Re: [moca] Locations?

[Pawel.Wojciechowski@xxxxxxx]

> for the sake of clarity, let me add that lsdpi enjoys the following law: 
> if all the free channels of a process P are explicitly located (i.e. in 
> the form a@s, being a the channel and s the location it belongs to), 
> then s[P] ~ r[P]. thus, the behaviour of the process does not depend on 
> where it is running.

In the Nomadic pi-calculus, you can create (global) channel names 
that can be effectively used only in the context of a given agent. 
Think of an agent as a "location" that can migrate between sites
(where sites represent physical machines).

In the Low-Level Nomadic Pict, you can write e.g. <a@s>c!v which
means "communicate 'v' using channel 'c' _located_ at agent 'a', 
where the agent is expected to be on site 's'.

The High-Level Nomadic Pict provides a higher level of abstraction
and you can simply write c@a!v instead, i.e. you do not need to specify 
the site name. 

The _distributed infrastructure_ is precisely the encoding which is
necessary to provide the high-level abstractions, expressed using 
the low-level abstractions. 

The choice or design of an infrastructure should address the problems
of the world you are in, in particular the model of failures, kind of 
network, etc. It is not realistic nor practical to abstract away from 
these problems (as Martin and others have pointed out). 

In Nomadic Pict we have adopted two levels of abstraction, thus providing 
open space (between the two levels) to be filled in with proper 
(algorithmic and semantically clean) definitions, expressed in the 
calculus, that give formal meaning to, e.g.:

- how the inter-site communication occurs, 
- how partial failures are tolerated (if they are tolerated), 
- what is the communication cost of migration/communication between 
  locations, 
- how agents contact local resources 
- etc.

	Pawel





  
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