Re: [Q]: Distributed System Annex DSA

[Jonas Nygren <jonas@joy.ericsson.se>]

Robert A Duff wrote:
> 
> In article <32343D67.5E2D@ehs.ericsson.se>,
> Jonas Nygren  <ehsjony@ehs.ericsson.se> wrote:
> >I referred to the wrong package in my first question which
> >made it self answering. Pls see below for correction.
> >
> >Jonas Nygren wrote:
> >>
> >> For Gnat there now is a release of support for DSA. So I thought
> >> it was time to get an understanding what DSA can do for me. I have
> >> a few questions around this topic after trying to understand what
> >> the Rational and the RM's texts on this feature. (If you can point me
> >> to some more good educational text for DSA I would appreciate that.)
> >>
> >> Now to my questions for which I use the example of Tape_Driver given
> >> in Annex E of the RM. I attach the code from the RM at the end of
> >> this posting.
> >>
> >> 1)  When I studied this example I got the impression that the three
> >>     parts Name_Server, Tape_Server and Tape_Client all could run
> >>     in different partitions (and on different nodes). What confuses
> >>     me is that the Name_Server does not have any 'pragma
> >
> >                      ^ ^  Ouch, it should have been Tape_Server
> 
> I see no Tape_Server in the code you quoted.  I assume you mean
> Tape_Driver?  (Ouch again?  ;-))

Yeah, I noticed my error just before leaving on a buisness trip and posted
my 'correction', in the taxi to the airport I got an uneasy feeling about
'Server' versus 'Driver', but since the taxi had no internet service I had 
no time to check up nor correct any errors a second time - then business class 
service made me forgetful.

> 
> >>     Remote_Call_Interface' and in the explaining text it is referred
> >>     to as a 'normal package'. Does this imply that Tape_Client and
> >>     Tape_Server must be co-located in the same partition?
> 
> Tape_Driver and Tape_Client do not need to be in the same partition.
> 
> >...If not,
> >>     how does the compiler know that Tape_Server's interfaces
> >>     (Copy, Rewind) should be constructed to receive remote calls?
> 
> It doesn't need to.  Tape_Ptr is a remote access type, because of where
> it's declared.  Normally, a pointer to a tagged type would be
> represented as a pointer to the object.  The object contains a tag,
> which is a pointer to the type descriptor, which contains pointers to
> all the primitive operations.  But for a remote access type, the
> compiler must do something different -- I think the best strategy is to
> make the pointer point to a dummy object (on the same partition as the
> pointer, of course).  Thus dummyy object has a tag, but instead of
> pointing to the "real" type descriptor, it points to one which has
> pointers to subprograms that send a message to the place where the
> "real" object is located.  The "real" object, off on the other node,
> doesn't need to know that it is being called remotely.
> 
> Make sense?

Yes. Almost like magic - perhaps the text should hav emphasized the 
importance of the 'remote pointer'. But: once told - never forget.

> 
> >> 2)  I have come to regard the package Tapes as the 'contract' of
> >>     a tape service. Would this be the right place to enforce
> >>     constraint and state checks to strengthen the consistency
> >>     of the interface. State information would then be stored in
> >>     the Tape (tagged) record. Are there any negative consequences
> >>     in this approach and are there better ways to do this?
> 
> Sounds reasonable.
> 
> >> 3)  With CORBA and other similar technologies there is often much
> >>     discussions on the need for threads in servers. My third question
> >>     is about if and how one can use Ada tasks to achieve this in
> >>     one remote active partition.
> >>
> >>     One example in the Rationale showed how one could link together
> >>     a number of active Worker partitions with a Controller partition
> >>     and how these partitions could be distributed on different nodes.
> >>     But nothing with tasks involved.
> >>
> >>     My question is really if it is possible to multi-plex several
> >>     service sessions on one partition. E.g. If Client A and B issues
> >>     requests to the server 'at the same time' - how do I design the
> >>     server so that these two calls can be overlapping and executed in
> >>     parallel.
> 
> You can declare multiple tasks in a partition, as usual.
> 
> Also, the PCS is normally implemented with tasks, so that multiple
> requests can be serviced simultaneously.

I dived into the Gnat DSA code and found in s-rpc.adb that a maximum of
127 anonymous tasks were created. Though, the stack size of these tasks
were fixed (~150 kb) and the configuration files (.cfg) did not seem to 
support a user defined stack size (well, 150 kb may seem enough in most cases
and also to much in other cases and finally to little in the odd case,
it would be nice if it was confgurable).

Though, DSA is really exciting. I believe it could provide the base for 
the 'killer' application Ada needs.

> 
> >> Any help appreciated!
> >>
> >> Thanks in advance,
> >>
> >> /jonas
> >>
> >> -- Tape example from Ada RM e.4.2
> >>
> >> (2)
> >>        package Tapes is
> >>           pragma Pure(Tapes);
> >>           type Tape is abstract tagged limited private;
> >>           -- Primitive dispatching operations where
> >>           -- Tape is controlling operand
> >>           procedure Copy (From, To : access Tape; Num_Recs : in Natural)
> >> is abstract;
> >>           procedure Rewind (T : access Tape) is abstract;
> >>           -- More operations
> >>        private
> >>           type Tape is ...
> >>        end Tapes;
> >> (3)
> >>        with Tapes;
> >>        package Name_Server is
> >>           pragma Remote_Call_Interface;
> >>           -- Dynamic binding to remote operations is achieved
> >>           -- using the access-to-limited-class-wide type Tape_Ptr
> >>           type Tape_Ptr is access all Tapes.Tape'Class;
> >>           -- The following statically bound remote operations
> >>           -- allow for a name-server capability in this example
> >>           function  Find     (Name : String) return Tape_Ptr;
> >>           procedure Register (Name : in String; T : in Tape_Ptr);
> >>           procedure Remove   (T : in Tape_Ptr);
> >>           -- More operations
> >>        end Name_Server;
> >> (4)
> >>        package Tape_Driver is
> >>          -- Declarations are not shown, they are irrelevant here
> >>        end Tape_Driver;
> >> (5)
> >>        with Tapes, Name_Server;
> >>        package body Tape_Driver is
> >>           type New_Tape is new Tapes.Tape with ...
> >>           procedure Copy
> >>            (From, To : access New_Tape; Num_Recs: in Natural) is
> >>           begin
> >>             . . .
> >>           end Copy;
> >>           procedure Rewind (T : access New_Tape) is
> >>           begin
> >>              . . .
> >>           end Rewind;
> >>           -- Objects remotely accessible through use
> >>           -- of Name_Server operations
> >>           Tape1, Tape2 : aliased New_Tape;
> >>        begin
> >>           Name_Server.Register ("NINE-TRACK",  Tape1'Access);
> >>           Name_Server.Register ("SEVEN-TRACK", Tape2'Access);
> >>        end Tape_Driver;
> >> (6)
> >>        with Tapes, Name_Server;
> >>        -- Tape_Driver is not needed and thus not mentioned in the
> >> with_clause
> >>        procedure Tape_Client is
> >>           T1, T2 : Name_Server.Tape_Ptr;
> >>        begin
> >>           T1 := Name_Server.Find ("NINE-TRACK");
> >>           T2 := Name_Server.Find ("SEVEN-TRACK");
> >>           Tapes.Rewind (T1);
> >>           Tapes.Rewind (T2);
> >>           Tapes.Copy (T1, T2, 3);
> >>        end Tape_Client;
> 
> - Bob