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Better comments.

This commit is contained in:
Tony Garnock-Jones 2017-08-27 08:18:11 -04:00
parent d0e803ac41
commit 5ce1cec2ea
2 changed files with 36 additions and 20 deletions
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42
racket/syndicate/examples/actor/example-demand-matcher-glitch-bug.rkt
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@ -1,10 +1,17 @@
#lang syndicate/actor
;; Example showing an interesting design flaw (?) (bug?) in the
;; old-school LLL demand-matcher when demand changes too quickly. The
;; port 6000 server is started, but by the time it starts monitoring
;; demand for its services, the demand is already gone, replaced with
;; demand for port 5999. This causes connections to be accepted on
;; port 6000 going nowhere.
;; Example showing the consequences of not honouring the requirement
;; of the current LLL demand-matcher that supply tasks must *reliably*
;; terminate when their demand is not present. In this case, demand
;; changes too quickly: it exists for long enough to start the task,
;; but is withdrawn before the task itself has a chance to detect it.
;; Because the task (as currently implemented) does not use the "learn
;; negative knowledge" pattern to detect the *absence* of some
;; assertion, it does not terminate as it is supposed to.
;;
;; Specifically, here, the port 6000 server is started, but by the
;; time it starts monitoring demand for its services, the demand is
;; already gone, replaced with demand for port 5999. This causes
;; connections to be accepted on port 6000 going nowhere.
;;
;; One "fix" is to use #:assertions to give the TCP listener actor
;; some initial interests, thus transferring responsibility
@ -12,15 +19,22 @@
;; this doesn't completely eliminate all possible instances where
;; demand may change too quickly. See example-demand-matcher-glitch-bug2.rkt.
;;
;; A speculative idea is to use a kind of contract-monitor to enforce
;; the invariant that demand *cannot* fluctuate too rapidly. One might
;; write that "if (listen 6000) is asserted, then if (listen 6000) is
;; retracted, (observe (listen 6000)) must have been asserted in the
;; causal history of the retraction", but what does "causal history"
;; mean, precisely? And how can it be soundly and efficiently tracked?
;; Of course, the real "fix" is for the TCP listener actor to use a
;; `flush!` to robustly detect that demand for its services no longer
;; exists even at startup time.
;;
;; The only "fix" that both solves the problem and is currently
;; implementable that I have thought of is to modify the
;; A speculative idea, if we set aside the (in principle) documented
;; requirement that the LLL demand-matcher places on its supply tasks,
;; is to use a kind of contract-monitor to enforce the invariant that
;; demand *cannot* fluctuate too rapidly. One might write that "if
;; (listen 6000) is asserted, then if (listen 6000) is retracted,
;; (observe (listen 6000)) must have been asserted in the causal
;; history of the retraction", but what does "causal history" mean,
;; precisely? And how can it be soundly and efficiently tracked?
;;
;; The only "fix" that solves the problem, is currently implementable,
;; and allows supply tasks to escape responsibility for noticing their
;; own superfluity that I have thought of is to modify the
;; demand-matcher to do something like `during/spawn` is doing, using
;; an auxiliary protocol to centralise tracking of demand and supply
;; at the demand-matcher rather than delegating it to the services.

14
racket/syndicate/examples/actor/example-demand-matcher-glitch-bug2.rkt
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@ -1,10 +1,12 @@
#lang syndicate/actor
;; Example showing an interesting design flaw (?) (bug?) in the
;; old-school LLL demand-matcher when demand changes too quickly. The
;; port 6000 server is started, but by the time it starts monitoring
;; demand for its services, the demand is already gone, replaced with
;; demand for port 5999. This causes connections to be accepted on
;; port 6000 going nowhere.
;; Example showing the consequences of not honouring the requirement
;; of the current LLL demand-matcher that supply tasks must *reliably*
;; terminate when their demand is not present. In this case, demand
;; changes too quickly: it exists for long enough to start the task,
;; but is withdrawn before the task itself has a chance to detect it.
;; Because the task (as currently implemented) does not use the "learn
;; negative knowledge" pattern to detect the *absence* of some
;; assertion, it does not terminate as it is supposed to.
;;
;; Demonstrates that the fix in commit 2a0197b isn't in general
;; sufficient.