Re: Distributed Systems Annex, data sharing between programs

[Pascal Obry <pascal@obry.net>]

Maciej,

> The solution is to stop thinking in terms of RPC (remote procesure

Right.

> calls), and start thinking in terms of messages. If you lift the
> concept of the message to the level of a design entity (yes,
> distribution is *not* transparent and it *cannot* be an afterthought),
> then some cool things are possible - just create thousands of messages
> and let them go. No need to create additional tasks and no need to
> wait for consecutive steps, as everything can happen in parallel
> naturally. The original problem becomes feasible even in the presence
> of partial failures, because separate message do not have to see their
> delays and their individual communication problems.

I don't agree, I've done that.

One partition keeps a list of "job" (what you call message) in a
protected object.

   package Job is
      pragma Remote_Types;

      type Object is tagged...

   package Job_Queue is
      pragma Remote_Call_Interface;

      function Get return Job.Object;

      ...


And then each remote partition get a new job when needed. This ensure
proper use of the resources as it naturally give good load balance. The
speed of the remote machine or network is not an issue in this design.

Again, I find Ada solution really good.

The only point is that the Job above must be large grained ones. But
this is true for every distributed applications where the cost of the
network transfer must not negate the computing speed up.

Also note that with the design above you don't need to go distributed
only. You can also create some tasks on a single application to get
multiple jobs on the same machine which is surely multicore, this also
ease the debugging.

Pascal.

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