#lang imperative-syndicate

(provide heartbeat)

(require "wire-protocol.rkt")

(require/activate imperative-syndicate/drivers/timer)

(define-logger syndicate/distributed)

;; TODO: move heartbeats to transport level, and use separate transport-activity timeouts from
;; message-activity timeouts. Using message-activity only has problems when messages are large
;; and links are slow. Also, moving to transport level lets us use e.g. WebSocket's ping
;; mechanism rather than a message-level mechanism.
(define (heartbeat who send-message teardown)
  (define period (ping-interval))
  (define grace (* 3 period))

  (log-syndicate/distributed-debug
   "Peer ~v heartbeat period ~ams; must not experience silence longer than ~ams"
   who period grace)

  (field [next-ping-time 0]) ;; when we are to send the next ping
  (field [last-received-traffic (current-inexact-milliseconds)]) ;; when we last heard from the peer

  (define (schedule-next-ping!)
    (next-ping-time (+ (current-inexact-milliseconds) period)))

  (on (asserted (later-than (next-ping-time)))
      (schedule-next-ping!)
      (send-message (Ping)))

  (on (asserted (later-than (+ (last-received-traffic) grace)))
      (log-syndicate/distributed-info "Peer ~v heartbeat timeout after ~ams of inactivity"
                                      who grace)
      (teardown))

  (lambda ()
    (schedule-next-ping!)
    (last-received-traffic (current-inexact-milliseconds))))