Dataflow concurrency and promises

Kris Zyp kris at sitepen.com
Tue Sep 29 21:07:25 PDT 2009


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Just FYI, there has also been significant discussion on CommonJS
regarding the use promises [1] and concurrency [2] in JavaScript,
CommonJS is focused on standardizing library APIs for JavaScript, in
particular on the server. Obviously on the server the concurrency
needs are extensive, and without the traditional browser platform, it
is has been important to define concurrency expectations. Generally
consensus has been around maintaining the worker API/model of the
browser, and employing promises  for the majority of asynchronous
functions (here is current API proposal/outline, albeit perhaps in
need of update [3]). I don't know that our discussions will provide
any significant revelations in these areas, it is primarily rehashing
well-covered ground. But, obviously, CommonJS would want to stay in
sync with EcmaScript plans.
[1]
http://groups.google.com/group/commonjs/browse_thread/thread/fa20fc6a3649b3a
<http://groups.google.com/group/commonjs/browse_thread/thread/fa20fc6a3649b3a>
[2]
http://groups.google.com/group/commonjs/browse_thread/thread/e93f73ef97e88439/50fc4d51eb0a9ee5
[3] http://wiki.commonjs.org/wiki/Promises
Kris

David-Sarah Hopwood wrote:
> Brendan Eich wrote:
>> Beyond this, concurrency via workers is great for certain use-cases but
>> not enough for others.
>>
>> In TC39 we are talking about formalizing the run-to-completion execution
>> model of JS, along with asynchronous message passing concurrency. In
>> particular, we're looking at Promises (precedent from E) and Futures
>> (differently, in MultiLisp and Alice-ML). At least one contributor on
>> es-discuss has advocated lower-level components such as dataflow
variables.
>
> That was presumably me:
> <https://mail.mozilla.org/pipermail/es5-discuss/2009-May/002557.html>.
> However, I don't agree that dataflow variables are "lower level".
>
>> It's too early to predict what we'll do but I hear strong consensus in
>> favor of asynchronous messaging and shared-nothing, with higher-level
>> abstractions such as Promises favored over lower-level concurrent
>> programming features such as dataflow variables.
>
> "Dataflow variable", "promise", and "future" are different kinds of delayed
> reference; they are at about the same level of abstraction, and are very
> similar to each other. The fact that different terminology is used for
> them that obscures the similarities is something of a historical accident.
>
> See <http://en.wikipedia.org/wiki/Futures_and_promises>. (Full disclosure:
> I wrote quite a bit of this article.)
>
> There are, however, some differences. Attempting to synchronously use
> an unbound dataflow variable will block until it is bound. Attempting
> to synchronously use an unbound (unresolved) promise will, at least in E,
> throw an exception. Note that in both cases, you can use a 'when' construct
> to wait for the delayed reference to become bound/resolved, and then the
> observable semantics are the same.
>
>
> Because the dataflow model (as supported in Oz) does not treat
> synchronously accessing an unbound delayed reference as an error,
> it is more general: it allows *either* a "dataflow style" of programming
> that depends on blocking, or an "event-loop style" that avoids blocking.
> An event-loop model (as supported in E) enforces the event-loop style.
>
> I am not arguing that a more general model is necessarily better.
> A less general model can in some cases be preferable because it can
> be an advantage for all code to be written in the same style.
>
> OTOH, the dataflow model does provide more flexibility: a programmer
> can choose to use a pure dataflow style, or a pure event-loop style,
> or to combine them. Combining them is not entirely without difficulty,
> but that difficulty is restricted to liveness issues rather than safety
> issues (that is, the combined programming style is still highly resistant
> to low-level race conditions, although more care is needed to avoid
> deadlocks). *If* a programmer chooses to use the combined style, for
> programs that need it, there is IMHO a significant payoff in
expressiveness.
>
> (This is the model that ideally I would like to be able to program in all
> the time. Of course you don't have to pay attention to my personal
> preference, but there is plenty of support for it among Oz programmers,
> and in the book "Concepts, Techniques, and Models of Computer
Programming".)
>
> The dataflow model potentially allows easier interoperation with APIs
> that are not based on event loops.
>
>
> It also seems that there is some confusion about "asynchronous messaging
> and shared-nothing". First, I agree completely that we want to support
> asynchronous message passing in JavaScript. In fact that combines very
> naturally with dataflow variables, just as well as it does with promises.
>
> "Shared-nothing" is a term used in the Erlang community to contrast
> message-passing models with shared-memory models. The term is
> inaccurate; it is not the case that processes "share nothing" in
> Erlang or in other message-passing languages. The restriction that
> this term is intended to refer to is that processes do not have
> synchronous shared access to the same mutable state.
>
> The following forms of sharing between processes are consistent with
> maintaining the advantages of message-passing models relative to
> shared-memory models:
>
>  - processes can share references to other processes.
>
>    In the case of a vat-based model (where vats are the units of
>    concurrency and each object belongs to a vat), an object can
>    have a reference to an object in another vat, but it can only
>    use the reference by *asynchronous* message passing. References
>    within the same vat can *also* be used via synchronous message
>    passing.
>
>    The model I would suggest for JavaScript would be like E in
>    this respect: it provides significant and useful additional
>    expressiveness. The partial isolation between vats, which
>    allows vats to fail or be destroyed independently, is also
>    very useful in a web context. For example the default behaviour
>    (when no additional vats are created explicitly), could be that
>    a vat is created for each JavaScript context.
>
>  - processes can have shared access to a single copy of a deeply
>    immutable structure. This is equivalent to copying the structure,
>    except for the important issue of reduced memory usage.
>
>  - processes can have shared access to declarative structures --
>    that is, structures that can be extended but not mutated.
>    This is in practice relatively easy to reason about, and does
>    not introduce the same programming difficulties as a
>    shared-memory model.
>
>    (It does introduce a limited form of nondeterminism: if two
>    processes attempt to make a conflicting extension, the program
>    will fail. This is a programming error. Programs without such
>    errors behave deterministically, and programs with such errors
>    deterministically fail, but the side-effects that occur before
>    they fail may be nondeterministic.)
>

- --
Kris Zyp
SitePen
(503) 806-1841
http://sitepen.com
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