[e-lang] Control flow considered harmful was: Proposal: Sugar-freewhens

Andreas Raab andreas.raab at gmx.de
Thu Dec 23 02:21:19 EST 2004

Hi Dean,

Very interesting, thank you so much! I will need some more time to chew on 
this (and likely come back with more questions) but one thing that strikes 
me in your argumentation is that (naively speaking) it looks as if these 
arguments all apply to a distributed system where certain properties (such 
as full order, or atomicity) are hard or impossible to achieve. Would you 
agree with this statement?

  - Andreas

----- Original Message ----- 
From: "Dean Tribble" <tribble at e-dean.com>
To: "Discussion of E and other capability languages" 
<e-lang at mail.eros-os.org>
Sent: Wednesday, December 22, 2004 10:46 PM
Subject: [e-lang] Control flow considered harmful was: Proposal: 

>>> <rant>
>>> When-chaining is essentially open-coding continuation-passing 
>>> control-flow. to achieve synchronous call.  It didn't make actors able 
>>> to succeed.  It doesn't work at a system level.  It doesn't work no 
>>> matter how much programmers new to distributed computing want it to.  It 
>>> doesn't work no matter how nice the syntax is for it.
>>> Dataflow works.
>>> </rant>
>> Say more about this. It's the first time I've heard this argumentation 
>> and clearly, given your strong feeling about it you've got some 
>> experience with both styles. I'm curious what exactly the problems at the 
>> system level are.
> Many of the systems, models, and alternatives explored before and during 
> the Joule effort involved variations of this sort.  Rather than get too 
> worked up, I'll just visit a bunch of different kinds of issues that show 
> up, and you can ask for expansion on ones that are not obvious in 
> retrospect.
> IMPLICIT FULL ORDER:  Control-flow ordering essentially imposes a total 
> order on a partially ordered set of statements.  You cannot tell what of 
> that imposed order is important, and what is incidental.  For example, 
> Marcs's and my sequential account examples were the same, whereas our 
> eventual variations differed in behavior under error.  That's because 
> there is an implicit coupling dependency on following statements that the 
> statement before them succeeded.  His service would only get invoked if 
> funds were available.  Did he intend that?  It matters a lot more when you 
> are crossing security boundaries.  This might argue for considering an 
> error construct that did *not* modify flow of control even in the 
> sequential case.
> PARTIAL ORDER: Distributed systems are partially ordered.  One of the 
> biggest and most frustrating values of Joule is that you could not get 
> away with anything.  Any assumption that the world was fully ordered 
> almost immeidately came back and bit you.  I was astonished at the number 
> of times I was assuming more order than was actually present in a 
> distributed system.  For example, if A send to B then C, and they both 
> send to D, what assurance do I have that B and C's events on D are ordered 
> in a particular way?  In the sequential world, I can be confident.  In the 
> distributed world, any model I have is likely to be wrong.  Unless of 
> course, each layer is carefully making sure to keep everythign fully 
> ordered.  Then see SNOWBALLING and DEADLOCK
> ATOMICITY LOSS:  In the trivial examples examined so far, there is not an 
> boject, and so there are no other side-effects.  That's simply not 
> realistic.  By moving code into a when, you've lost the atomicity that 
> enables simple coordination in a chaotic world.  For example, if you need 
> to atomically update your object state to spend the money and get the 
> result of the service, then the service invocation cannot be in the nested 
> when clause with the option activation, because otherwise another event 
> could get in there and change the object, its account, the service, etc. 
> The simple atomicity in E of being able to do things and put your 
> invariants back to right before taking another external event is a huge 
> source of power, simplicity, and robustness.
> DEADLOCK: everything in a when block is "waiting" for the event that calls 
> back to that when.  By using that for control-flow ordering, you 
> essentially map the control-flow restrictions into dataflow.  That simply 
> means you bring all the deadlock problems into the dataflow world, so 
> emergent deadlock starts to appear again.  The observer example in MarkM's 
> CML response is of that flavor.
> SNOWBALLING: the ordering trick kinda works within a method.  How do you 
> ensure that operations expressed byt he caller of a method happen *after* 
> the actions (now embedded in when clauses) that are spawned by the method. 
> Well, you can pass in a thunk, or always pass back just the right promise 
> and have the caller do a when on that.  So which libraries should we make 
> follow that pattern?  You pretty quickly need most of your libraries to 
> follow the same pattern, or they cannot be used in the distributed 
> context.  Then, you are programming in the actors model, in which most 
> returns are via callbacks rather than simple return.  That's a much harder 
> model to understand.  Similarly, all the services that you call need to 
> work together to implementation the IMPLICIT FULL ORDER so that the natral 
> PARTIAL ORDER of a distributed system doesn't surprise you.
> INHERITANCE:  The same pattern issue arises in trying to use inheritance 
> or other object patterns.  You can no loner reliably call your superclass; 
> you start needing to eventually send to your superclass, with all the 
> additional coordination overhead.
> DEPENDENCE:  Since your forward computation is now dependent on the 
> resolution of results from a call, your termination becomes dependent on 
> the return of another service.  That fine within a single vat (because it 
> could just infinite loop), but not fine between services.  Thus, you need 
> to avoid such dependencies.  They lead to brittleness and cascading 
> failures.
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