Home > Uncategorized > Distributed Programming Language -> What is it exactly ?

Distributed Programming Language -> What is it exactly ?

There are already quite a few distributed programming languages out there. The most popular being probably Erlang, although it s not a “network transparent” or “distribution transparent” language. On the other hand Mozart ( which I didnt try yet, but probably will very soon ), seems to have some mechanics for transparent distribution, but with the cost of lot of semantics to be learned. And, well to be honest I never heard about it so far, although I am sure there must be clever systems in it… Why is that ?

The difficulty in finding a proper distributed language for your application comes from the fact, that you can write distributed application as soon as you have any “socket related” feature in your language. However the time you need to write such an application in a language that hasnt been designed for it, is humongous. In addition, in the real world, the necessity or distributed languages is arguable right now, because companies dont see much applications for it just yet. Without the wheel nobody thought about cars I bet 😉

The main question I want to discuss here is : “What does a distributed programming language need in its very core to be useful, widely used and accepted by usual developers ??”

=> To be used it needs to be useful, yet simple, so most developers used to imperative programming can grasp it quickly, and do something useful with it.
In traditional local programming, the execution is done on a CPU, one instruction after another.
In concurrent / distributed programming, the execution is done on multiple CPU, many instructions at the same time ( maybe? ) and we do need a way to harness that. It can be an extremely complex “concurrency graph” of relations between different instructions set in different locations. One of the way to harness it with a very fine-grained control system could be based on “Causality-tracking” algorithm, that are already in use by TeleCom Companies, for mobile phones related applications.

In traditional ( imperative and functional ) programming, we are writing instructions in a structured way, that we can represent as a decision tree. Problems arises when you have to mix the “causality graph” of relations between different instructions or events in the different location, and that decision tree.
If I try to write ( in C++ like syntax) instructions like :

var a = 5; if ( condition )
//I am asking remote processB to increment my variable "a"
{ remote_execute ( incr(a), processB); }

//I expect a to be incremented -> MISTAKE !!!
{ getfrom(a, processB ); }

I am making an obvious mistake, because in the “else” block, my variable a is not going to be incremented, and even more, it might not be known by processB at all. Even if it is a programmer error, we do need a way to tell him early on it s an error, or better, make it impossible for someone to write such a code. And this might be the most obvious error that can happen. If you have written multithreaded applications in the past, you know how troublesome it can quickly become.

The main thing behind compiled languages, is that they try to warn the user during compilation of all the possible errors. And they cant prevent most concurrency errors, because these are very dependent on other factors ( execution environment, ordering of instructions, etc. ) that the compiler cannot have knowledge of. The causality tree of a concurrent program is unveiling during the execution, there is no static representation of it that we can code. And even if we could, it would be a big hassle to the programmer who would probably not even want to think about it, since it s not related to his problem, as long as performance is satisfactory.

To conclude, to be useful, a distributed language needs to :
– remain simple and efficient, maybe simplifying the different ways to have concurrency( threads process, etc. )  and making it more transparent.
– harness concurrency with transparent causality tracking, ( simplicity : transparent, efficiency : allowing concurrent events – partial order )
– find simple and intuitive ways to mix that “dynamic” causality tree with the “static” decision tree. This is a very tricky part. Some system have concurrency controling imperative algorithms( BOINC ), some have imperative control over lightweight concurrency ( multithreads )
– be based on “evaluation”, just like script engines, because we already have a running system, and we can dynamically check for concurrency errors using it. It s also a good sandbox for concurrency algorithm tests.

=> To be widely accepted it needs to do *what we are already doing* but better, faster, stronger, even if we are not already doing distributed applications…
The most widespread “development” and “application writing” activity might be website development nowadays. It make sense in a way because it s related to distribution, people interaction, and just like with distributed languages, few years before the invention of the web, all the nice web applications we use on a daily basis right now would have been very hard to imagine.
To be able to compete with traditional imperative local languages, it should have decent performance on local CPU execution. Probably with implicit threading when it is detected possible, since nowadays most new machines have multiples CPU. It should also be quite familiar and intuitive to get used to it quickly. We probably need to keep and use most of the traditional control instructions ( if/else, for, while, etc. )

A good way for people to be able to use a quite low-level language for what they want is to allow “modules” or “plugins” to add high level functionality to it. the core language should however provide interface to most devices.

The road to an accepted distributed language seems still pretty long, but it s definitively an interesting one indeed…

Does someone already a knows a Distributed Language that fits all his needs ? Does anyone has other / better ideas for distributed language features ?
I wonder when the first truly transparent distributed language is going to arrive and be widely used…

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