76 lines
2.8 KiB
Racket
76 lines
2.8 KiB
Racket
;;; SPDX-License-Identifier: LGPL-3.0-or-later
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;;; SPDX-FileCopyrightText: Copyright © 2010-2021 Tony Garnock-Jones <tonyg@leastfixedpoint.com>
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#lang syndicate
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(provide (all-defined-out))
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(require "../functional-queue.rkt")
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(define-logger syndicate/protocol/credit)
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;; (credit* Any (U Number 'reset))
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;; (credit Any ... (U Number 'reset))
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;;
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;; Send this message to issue `amount` units of credit (in the context
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;; of credit-based flow control) to the given `context`.
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;;
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;; A `context` may identify any essentially asynchronous stream where
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;; either the possibility of overwhelming a consumer exists, or the
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;; need for occasionally changing the settings of a producer in an
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;; atomic way exists. For example, reading HTTP headers proceeds
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;; line-by-line until the body is reached, at which point it proceeds
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;; byte-by-byte.
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;;
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;; The `amount` may either be a number or `'reset`, which should zero
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;; out (discard) any available credit. In particular, it may be
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;; `+inf.0`, effectively turning credit-based flow control off for the
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;; named context.
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;;
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;; See also https://eighty-twenty.org/2011/05/15/origins-of-ack-and-flow-control.
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;;
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(message-struct credit* (context amount))
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(define-match-expander credit
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(syntax-rules () [(_ context ... amount) (credit* (list context ...) amount)])
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(syntax-rules () [(_ context ... amount) (credit* (list context ...) amount)]))
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(define (issue-credit! #:amount [amount 1] . context)
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(send! (credit* context amount)))
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(define (issue-unbounded-credit! . context)
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(send! (credit* context +inf.0)))
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(define (make-flow-controlled-sender . context)
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(make-flow-controlled-sender* context))
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(define (make-flow-controlled-sender* context)
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(field [q (make-queue)]
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[item-credit 0])
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(when (log-level? syndicate/protocol/credit-logger 'debug)
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(begin/dataflow
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(log-syndicate/protocol/credit-debug
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"context ~a, queue length ~a, credit ~a"
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context
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(queue-length (q))
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(item-credit))))
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(begin/dataflow
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(when (and (positive? (item-credit))
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(not (queue-empty? (q))))
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(define-values (item new-q) (dequeue (q)))
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(send! item)
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(q new-q)
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(item-credit (- (item-credit) 1))))
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(on (message (credit* context $amount))
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(item-credit (if (eq? amount 'reset) 0 (+ (item-credit) amount))))
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(lambda (item) (q (enqueue (q) item))))
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;; It's quite possible that credit-based flow control is not the right
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;; approach for Syndicate. Using assertions that describe the content
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;; of a stream more relationally ought to allow "replay" of
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;; information in different contexts; though the trade-off is not only
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;; reduced performance, but a need to garbage-collect
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;; no-longer-interesting portions of the stream; that is,
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;; *acknowledgements*. In a reliable-delivery context, it would appear
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;; that at least one of acks or flow-control is required! (?!?)
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