

1. INTRODUCTION

This directory contains an example that illustrates how to use both
threaded and reactive concurrency mechanisms in ACE.  The example
application schedules and processes heterogenerous user input and
timer-based events in the context of a bounded packet relay mechanism.

In this example, a transmission begins, packets arrive from an input device
object, and are transferred to an output device object by a relay object at
a specified pace.  The transfer continues until all packets have been
relayed, a duration limit expires, or the transmission is cancelled.

User input is handled concurrently with a running transmission.  You
can run a transmission, cancel a transmission, change transmission
parameters, view statistics from the most recent transmission, or exit
the program, using selections from an interactive text-based menu.
In addition, the example program can be run in batch mode, with the
appropriate commands piped to the program's standard input stream.

Transmission parameters are intialized to default values.  Transmission
parameter values persist until/unless they are subsequently modified by an
appropriate command.  If an invalid value for a command is given, or a run
or report command is issued while a transmission is in progress, the
offending command has no effect, and an error message is generated.

2. USER INTERFACE

Commands that can be given to the program include the following:

Settings commands:

  1 <number of packets to relay in one transmission>

     Minimum value is 1 packet, defaults to 1000 packets.

  2 <input packet arrival period (in usec)>

     Minimum value is 1 usec, defaults to 10000 usec (10 msec).

  3 <output packet transmission period (in usec)>

     Minimum value is 1 usec, defaults to 10000 usec (10 msec).

  4 <limit on duration of transmission (in usec)>

     Minimum value is 1 usec, defaults to 20000000 usec (20 sec).
     A value of 0 is also a valid input, in which case no limit
     will be placed on the duration of the transmission (it will
     end when all packets have been relayed from the input device
     to the output device).

  5 <logging level>

     0 - does no logging
     1 - logs packets created by the input device
     2 - logs packets consumed by the output device
     4 - prints contents of packets consumed by the output device

     To set several flags, pass their sum as the logging level value:
     e.g., a logging level value of 7 turns on all possible logging.

Action commands:

  6 - runs a transmission using the current settings

  7 - cancels a transmission if there is one running

  8 - reports statistics from the most recent transmission

  9 - quits the program

3. APPLICATION DESIGN

3.1. KEY COMPONENTS

The design of this example application consists of four main
components: the driver object, the relay object, the input device
object, and the output device object.

The driver object is responsible for receiving user input and overall handling
of user input commands.  The driver object is active, with separate threads
for receiving user input and managing its transmission timer queue.  This
allows the user to issue commands while a transmission is in progress.  The
driver object uses an ACE_Thread_Timer_Queue_Adapter, which is derived from
ACE_Task_Base.  The driver object starts another active object, called
User_Input_Task, which is also derived from ACE_Task_Base.  This allows both
the timer queue and the user input object to be made active, running in their
own threads of control.

The relay object is passive, managing a message queue and necessary
locks to allow safe access from multiple threads.  It provides methods
to receive and enqueue a mesage from the input device, dequeue a
message and send it to the output device, and to start or end a
transmission.  It uses ACE_Message_Queue (which contains ACE_Message_Block
objects) and ACE_Thread_Mutex objects to implement this functionality.

The input object is active, managing timeouts and input events in its
own thread.  The input object is also reactive, using an ACE_Reactor to
allow response to multiple input handles as well as to do polling at
specific timeouts.  The input pseudo-device wrapper in this example
does not make use of input handles and only does timed polling, but
extending this only requires registering the appropriate input handles
and handlers with the reactor.  The input object is derived from
ACE_Task_Base, and is activated by the relay object when a new
transmission starts.  The input object packages each data packet in an
ACE_Message_Block before sending it to the relay object.

The output object is passive.  If logging is turned on, it will report
the arrival time, relative to the start of the transmission, of each
output message, and the contents of that message.  The output object
will also "consume" each ACE_Message_Block passed to it, calling
delete on the passed pointer.

3.2. RUN-TIME CHARACTERSITICS

When the user initiates a transmission, the appropriate settings are passed
by the driver to the relay object's start transmission method. The relay
object tries to start a new transmission.  If another transmission is in
progress, the method returns an error.  Otherwise, the relay object's start
transmission method initializes itself and the input and output device
objects, activates the input device object, and stores the handle for
the new input device thread.

The driver then constructs a timeout handler with a count of the
number of messages to relay and a send timeout value, and pushes a
timer with this handler onto the timer queue.  If there is a limit on
the duration of the transmission, the driver constructs a different
handler for end-of-transmission, and pushes a timer for the end of
the transmission with this handler onto the timer queue as well.  When
the user issues a cancel transmission command, the driver calls the
relay's end transmission method.

When a send timeout expires, the handler (running in the timer queue
thread) calls the send method of the relay.  If there are any enqueued
messages from the input device object in its queue, the relay object's
send method will dequeue a message, pass it to the output device
object, and return success.  If there are no messages in the queue,
the relay object's send method will return failure.  If the send was
successful, the handler will decrement its count of remaining
messages.  If the count is greater than zero, the handler will
register a new send timer for itself and exit.  If the count is zero,
then the handler will call the relay's end transmission method, clear
the timer queue, and mark itself inactive before exiting.

Similarly, if the end-of-transmission timer expires before all packets
have been sent, that handler will call the relay's end transmission
method, clear the timer queue, release the semaphore, and then exit.

When the relay's end transmission method is called, it marks the relay
itself inactive, and calls the input device's deactivate method, which
sets the input device's activity flag to inactive (see below).  The
relay's end transmission method then waits until the input device thread
exits, before returning.

While it is still active, the input device thread blocks on a reactor
timeout for the duration it was given by the relay.  When the timeout
expires, the input device checks a flag to see if it is still active.
If the flag says the input device is inactive, the thread simply
exits.  This allows cancellation of the input device when a
transmission has ended.  If the flag says it is still active, the
thread builds a new character buffer, and wraps this with a new
ACE_Message_Block.  The input device passes this message block to the
execution method of the send input message command object with which
it was constructed. This level of indirection allows the input device
to be used with arbitrary types, so that it could for example be
connected directly to an output device.  The input device also
maintains a count of the number of messages it has sent.  Once the
input device has sent all its messages, it marks itself inactive, and
its thread simply exits.

4. ACCESSING THE SOURCE CODE

The files for this example are located in
$ACE_ROOT/examples/Bounded_Packet_Relay in the latest release of ACE,
which is located at

http://www.cs.wustl.edu/~schmidt/ACE_wrappers/ACE.tar.gz

Source Files: Thread_Bounded_Packet_Relay.h
              Thread_Bounded_Packet_Relay.cpp
              BPR_Driver.h
              BPR_Driver.cpp
              BPR_Driver_T.h
              BPR_Driver_T.cpp
              bpr_thread.cpp

Make file:    Makefile

Doc file:     README (this file)

Executable:   bpr_thread



