[squeak-dev] Re: Future examples (Re: Inbox: #future keyword for asynchronous message invocation)

Andreas Raab andreas.raab at gmx.de
Thu Dec 17 22:41:05 UTC 2009

A combined response:

Stephen Pair wrote:
 > What is the scope of this safety with respect to shared state and
 > mutation?  Are you essentially treating the entire image as a vat (in
 > the E sense)?  Is it per morphic project?  Or something else?

It depends on the implemenation. Josh's version is deliberately simple 
to get people accustomed to the idea and uses 
Project>>addDeferredUIMessage: for delivery. We'll definitely want to 
improve on that and introduce separate event loops and possibly even 
VATs but I think going slowly is advantageous because it helps people to 

Colin Putney wrote:
 > [regarding the bug] I'll take that as a challenge. :-) I assume that 
If the mouse button is released then keepScrolling will be set to false. 
  If the button is then pressed again before the delay wakes up, you'll 
have two processes doing the scrolling. It would seem that they'd cause 
scrolling to happen twice as fast.

Good one! But this could conceivably be fixed by implementing 
#finishedScrolling properly (for a certain meaning of "properly"). The 
bug I'm referring to is that in

    self future setValue: (value + scrollDelta + 0.000001 min: 1.0).

we're reading both value and scrollDelta without synchronization. 
Properly written this should look more like:

    [self setValue: (value + scrollDelta + 0.000001 min: 1.0)] future value.

to defer binding value and scrollDelta but I'm sure you can see why I'm 
preferring the alternative for illustrating the concept. It does point 
an interesting property of future messages namely that arguments are 
bound early by default. This is a question of choice but I find it in 
practice to be advantageous because the cases where you'd want to late 
bind them are relatively rare.

Bert Freudenberg wrote:
 > Do you have experience with using alternate main loops? Like, in a 
server where you might want to block the UI thread, we still might want 
to drain the future queue.

Yes. We use EventLoop instances to encapsulate processes and their 
message queues. Those run in separate processes and communicate via 
future messages. There is *lot* of those in our servers (thus the point 
about lock-free communication; regardless where you are you can always 
just fire a foo future bar without worrying about locking etc).

Igor Stasenko wrote:
 > Hmm.. can't see how futures helping to deal with concurrency. Unless
 > there some details which i don't see.
 > A semantics of 'future' is guarantee that message will be sent
 > eventually in future, but there is no need to guarantee that this
 > message order will be preserved e.g.:

Messages delivered from the same unit of concurrency (A) and being sent 
to the same unit of concurrency (B) are ordered. As a consequence a 
series of
	self future foo.
	self future bar.
	self future baz.
is always well-ordered (foo first, then bar, then baz). For the example 
(all messages inside a single concurrency unit) the ordering is even 
more strict than that: bar and baz *will* get executed before any future 
messages sent from executing foo.

 > And if some other code poking with your data and interrupted to handle
 > future message send, you still might need to use a synchronization, if
 > both accessing same state.

Yes, without stronger encapsulation (which we use for example in Croquet 
islands) there is still the chance to introduce "accidental sharing" 
(just as illustrated in the bug above). However, the main advantage is 
that in practical situation it's *always* safe to just use "self future 
foo" if you don't know whether the code you're is executed from the same 
concurrency unit or not. Classic example: If you don't know if the 
logging code can be executed from some other thread, just use 
"Transcript future show: 'Hello World'". This is safe no matter if you 
run it from a background process or from the Morphic UI process.

   - Andreas

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