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Profiling programs
==================

In this example, we will illustrate the use of:

-  events
-  monitors

by defining a simple profiler that prints the starting and ending time
for processing a message sent to an object.

.. _profiling_events:

Messages as events
------------------

In a pure object-oriented system, all computations start by sending
messages to objects. We can thus define an *event* as the sending of a
message to an object. An event can then be specified by the tuple
``(Object, Message, Sender)``. This definition can be refined by
interpreting the sending of a message and the return of the control to
the object that has sent the message as two distinct events. We call
these events respectively ``before`` and ``after``. Therefore, we end up
by representing an event by the tuple
``(Event, Object, Message, Sender)``. For instance, if we send the
message:

.. code-block:: text

   | ?- foo::bar(X).

   X = 1
   yes

the two corresponding events will be:

.. code-block:: text

   (before, foo, bar(X), user)
   (after, foo, bar(1), user)

Note that the second event is only generated if the message succeeds. If
the message as a goal has multiple solutions, then an ``after`` event
will be generated for each solution.

Events are automatically generated by the message-sending mechanisms for
each public message sent using the
:ref:`control_send_to_object_2` operator.

.. _profiling_monitors:

Profilers as monitors
---------------------

A monitor is an object that reacts whenever a spied event occurs. The
monitor actions are defined by two event handlers:
:ref:`methods_before_3` for ``before`` events
and :ref:`methods_after_3` for ``after``
events. These predicates are automatically called by the message-sending
mechanisms when an event registered for the monitor occurs. These event
handlers are declared as public predicates in the ``monitoring``
built-in protocol.

In our example, we need a way to get the current time before and after
we process a message. We will assume that we have a ``time`` object
implementing a ``cpu_time/1`` predicate that returns the current CPU
time for the Prolog session:

::

   :- object(time).

       :- public(cpu_time/1).
       :- mode(cpu_time(-number), one).
       ...

   :- end_object.

Our profiler will be named ``stop_watch``. It must define event handlers
for the ``before`` and ``after`` events that will print the event
description (object, message, and sender) and the current time:

::

   :- object(stop_watch,
       % event handler predicates protocol
       implements(monitoring)).

       :- uses(time, [cpu_time/1]).

       before(Object, Message, Sender) :-
           write(Object), write(' <-- '), writeq(Message),
           write(' from '), write(Sender), nl, write('STARTING at '),
           cpu_time(Seconds), write(Seconds), write(' seconds'), nl.

       after(Object, Message, Sender) :-
           write(Object), write(' <-- '), writeq(Message),
           write(' from '), write(Sender), nl, write('ENDING at '),
           cpu_time(Seconds), write(Seconds), write(' seconds'), nl.

   :- end_object.

After compiling and loading the ``stop_watch`` object (and the objects
that we want to profile), we can use the :ref:`predicates_define_events_5`
built-in predicate to set up our profiler. For example, to profile all
messages that are sent to the object ``foo``, we need to call the goal:

.. code-block:: text

   | ?- define_events(_, foo, _, _, stop_watch).

   yes

This call will register ``stop_watch`` as a monitor to all messages sent
to object ``foo``, for both ``before`` and ``after`` events. Note that
we say "as a monitor", not "the monitor": we can have any number of
monitors over the same events.

From now on, every time we sent a message to ``foo``, the ``stop_watch``
monitor will print the starting and ending times for the message
execution. For instance:

.. code-block:: text

   | ?- foo::bar(X).

   foo <-- bar(X) from user
   STARTING at 12.87415 seconds
   foo <-- bar(1) from user
   ENDING at 12.87419 seconds

   X = 1
   yes

To stop profiling the messages sent to ``foo`` we use the
:ref:`predicates_abolish_events_5` built-in predicate:

.. code-block:: text

   | ?- abolish_events(_, foo, _, _, stop_watch).

   yes

This call will abolish all events defined over the object ``foo``
assigned to the ``stop_watch`` monitor.

Summary
-------

-  An event is defined as the sending of a (public) message to an
   object.

-  There are two kinds of events: ``before`` events, generated before a
   message is processed, and ``after`` events, generated after the
   message processing has completed successfully.

-  Any object can be declared as a monitor to any event. A monitor shall
   reference the ``monitoring`` built-in protocol in the object opening
   directive.

-  A monitor defines event handlers, the predicates
   :ref:`methods_before_3` and :ref:`methods_after_3`, that are
   automatically called by the runtime engine when a spied event occurs.

-  Three built-in predicates, :ref:`predicates_define_events_5`,
   :ref:`predicates_current_event_5`, and
   :ref:`predicates_abolish_events_5`,
   enables us to define, query, and abolish both events and monitors.
