[TYPES] Types in distributed systems

"Ionuț G. Stan" ionut.g.stan at gmail.com
Thu Aug 28 22:24:22 EDT 2014


First, let me acknowledge upfront that I'm a complete dilettante when it 
comes to type theory (I'm still working through the first chapters of 
TaPL). That means that what I'm about to ask will be either stupid or 
extremely inexact. Nevertheless, I'm curious.

Now, onto my question...

The TL;DR version is: how does one specify types in a distributed 
programming model, like actors? And how much can we trust these types?

I was debating today, on Twitter [1], how to type the sets of messages 
two actors can exchange; a producer (P) and a consumer (C).

Let's say that the possible set of messages produced by P is M. We would 
like, by means of a type system, to know that C can handle: 1) just 
messages of type M and 2) all possible messages of type M.

M could be represented by a sum type. Let's consider this particular one:

M1 = A | B | C

In a closed world this makes complete sense (to me, at least) and it's 
easy to verify statically. But in an open world setting, like a 
distributed system, where P and C are on different machines that may be 
upgraded separately, things look harder to me. You may guarantee 
statically that the two properties are met, but at runtime there may 
appear race conditions that violate the second property.

For example, we deploy P1 and C1, both agreeing on M1. Next, we add a 
new variant to M1:

M2 = A | B | C | D

P1 and C1 are updated accordingly and we get P2 and C2, which we try to 
deploy, but a race condition appears and P2 send message D to C1. 
Obviously, C1 does not understand it. Even though the type system told 
us that C1 can handle all variants of M, it can't actually.

A similar scenario appears when removing one of the variants of M1.

Is there any typing approach to this kind of problem?

It looks to me that types would have to include a version as well and 
all runtime communication between different versions should be 
prohibited by having versioned communication channels.

Does anyone have any insights or pointers to articles, papers or books 
that discuss this sort of problem?

Thank you for reading this!

[1]: https://twitter.com/shajra/status/504568858967953408

Ionuț G. Stan  |  http://igstan.ro

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