2017-08-22 20:53:57 +00:00
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#lang syndicate/actor
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2017-08-27 12:18:11 +00:00
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;; Example showing the consequences of not honouring the requirement
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;; of the current LLL demand-matcher that supply tasks must *reliably*
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;; terminate when their demand is not present. In this case, demand
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;; changes too quickly: it exists for long enough to start the task,
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;; but is withdrawn before the task itself has a chance to detect it.
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;; Because the task (as currently implemented) does not use the "learn
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;; negative knowledge" pattern to detect the *absence* of some
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;; assertion, it does not terminate as it is supposed to.
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;;
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;; Specifically, here, the port 6000 server is started, but by the
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;; time it starts monitoring demand for its services, the demand is
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;; already gone, replaced with demand for port 5999. This causes
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;; connections to be accepted on port 6000 going nowhere.
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2017-08-22 20:53:57 +00:00
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;;
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2017-08-27 11:50:09 +00:00
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;; One "fix" is to use #:assertions to give the TCP listener actor
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;; some initial interests, thus transferring responsibility
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;; atomically. This has been implemented (in commit 2a0197b). However,
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;; this doesn't completely eliminate all possible instances where
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;; demand may change too quickly. See example-demand-matcher-glitch-bug2.rkt.
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;;
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2017-08-27 12:18:11 +00:00
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;; Of course, the real "fix" is for the TCP listener actor to use a
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;; `flush!` to robustly detect that demand for its services no longer
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;; exists even at startup time.
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;;
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;; A speculative idea, if we set aside the (in principle) documented
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;; requirement that the LLL demand-matcher places on its supply tasks,
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;; is to use a kind of contract-monitor to enforce the invariant that
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;; demand *cannot* fluctuate too rapidly. One might write that "if
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;; (listen 6000) is asserted, then if (listen 6000) is retracted,
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;; (observe (listen 6000)) must have been asserted in the causal
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;; history of the retraction", but what does "causal history" mean,
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;; precisely? And how can it be soundly and efficiently tracked?
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2017-08-27 11:50:09 +00:00
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;;
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2017-08-27 12:18:11 +00:00
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;; The only "fix" that solves the problem, is currently implementable,
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;; and allows supply tasks to escape responsibility for noticing their
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;; own superfluity that I have thought of is to modify the
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2017-08-27 11:50:09 +00:00
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;; demand-matcher to do something like `during/spawn` is doing, using
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;; an auxiliary protocol to centralise tracking of demand and supply
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;; at the demand-matcher rather than delegating it to the services.
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2017-08-22 20:53:57 +00:00
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(require syndicate/protocol/advertise)
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(require/activate syndicate/drivers/tcp)
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(struct listen-port (number) #:prefab) ;; assertion
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(define default-port 6000)
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(spawn #:name 'connection-acceptor
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(define/query-value port default-port (listen-port $v) v)
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(assert (advertise (observe (tcp-channel _ (tcp-listener (port)) _))))
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(during (advertise (tcp-channel $c (tcp-listener (port)) _))
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(let ((accepted-port (port))) ;; capture field value at time of connect
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(assert (advertise (tcp-channel (tcp-listener accepted-port) c _)))
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(on-start
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(printf "Accepted connection from ~v on port ~a\n" c accepted-port)
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(send! (tcp-channel (tcp-listener accepted-port) c #"Hello!\n")))
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(on-stop
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(printf "Closed connection ~v on port ~a\n" c accepted-port)))))
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(spawn #:name 'configuration-provider
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(assert (listen-port 5999)))
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