Route of a message through the system

Let's examine the flow of a message sent from user A to user B, both of whom are served by the same domain.

Note that hooks are called at various stages of routing - they perform many tasks, in fact, many MongooseIM functionalities are implemented through hooks & handlers. For a general introduction to hooks, see Hooks and Handlers; to get a closer look at a core few, see hooks description.

1. Receiving the stanza

User A's ejabberd_receiver receives the stanza and passes it to ejabberd_c2s.

2. Call to user_send_packet

Upon some minimal validation of the stanza, a hook user_send_packet is called. This is handled by a couple of modules which subscribe to this hook. Those modules do various complementary tasks, like storing the message in an archive, sending carbon copies, etc.

3. Privacy lists and ejabberd_router:route/3

The stanza is checked against any privacy lists in use and, in the case of being allowed, it will be routed by ejabberd_router:route/3. This also takes into account "blocking commands", which are part of the privacy system.

4. Chain of routing

Further on, the behaviour is configurable: ejabberd_router:route/3 passes the stanza through a chain of routing modules and applies Mod:filter/3 and Mod:route/3 from each of them. Each of those modules has to implement xmpp_router behaviour.

There are a few actions available to the module:

• it can drop or route the stanza,
• it can pass the stanza on unchanged or modify it and pass the result on.

A set of routing modules can be set in configuration as routing_modules. The default behaviour is the following:

• mongoose_router_global: runs a global filter_packet hook
• mongoose_router_external_local: checks if there is an external component registered for the destination domain on the current node, possibly routes the stanza to it
• mongoose_router_external: checks if there is an external component registered for the destination domain on any node in the cluster, possibly routes the stanza to it
• mongoose_router_localdomain: checks if there is a local route registered for the destination domain (i.e. there is an entry in mnesia route table), possibly routes the stanza to it
• ejabberd_s2s: tries to find or establish a connection to another server and send the stanza there

5. Look up external_component and route

An external component and a local route are obtained by looking up external_component and route mnesia tables, respectively. The items stored in the tables provide funs to call and MFs to apply:

```erlang
(ejabberd@localhost)2> ets:tab2list(route).
[{route,<<"vjud.localhost">>,
        {apply_fun,#Fun<ejabberd_router.2.123745223>}},
 {route,<<"muc.localhost">>,
        {apply_fun,#Fun<mod_muc.2.63726579>}},
 {route,<<"localhost">>,{apply,ejabberd_local,route}}]
```

Here we see that for a domain "localhost" ejabberd_local:route()is called. Routing the stanza there means calling mongoose_local_delivery:do_route/5, which calls filter_local_packet hook and, if passed, runs the fun or applies the handler. In most cases, the handler is ejabberd_local:route/3.

6. ejabberd_local to ejabberd_sm

ejabberd_local routes the stanza to ejabberd_sm given it has at least a bare JID as the recipient.

7. ejabberd_sm

ejabberd_sm determines the available resources of User B, takes into account their priorities and whether the message is addressed to a particular resource or a bare JID. It appropriately replicates (or not) the message and sends it to the recipient's ejabberd_c2s process(es).

In case no resources are available for delivery (hence no ejabberd_c2s processes to pass the message to), offline_message_hook is run.

8. ejabberd_c2s

ejabberd_c2s verifies the stanza against relevant privacy lists and sends it to the socket. user_receive_packet hook is run to notify the rest of the system about the stanza delivery to User B.