
                               Acc,
                               [From, To, Packet])

The extra level of indirection introduced by this call gives the flexibility to determine at runtime what code actually gets run at this point. offline_message_hook is just the name of the hook (in other words of the event that is being signalled); From, To and Packet are the arguments passed to the handler just as they would in case of the function being called directly; LServer is the XMPP domain for which this hook is signalled.

Getting results from handlers

Hook handlers are called by "folding". This means that each handler on a list is passed a set of arguments and an initial value that it then modifies, returns and hands over to the next handler in line.

A simple example would look like this:

ListOfSomething = ejabberd_hooks:run_fold(a_certain_hook,
                                          StateData#state.server,
                                          [],
                                          [StateData#state.user,
                                           StateData#state.server])

The initial value of the accumulator being passed through the sequence of handlers (in this case an empty list []) is inserted between the XMPP domain StateData#state.server and handler arguments. In between the XMPP domain (StateData#state.server) and handler arguments is the initial value of the accumulator being passed through the sequence of handlers is inserted - in this case an empty list ([]).

Sidenote: Folds

If you haven't encountered the term fold before, think of it as reduce (like Array.reduce) in Ruby-speak, roughly equivalent to the Reduce step in MapReduce, sometimes called accumulate, aggregate or compress. See Wikipedia for more.

Using accumulators

MongooseIM uses a dedicated data structure to accumulate results (see "Accumulators"). This data structure is implemented in the mongoose_acc module, which has a map-like interface: it has get/2, get/3, put/3 etc. It is instantiated with an incoming stanza, passed along throughout the processing chain, supplied to and returned from hook calls, and terminated when stanza is leaving MongooseIM. If hook handlers are supposed to return some value they put it into the accumulator.

The right way to use hooks is therefore:

  • create a handler which takes and returns an accumulator
  • take an accumulator if available, or instantiate a new one
  • call run_fold giving your acc as the accumulator, plus some extra arguments as needed
  • take return value from the acc the hook call returned
  • pass the modified accumulator on

Handlers should store their return values in the accumulator; there are three ways to do it: if it is a one-off value whch doesn't need to be passed on along with the accumulator (can be overwritten any time), use mongoose_acc:put(result, Value, Acc) if the value is to be passed on to be reused within the user's session use mongoose_acc:put(Key, Value, Acc) * if the value should be passed on to the recipient's session, pubsub node etc. use mongoose_acc:add_prop(Key, Value, Acc)

A real life example, then, with regard to mod_offline is the resend_offline_messages_hook run in ejabberd_c2s:

Acc1 = ejabberd_hooks:run_fold(resend_offline_messages_hook,
                               StateData#state.server,
                               Acc,
                               [StateData#state.user, StateData#state.server]),
Rs = mongoose_acc:get(offline_messages, Acc1, []),

Sidenote: something deprecated

Occassionally you may find some calls to ejabberd_hooks:run/3 in the MongooseIM source code. Under the hood it calls the same handlers with an empty accumulator. This is deprecated and some day will be removed.

Error handling in hooks

Hooks are meant to decouple modules; in other words, the caller signals that some event took place or that it intends to use a certain feature or a set of features, but how and if those features are implemented is beyond its interest. Fro that reason hook don't use the "let it crash" approach. Instead it is rather like "fire-and-forget", more similar in principle to the Pid ! signal way.

In practical terms: if a handler throws an error the hook machine logs a message and proceeds to the next handler with unmodified accumulator. If there is no handlers registered for a given hook, the run_fold call has simply no effect.

Sidenote: Code yet to be written

Let's imagine, that when building a minimum viable product we settle on using mod_offline for delayed delivery of messages to unavailable clients. However, while the product evolves (or the relevant client software catches up) we might drop mod_offline in favour of a more sophisticated solution like Message Archive Management which would require a different action to be taken at the same point. Thanks to loose coupling and ejabberd_hooks it's possible to turn off mod_offline and turn on mod_mam without changing a single line of code in ejabberd_sm.

The only required change is to the configuration (apart from deploying the new module) which can even be performed at runtime - without restarting the server.

Sidenote: Multiple Domains

A MongooseIM cluster may serve more than one domain at the same time. E.g. it's quite common that services like Multi User Chat or Publish-Subscribe are available as subdomains of the main XMPP domain served by an installation.

Registering hook handlers

In order to store a packet when ejabberd_sm runs offline_message_hook the relevant module must register a handler for this hook. To attain the runtime configurability the module should register the handlers when it's loaded and unregister them when it's unloaded. That's usually done in, respectively, start/2 and stop/1 functions. Here's the relevant snippet from mod_offline:start/2:

ejabberd_hooks:add(offline_message_hook, Host,
                   ?MODULE, store_packet, 50)

It's clearly visible that the handler is added to offline_message_hook.

Host corresponds to LServer used in the aforementioned call to ejabberd_hooks:run_fold, i.e. it's the XMPP domain for which the handler is to be executed.

The handler itself is specified as a module-function pair; the arity of the function is neither specified at registration nor verified when calling the handler, so be careful to pass the appropriate number of arguments to ejabberd_hooks:run_fold - otherwise the handler will crash.

Multiple handlers may be registered for the same hook. The last argument, 50, is the sequence number of this handler in the handler chain. The higher the number, the later in the sequence the handler will be executed. It's reasonable to keep this number small (e.g. in the range 0-100), though there's no real limit other than the size of the integer type in the Erlang VM.

Unregistering handlers

Pluggability also requires the components to be unpluggable at will. For that purpose there's the option to unregister a hook handler. It's exercised as follows in mod_offline:stop/1:

ejabberd_hooks:delete(offline_message_hook, Host,
                      ?MODULE, store_packet, 50)

The arguments are exactly the same as passed to ejabberd_hooks:add/5.

Sidenote: Metrics

Every time a hook is run, a corresponding metric of the same name in the same host is incremented by one. There are some exceptions though as some metrics were implemented before the generic hook metrics. List of hooks not updating generic metrics can be found in the mongoose_metrics:hook_to_name/1 function. Such skipped hooks update metrics defined in the mongoose_metrics_hooks module.

Writing handlers

The signature of a handler has to follow three rules:

  • correct arity (the numer of args passed to run_fold + 1)
  • the first arg is a mongoose_acc
  • returns mongoose_acc

Let's look at this example, from MongooseIM codebase:


process_iq_get(Acc, #jid{ lserver = FromS } = From, To, #iq{} = IQ, _ActiveList) ->
    MUCHost = gen_mod:get_module_opt_subhost(FromS, ?MODULE, default_host()),
    case {mod_muc_light_codec_backend:decode(From, To, IQ),
          gen_mod:get_module_opt_by_subhost(MUCHost, ?MODULE, blocking, ?DEFAULT_BLOCKING)} of
        {{ok, {get, #blocking{} = Blocking}}, true} ->
            Items = mod_muc_light_db_backend:get_blocking(jid:to_lus(From), MUCHost),
            mod_muc_light_codec_backend:encode(
              {get, Blocking#blocking{ items = Items }}, From, jid:to_lus(To),
              fun(_, _, Packet) -> put(encode_res, Packet) end),
            #xmlel{ children = ResponseChildren } = erase(encode_res),
            Result = {result, ResponseChildren},
            {stop, mongoose_acc:put(iq_result, Result, Acc)};
        {{ok, {get, #blocking{}}}, false} ->
            Result = {error, ?ERR_BAD_REQUEST},
            {stop, mongoose_acc:put(iq_result, Result, Acc)};
        _ ->
            Result = {error, ?ERR_BAD_REQUEST},
            mongoose_acc:put(iq_result, Result, Acc)
    end.

As seen in this example, a handler receives an accumulator, puts some value into it and returns it for further processing.

There's also one important feature to note: in some cases our handler returns a tuple {stop, Acc}. This skips calling the latter actions in the handler sequence, while the call to run_fold returns the Acc. If a handler returns just an atom stop, then all later actions are skipped and run_fold returns stopped.

Watch out! Different handlers may be registered for the same hook - the priority mechanism orders their execution. If a handler returns stop but runs early in the handler chain, it may prevent some other handler from running at all! That might or might not be intentional. It may be especially surprising in case of handlers from different modules registered for the same hook. Always ensure what handlers are registered for a given hook (grep is your friend) and that you understand their interdependencies.

Hooks list and how to extract it

The following command should give you a list of all the hooks available in MongooseIM (and some garbage filtering out automatically isn't worth the effort):

$ find ejabberd/src/ -name '*.erl' -print | xargs ./find-hooks.awk \
> | sort | uniq
# ... snip out the ~5 lines of garbage ...
adhoc_local_commands
adhoc_local_items
...
...
...
webadmin_user_parse_query

Refer to grep/ack to find where they're used.

Here are the contents of find-hooks.awk:

#!/usr/bin/env awk -f
BEGIN {
    RS=")"
    ORS=""
    FS="[ (,]"
}

$0 ~ /ejabberd_hooks/ {
    found = -1
    for (i = 1; i < NF; i++) {
        if ($i ~ /ejabberd_hooks/) {
            found = i
        }
    }
    if (found != -1 && $(found+1) != "")
        print $(found+1)"\n"
}

Creating your own hooks

There's no special function or any setup necessary to create a new hook. The only thing that needs to be done is calling ejabberd_hooks:run/3 or ejabberd_hooks:run_fold/4 with the name of the new hook and relevant arguments.

Of course, as long as no module registers handlers for this hook just running, it won't have any effects.

Similar is the case when a module registers handlers for some hook, but that hook is never run in the code. That won't have an effect either.

The following is a self-contained example of a module which both runs and registers a few handlers for a completely new hook. The handlers are run sequentially using disparate priorities and passing over an accumulator value. One of the handlers stops the handler execution chain prematurely by returning {stop, NewVal}. It's also possible to try out what happens when the same hook is run with different XMPP domains by passing an argument to run_custom_hook/1 - we'll see that the handlers are registered for a particular domain only.

At the end, you can see a printout of an accumulator with some debugging info.

To cut the long story short:

-module(testhooks).
-author("bartek").

-behaviour(gen_mod).

-include("ejabberd.hrl").

%% API
-export([run_custom_hook/1]).

%% gen_mod callbacks
-export([start/2,
         stop/1]).

%% Hook handlers
-export([first_handler/2,
         stopping_handler/2,
         never_run_handler/2]).

start(Host, _Opts) ->
    ejabberd_hooks:add(custom_new_hook, Host, ?MODULE, first_handler, 25),
    ejabberd_hooks:add(custom_new_hook, Host, ?MODULE, stopping_handler, 50),
    ejabberd_hooks:add(custom_new_hook, Host, ?MODULE, never_run_handler, 75).

stop(Host) ->
    ejabberd_hooks:delete(custom_new_hook, Host, ?MODULE, first_handler, 25),
    ejabberd_hooks:delete(custom_new_hook, Host, ?MODULE, stopping_handler, 50),
    ejabberd_hooks:delete(custom_new_hook, Host, ?MODULE, never_run_handler, 75).

run_custom_hook(Host) ->
    Acc = mongoose_acc:new(),
    Acc1 = mongoose_acc:put(value, 5, Acc),
    ResultAcc = ejabberd_hooks:run_fold(custom_new_hook, Host, Acc1, [2]),
    ResultValue = mongoose_acc:get(value, ResultAcc),
    ?INFO_MSG("Final hook result: ~p", [ResultValue]),
    ?INFO_MSG("Returned accumulator: ~p", [ResultAcc]).

first_handler(Acc, Number) ->
    V0 = mongoose_acc:get(value, Acc),
    Result = V0 + Number,
    ?INFO_MSG("First handler~n"
    "  value: ~p~n"
    "  argument: ~p~n"
    "  will return: ~p",
        [V0, Number, Result]),
    mongoose_acc:put(value, Result, Acc).

stopping_handler(Acc, Number) ->
    V0 = mongoose_acc:get(value, Acc),
    Result = V0 + Number,
    ?INFO_MSG("Stopping handler~n"
    "  value: ~p~n"
    "  argument: ~p~n"
    "  will return: ~p",
        [V0, Number, Result]),
    {stop, mongoose_acc:put(value, Result, Acc)}.

never_run_handler(Acc, Number) ->
    ?INFO_MSG("This hook won't run as it's registered with a priority bigger "
    "than that of stopping_handler/2 is. "
    "It doesn't matter what it returns. "
    "This text should never get printed.", []),
    Acc * Number.

The module is intended to be used from the shell for educational purposes:

(ejabberd@localhost)1> gen_mod:is_loaded(<<"localhost">>, mod_hook_example).
false
(ejabberd@localhost)2> gen_mod:start_module(<<"localhost">>, mod_hook_example, [no_opts]).
ok
(ejabberd@localhost)3> gen_mod:is_loaded(<<"localhost">>, mod_hook_example).
true
(ejabberd@localhost)4> ejabberd_loglevel:set_custom(mod_hook_example, 4).
{module,ejabberd_logger}
(ejabberd@localhost)5> mod_hook_example:run_custom_hook(<<"localhost">>).
ok
(ejabberd@localhost)6>
=INFO REPORT==== 30-Oct-2013::15:33:36 ===
I(<0.47.0>:mod_hook_example:51) : First handler
value: 5
argument: 2
will return: 7

=INFO REPORT==== 30-Oct-2013::15:33:36 ===
I(<0.47.0>:mod_hook_example:61) : Stopping handler
  value: 7
  argument: 2
  will return: {stop,9}

=INFO REPORT==== 30-Oct-2013::15:33:36 ===
I(<0.47.0>:mod_hook_example:31) : Final hook result: 9
I(<0.47.0>:mod_hook_example:32) : Returned accumulator: #{handlers_run => [{custom_new_hook,testhooks,first_handler}],
                                                          hooks_run => [custom_new_hook],
                                                          mongoose_acc => true,
                                                          value => 9}

(ejabberd@localhost)6> mod_hook_example:run_custom_hook(<<"another-domain">>).
ok
(ejabberd@localhost)7>
=INFO REPORT==== 30-Oct-2013::15:33:41 ===
I(<0.47.0>:mod_hook_example:45) : Final hook result: 5
I(<0.47.0>:mod_hook_example:46) : Returned accumulator: #{handlers_run => [{custom_new_hook,testhooks,first_handler}],
                                                          hooks_run => [custom_new_hook],
                                                          mongoose_acc => true,
                                                          value => 9}

(ejabberd@localhost)7> gen_mod:stop_module(<<"localhost">>, mod_hook_example).
{atomic,ok}
(ejabberd@localhost)8>