Re: Distributed Systems Annex, data sharing between programs

[tmoran@acm.org]

> .. between partitions (of the same program?) transparently, and also to share
> data in Shared_Passive partitions transparently, without users having to
> worry about the representation.  So it would seem like a natural extension
> to provide this sort of communication between "programs".  The mechanisms
> would already exist.

    If program A consists of Procedure Main_A and a separate partition
Server, while program B is Procedure Main_B and also a separate Server,
you can write a small program AB which simply starts up two tasks.  One of
those tasks calls Main_A and the other Main_B and there you have
effectively the two programs running as a single, multitasking, program.
Separate it into partitions and you have the original two programs running
as a distributed program.  (Server would of course have to be re-entrant
whether it's called by two tasks in a traditional single physical
partition, or by two different active partitions.)

> On the other hand, you have a point that writing a server that uses a
> feature like this would limit the programs that could use the service to
> those written in Ada.
   Or any program with the needed Ada interface layer.

> .. now, if we repartition the program in such a way that a given
> operation (Do_Something_For_Me) ends up in a separate partition, it
> might become potentially blocking by virtue of the hidden physicality
> of remote call. In short, it might become a I/O operation even though
> it is not visible anywhere. If such operation is used within a
> protected body, then the program can be legal or illegal depending on
> the partitioning scheme, which is not expressed in code.
    "All forms of remote subprogram calls are potentially blocking
operations."  So the final physical partitioning may decide whether a
particular call is actually blocking, but if it's a remote subprogram
call, even if the whole program is physically in one partition, then
the call is "potentially blocking" and thus illegal from a protected type.