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@page PP_Memory_Management Memory Management Rules for TAO's Pluggable Protocol Framework

@section background Background

    This document proposes a clearer set of memory management rules
    for the pluggable protocols framework.
    To understand this proposal some basic background on how does the
    pluggable protocol framework works, and how each abstraction
    relates to the other components in the ORB.

    The pluggable protocol framework uses the Acceptor and Connector
    patterns, unlike ACE, however, it must treat all of them
    homogenously.
    The basic abstraction in TAO's pluggable protocol framework is the
    <CODE>TAO_Transport</CODE>,
    an instance of this class represents a single connection, for
    example, the IIOP plugin uses one instance of TAO_Transport for
    each socket.
    To integrate this abstraction with the ACE_Reactor framework,
    all the protocols implemented so far use
    specializations of the ACE_Svc_Handler class.
    However, the original design considered the possibility of using
    protocols without any ACE abstractions, though in practice this
    hasn't happened so far,
    all changes to the framework should keep this possibility open.

    This is the main source of memory management problems in the
    pluggable protocol framework:
    a single entity (a connection) is represented by two instances of
    two separate classes.  On one side the ORB uses an instance of the
    TAO_Transport abstraction, on the other the Reactor uses an
    instance of an ACE_Svc_Handler.

    To complicate matters even further the ORB caches both passively
    accepted and actively established connections.
    The actively established connections are cached by the client-side
    to minimize or amortize the cost of connection establishment.
    The passively accepted connections are kept in the same cache
    mainly to support bi-directional GIOP, however, they also allow us
    to close both accepted and established idle connections using a
    single component, this is useful when the ORB shutdowns, but it is
    crucial in the implementation of connection recycling strategies,
    where the total number of connections kept by the ORB must be
    known.

    The design must also support multithreaded clients and servers, in
    both cases multiple threads may be using a connection
    simultaneously, for example, multiple client threads can be
    waiting for replys over the same connection, or multiple server
    threads can be servicing requests received on the same connection,
    or, if bi-dir GIOP is enabled, maybe a mix of both.

    Some aspects of the GIOP protocol require special treatment of
    connections with pending requests, both on the server and client
    side.
    On the server side connections that have pending requests cannot
    be closed (section 15.5.1.1 in the CORBA/IIOP 2.4 specification),
    therefore, the ORB needs to know how many requests are pending, at
    all times.
    Despite this, it is possible that the underlying connection is
    broken, for example, because the client crashed.  In such cases,
    the ORB should be able to reclaim the OS resources, but the
    TAO_Transport must remain valid until the upcall threads finish.
    Similarly, the client side should be able to distinguish between
    orderly and abortive disconnects, essentially the ORB needs to
    know if a <CODE>CloseConnection</CODE> message has been received.

    Finally we must never forget that the ORB can be used in
    thread-per-connection mode.  In this concurrency model there is no
    reactor used to detect when the connection can accept more input,
    though normally this is a global setting, it is possible for a
    pluggable protocol to *always* work in thread-per-connection and
    no other architecture.
    Similarly, the ORB can be configured to wait for replys using
    read() operations, instead of the more generic wait-on-reactor or
    wait-on-leader-follower strategies.
    Therefore, we cannot always rely on the Reactor framework to
    perform all the memory management for us.

@section requirements Requirements

    To summarize, the TAO_Transport class should:

    - Not be deleted until all threads using it is released by all the
      threads using it.
    - As many OS and ORB resources must be released when the ORB
      detects that the connection has been terminated.  For example,
      the socket should be closed and the ACE_Svc_Handler, if any,
      should be destroyed.
    - Not be deleted until it is removed from the connection cache.
    - Support a mechanism to proactively close the connection.
    - Keep track of the number of pending requests in a connection.

@section rules Memory Management Rules

    Instances of TAO_Transport are reference counted,
    this is a simple way to share it among the threads using it to
    send or receive invocations, the cache, and the potential
    connection handler using it.
    However, the service handler should follow the standard rules for
    the Reactor, i.e. the Reactor owns it, and is destroyed as soon as
    the connection is closed.

    The underlying connection can be closed by the remote peer,
    in this case, either the Reactor or the thread blocked on
    <CODE>read()</CODE> will detect the problem.
    Following the normal conventions, the ACE_Svc_Handler would be
    closed as soon as this is detected.
    The corresponding TAO_Transport must be informed, otherwise it
    could attempt to use a connection already closed.

    Finally if the connection is proactively closed, the TAO_Transport
    informs the ACE_Svc_Handler, at this point the ACE_Svc_Handler
    commits suicide by removing itself from the reactor.
    Notice that it must still callback the TAO_Transport in this
    case.

@section data Processing Incoming and Outgoing Data

    The final aspect to consider is the processing of incoming and
    outgoing data.  We are still working on this problem, but the
    current approach is more complex than it has to be.
    The usual path is as follows: the Reactor signals (via
    handle_input) that there is some data available in the socket.
    The message is forwarded from the ACE_Svc_Handler to the
    TAO_Transport, then to several helper classes in the pluggable
    protocol framework.  Eventually a method is invoked on the
    TAO_Tranport to read the actual data, this forwards on the
    ACE_Svc_Handler (again), and eventually returns.

    A much simpler approach would be to read the data on the
    handle_input() method itself, and forward the data up the stream.

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