Stream management in Ada

Stream management in Ada

[PSI%SNMSN1::PSI%DSAVX2::WILLY@slb-test.CSNET]

Forwarded for: Hever Bitteur (BITTEUR%M_SFMVX1@SLB-DOLL.CSNET)

	I would be very interested to hear about any one who has
already worked on this problem, especially if neat solutions
taking advantage of high level concept of Ada rendezvous, have
already been found. 

	The application deals with various tasks. A task uses
some data as input and produces some data as output, which can be
in turn used partially or totally by another task, and so on. A
feature common to all these pieces of data is that they behave
like flows of data : typically, we can have sampled raw data
which come from sensors, and which are passed through filters to
give engineering data, which in turn can be used to compute
derived variables, and so on. 

	Of course, transmission of data can be achieved with
direct rendezvous between a producer and a consumer task. The
problem is that we rapidly get into a mess of tasks all
interconnected, where any modification is painful, and where it
is difficult to add a new task, since we have to connect this new
task to any relevant producer task and any relevant consumer
task. 

	One more problem is that this design does not insure that
all the data used by a task at a given time is consistent : there
may be some dephasing between this and this variable. 

	A better approach seems to organize the task activities
around one or several "Stream Managers", whose function is to
store written data, to forward it to tasks interested in it, and
to handle the data consistency in a centralized manner. Thus, the
design looks like a data bus, where tasks (writers and/or
readers) are plugged on. Any new task just have to know on which
bus(es) it has to be plugged. 

	I have heard of implementations of this concept in
"classical" real time, using queues, events and shared memory.
Since this is to be integrated in a multi-tasking Ada
application, I am looking for a pure Ada solution. Of course I
would like to organize such a (generic) stream manager as a pure
server, which could be efficiently used by other Ada tasks. 

 	Thank you for any help.

Re: Stream management in Ada

[Ian Kaplan]

    It was stated in the original article that a "pure Ada" solution was
    sought.  I am not sure what exactly this means.  Presumable if it is
    coded in Ada it is "pure", but I think that the author ment more than
    this.

    At the heart of the Ada process model is Hoare's Communicating
    Sequential Processes (CSP) scheme.  Pure process model algorithms are
    not very good at handling steam applications.  Dataflow is very good at
    handling streams, however.  The Lucid programming language is designed
    for stream processing.  See "Lucid" by Ashcroft and Wadge, Academic 
    Press.  For a brief discussion of the dataflow and process model, see
    "Programming the LOral LDF 100 Dataflow Machine", by Ian Kaplan, which 
    should appear in SigPlan Notices around May.  
    
    "Processes" in dataflow communicate asynchronously.  This can be 
    simulated in a process based language by placing a mail box
    between the sender and the receiver.  The rendezvous are with the mail
    box.  The mail box not only allows asynchronous operation (e.g.,
    pipelining) but it also functionally decouples the two communicating
    processes.  

    Dataflow is not without its problems however.  The mailbox must provide
    buffering.  If the data producer is faster than the data consumer the
    mailbox buffers will fill up.  When this happens the producer must
    suspend.  This is what happens with UNIX pipes.  For an excellent
    discussion of UNIX pipes and their implementation see Chapter 2 of
    "Operating Systems: design and implementation" by Andrew S. Tanenbaum,
    Prentice-Hall.

                 Until March 27, 1987

		     Ian Kaplan
		     Loral Dataflow Group
		     Loral Instrumentation
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