.. mode: -*- rst -*-

Client message protocol
=======================

:Tag: design.mps.message
:Author: David Jones
:Date: 1997-02-13
:Status: complete document
:Revision: $Id$
:Copyright: See `Copyright and License`_.
:Index terms:
   pair: messages; design
   single: client message protocol


Introduction
------------

_`.intro`: The client message protocol provides a means by which
clients can receive messages from the MPS. The motivating use case is
finalization notification (see design.mps.finalize_), but the
mechanism is also used for feedback about collections.

.. _design.mps.finalize: finalize

_`.contents`: This document describes the design of the external and
internal interfaces and concludes with a sketch of an example design
of an internal client. The example is that of implementing
finalization using the MRG pool.

_`.readership`: Any MPS developer.


Requirements
------------

_`.req.synchronous`: The message protocol must be synchronous with the
client program: that is, the client program must be able to choose
when to collect and act on messages. Justification: [Boehm_2002]_
shows that asynchronous finalization is impossible to implement
correctly.

_`.req.reliable`: Posting a message must be reliable: that is, it must
not fail for a dynamic reason such as running out memory to store the
message. Justification: messages can't be used to implement
finalization unless the messages can be delivered reliably.

_`.req.extensible.types`: The message mechanism must be extensible
with new types of message in future versions of the MPS, without
breaking client programs that do not receive those types of message.

_`.req.resources`: It follows from `.req.extensible.types`_ that
messages must not use resources unless the client program has
requested them (otherwise resources would leak in client programs that
have not been updated to handle new types of message).

_`.req.extensible.fields`: It must be possible to add new fields to
existing types of message in future versions of the MPS, without
breaking client programs that do not receive those types of message.


Design
------

_`.sol.synchronous`: Messages are stored on a ring belonging to the
arena. An interface is provided that allows the client program to
collect messages from the ring at a time of its choosing.

_`.sol.reliable`: The memory needed for the message is allocated at an
earlier point in time, when it possible to communicate an allocation
failure via a result code. In particular, space for a finalization
message is allocated when the client program calls ``mps_finalize()``,
and space for trace messages is allocated in the arena (there can be
at most one instance of each message per trace, and the maximum number
of traces is known statically).

_`.sol.resources`: Messages are not posted unless they belong to a
type that has been enabled by the client program calling
``mps_message_enable()``. This means that message types that are not
understood by the client program are not posted and use no resources.

_`.sol.extensible.fields`: Message fields are retrieved by calling
accessor functions.


External interface
------------------


Functions
.........

_`.if.fun`: The following functions are provided:

_`.if.fun.poll`: ``mps_message_poll()`` sees whether there are any
messages pending. Returns 1 only if there is a message on the queue of
arena. Returns 0 otherwise.

_`.if.fun.enable`: ``mps_message_type_enable()`` enables the flow of
messages of a certain type. The queue of messages of a arena will
contain only messages whose types have been enabled. Initially all
message types are disabled. Effectively this function allows the
client to declare to the MPS what message types the client
understands.

_`.if.fun.disable`: ``mps_message_type_disable()`` disables the flow
of messages of a certain type. The antidote to
``mps_message_type_enable()``. Disables the specified message type.
Flushes any existing messages of that type on the queue, and stops any
further generation of messages of that type. This permits clients to
dynamically decline interest in a message type, which may help to
avoid a memory leak or bloated queue when the messages are only
required temporarily.

_`.if.fun.get`: ``mps_message_get()`` begins a message "transaction".
If there is a message of the specified type on the queue then the
first such message will be removed from the queue and a handle to it
will be returned to the client via the ``messageReturn`` argument; in
this case the function will return ``TRUE``. Otherwise it will return
``FALSE``. Having obtained a handle on a message in this way, the
client can use the type-specific accessors to find out about the
message. When the client is done with the message the client should
call ``mps_message_discard()``; failure to do so will result in a
resource leak.

_`.if.fun.discard`: ``mps_message_discard()`` ends a message
"transaction". It indicates to the MPS that the client is done with
this message and its resources may be reclaimed.

_`.if.fun.type.any`: ``mps_message_queue_type()`` determines the type
of a message in the queue. Returns ``TRUE`` only if there is a message
on the queue of arena, and in this case updates the ``typeReturn``
argument to be the type of a message in the queue. Otherwise returns
``FALSE``.

_`.if.fun.type`: ``mps_message_type()`` determines the type of a
message (that has already been got). Only legal when inside a message
transaction (that is, after ``mps_message_get()`` and before
``mps_message_discard()``). Note that the type will be the same as the
type that the client passed in the call to ``mps_message_get()``.


Types of messages
.................

_`.type`: The type governs the "shape" and meaning of the message.

_`.type.int`: A message type is an integer belonging to the
``MessageType`` enumeration.

_`.type.semantics`: A type indicates the semantics of the message.

_`.type.semantics.interpret`: The semantics of a message are
interpreted by the client by calling various accessor methods on the
message.

_`.type.accessor`: The type of a message governs which accessor
methods are legal to apply to the message.

_`.type.finalization`: There is a finalization type,
``MessageTypeFINALIZATION``.

_`.type.finalization.semantics`: A finalization message indicates that
an object has been discovered to be finalizable (see
design.mps.poolmrg.def.final.object_ for a definition of finalizable).

.. _design.mps.poolmrg.def.final.object: poolmrg#.def.final.object

_`.type.finalization.ref`: The accessor function
``mps_message_finalization_ref()`` retrieves the reference to the
object which is finalizable.

_`.type.finalization.ref.scan`: Note that the reference returned
must be stored in scanned memory.



Internal interface
------------------

Types
.....

``typedef struct MessageStruct *Message``

_`.message.type`: ``Message`` is the type of messages.

_`.message.instance`: Messages are instances of Message Classes.

_`.message.concrete`: Concretely a message is represented by a
``MessageStruct``. A ``MessageStruct`` has the usual signature field
(see design.mps.sig_). A ``MessageStruct`` has a type field which
defines its type, a ring node, which is used to attach the message to
the queue of pending messages, a class field, which identifies a
``MessageClass`` object.

.. _design.mps.sig: sig

_`.message.intent`: The intention is that a ``MessageStruct`` will be
embedded in some richer object which contains information relevant to
that specific type of message.

_`.message.struct`: The structure is declared as follows::

    typedef struct mps_message_s {
      Sig sig;                      /* <design/sig/> */
      Arena arena;                  /* owning arena */
      MessageClass klass;           /* Message Class Structure */
      Clock postedClock;            /* mps_clock() at post time, or 0 */
      RingStruct queueRing;         /* Message queue ring */
    } MessageStruct;

``typedef struct MessageClassStruct *MessageClass``

_`.class`: A message class is an encapsulation of methods. It
encapsulates methods that are applicable to all types of messages
(generic) and methods that are applicable to messages only of a
certain type (type-specific).

_`.class.concrete`: Concretely a message class is represented by a
``MessageClassStruct`` (a struct). Clients of the Message module are
expected to allocate storage for and initialise the
``MessageClassStruct``. It is expected that such storage will be
allocated and initialised statically.

_`.class.one-type`: A message class implements exactly one message
type. The identifier for this type is stored in the ``type`` field of
the ``MessageClassStruct``. Note that the converse is not true: a
single message type may be implemented by two (or more) different
message classes (for example: for two pool classes that require
different implementations for that message type).

_`.class.methods.generic`: The generic methods are as follows:

* ``delete`` -- used when the message is destroyed (by the client
  calling ``mps_message_discard()``). The class implementation should
  finish the message (by calling ``MessageFinish()``) and storage for
  the message should be reclaimed (if applicable).

_`.class.methods.specific`: The type specific methods are:

_`.class.methods.specific.finalization`: Specific to
``MessageTypeFINALIZATION``:

* ``finalizationRef`` -- returns a reference to the finalizable object
  represented by this message.

_`.class.methods.specific.gc`: Specific to ``MessageTypeGC``:

* ``gcLiveSize`` -- returns the number of bytes (of objects) that were
  condemned by the trace but survived.

* ``gcCondemnedSize`` -- returns the number of bytes condemned by the
  trace.

* ``gcNotCondemnedSize`` -- returns the number of bytes (of
  objects) that are collectable but were not condemned by the trace.

_`.class.methods.specific.gcstart`: Specific to ``MessageTypeGCSTART``:

* ``gcStartWhy`` -- returns an English-language description of the
  reason why the trace was started.

_`.class.sig.double`: The ``MessageClassStruct`` has a signature field
at both ends. This is so that if the ``MessageClassStruct`` changes
size (by adding extra methods for example) then any static
initializers will generate errors from the compiler (there will be a
type error causes by initialising a non-signature type field with a
signature) unless the static initializers are changed as well.

_`.class.struct`: The structure is declared as follows::

    typedef struct MessageClassStruct {
      Sig sig;                      /* <design/sig/> */
      const char *name;             /* Human readable Class name */

      MessageType type;             /* Message Type */

      /* generic methods */
      MessageDeleteMethod delete;   /* terminates a message */

      /* methods specific to MessageTypeFINALIZATION */
      MessageFinalizationRefMethod finalizationRef;

      /* methods specific to MessageTypeGC */
      MessageGCLiveSizeMethod gcLiveSize;
      MessageGCCondemnedSizeMethod gcCondemnedSize;
      MessageGCNotCondemnedSizeMethod gcNotCondemnedSize;

      /* methods specific to MessageTypeGCSTART */
      MessageGCStartWhyMethod gcStartWhy;

      Sig endSig;                   /* <design/message/#class.sig.double> */
    } MessageClassStruct;

_`.space.queue`: The arena structure is augmented with a structure for
managing for queue of pending messages. This is a ring in the
``ArenaStruct``::

    struct ArenaStruct
    {
      ...
      RingStruct messageRing;
      ...
    }


Functions
.........

``void MessageInit(Arena arena, Message message, MessageClass klass, MessageType type)``

_`.fun.init`: Initializes the ``MessageStruct`` pointed to by
``message``. The caller of this function is expected to manage the
store for the ``MessageStruct``.

``void MessageFinish(Message message)``

_`.fun.finish`: Finishes the ``MessageStruct`` pointed to by
``message``. The caller of this function is expected to manage the
store for the ``MessageStruct``.

``void MessagePost(Arena arena, Message message)``

_`.fun.post`: Places a message on the queue of an arena.

_`.fun.post.precondition`: Prior to calling the function, the
``queueRing`` field of the message must be a singleton
(design.mps.ring.def.singleton_). After the call to the function the
message will be available for MPS client to access. After the call to
the function the message fields must not be manipulated except from
the message's class's method functions (that is, you mustn't poke
about with the ``queueRing`` field in particular).

.. _design.mps.ring.def.singleton: ring#.def.singleton

``void MessageEmpty(Arena arena)``

_`.fun.empty`: Empties the message queue. This function has the same
effect as discarding all the messages on the queue. After calling this
function there will be no messages on the queue.

_`.fun.empty.internal-only`: This functionality is not exposed to
clients. We might want to expose this functionality to our clients in
the future.


Message life cycle
------------------

_`.life.alloc`: Space for the message structure is allocated at the
earliest point in time when the MPS knows that the message might be
needed.

_`.life.init`: The message structure is initialized by calling
``MessageInit()``.

_`.life.post`: The message is posted on the arena's message queue by
calling ``MessagePost()``.

_`.life.get`: The client program retrieves the message by calling ``mps_message_get()``.

_`.life.discard`: The client program indicates that it is finished
with the message by calling ``mps_message_discard()``.

_`.life.reuse`: The MPS may reuse the message structure, in which case
the lifecycle continues from `.life.post`_.

_`.life.delete`: When the MPS no longer needs the message structure,
its ``delete`` method is called.


References
----------

.. [Boehm_2002] Hans-J. Boehm. 2002. "`Destructors, Finalizers, and
   Synchronization
   <http://www.hpl.hp.com/techreports/2002/HPL-2002-335.html>`_". HP
   Labs technical report HPL-2002-335.


Document History
----------------

- 1997-02-13 David Jones. incomplete document.

- 2002-06-07 RB_ Converted from MMInfo database design document.

- 2006-10-25 Richard Kistruck. Created guide.

- 2006-12-11 Richard Kistruck. More on lifecycle; unmention evil hack
  in initial design.

- 2008-12-19 Richard Kistruck. Simplify and clarify lifecycle. Remove
  description of and deprecate re-use of messages.

- 2013-05-23 GDR_ Converted to reStructuredText.

.. _RB: https://www.ravenbrook.com/consultants/rb/
.. _GDR: https://www.ravenbrook.com/consultants/gdr/


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---------------------

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