xbus is yet an other message bus for erlang, it is based on tree_db, and can subscribe on topic patterns among a limited set of features.

 Start the xbus application

Subscribe to messages sent by calling pub. For example

> xbus:sub("xbus.*.message").
> xbus:pub("xbus.a.message", "Hello").
> flush().
Shell got {xbus,<<"xbus.*.message">>,
            #{timestamp => 1494846399372848,
              topic => <<"xbus.a.message">>,
              value => "Hello"}}

Note that the pattern key is always reported as binary.

Remove a subscription. If the same process has several subscriptions under the same pattern, all are removed.

Broadcast a message on the message bus.

> xbus:pub("a.topic", 99).

An optional time stamp maybe supplied, and should in this case be xbus:timestamp() or tree_db_bin:timestamp() which is in unix micro seconds.

pub_meta will publish (and retain) meta information.

> xbus:pub_meta("a.topic", [{comment,"A comment"},{unit,"%"}]).

The meta {persistent, true} will declare that the topic values are stored in persistent storage. A sys.config is needed to open a dets file. The number of retained values may be declared as {retain,N} when N>=0. The default is to ratain one value. If no retain is wanted the {retain,0} is used. retained values may be combined with persistent storage.

Subscribe to meta declaration sent by calling pub_meta. For example

> xbus:sub_meta("xbus.*.message").
> xbus:pub_meta("xbus.a.message", [{unit,"mm"}]).
> flush().
> Shell got {xbus_meta,<<"xbus.*.message">>,
                  #{timestamp => 1494848025006097,
                  topic => <<"xbus.a.message">>,
                  value => [{unit,"mm"}]}}

Patterns in xbus may contain * or ? matching any number of, and one, number of components respectivly. The patterns must replace a complete component and can not match inside components, so a.b is not a topic pattern but a.*.b is.

{xbus,
   [%% declare meta information about initial topics
    {persistent, false},   %% default is not to store (using dets)
    {file, "xbus_retain"}, %% default filename for persistence, if used.
    {topics,[
       {<<"sensor.1.temperature">>, [{unit,"C"},{persistent,true},{retain,100}]},
       {<<"sensor.2.motion">>, [{unit,"boolean"}]}
]}]}.