93 lines
3.3 KiB
JavaScript
93 lines
3.3 KiB
JavaScript
//
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//
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//
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// Heartbeats. In AMQP both clients and servers may expect a heartbeat
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// frame if there is no activity on the connection for a negotiated
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// period of time. If there's no activity for two such intervals, the
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// server or client is allowed to close the connection on the
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// presumption that the other party is dead.
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//
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// The client has two jobs here: the first is to send a heartbeat
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// frame if it's not sent any frames for a while, so that the server
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// doesn't think it's dead; the second is to check periodically that
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// it's seen activity from the server, and to advise if there doesn't
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// appear to have been any for over two intervals.
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//
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// Node.JS timers are a bit unreliable, in that they endeavour only to
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// fire at some indeterminate point *after* the given time (rather
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// gives the lie to 'realtime', dunnit). Because the scheduler is just
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// an event loop, it's quite easy to delay timers indefinitely by
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// reacting to some I/O with a lot of computation.
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//
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// To mitigate this I need a bit of creative interpretation:
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//
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// - I'll schedule a server activity check for every `interval`, and
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// check just how much time has passed. It will overshoot by at
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// least a small margin; modulo missing timer deadlines, it'll
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// notice between two and three intervals after activity actually
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// stops (otherwise, at some point after two intervals).
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//
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// - Every `interval / 2` I'll check that we've sent something since
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// the last check, and if not, send a heartbeat frame. If we're
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// really too busy to even run the check for two whole heartbeat
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// intervals, there must be a lot of I (but not O, at least not on
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// the connection), or computation, in which case perhaps it's best
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// the server cuts us off anyway. Why `interval / 2`? Because the
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// edge case is that the client sent a frame just after a
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// heartbeat, which would mean I only send one after almost two
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// intervals. (NB a heartbeat counts as a send, so it'll be checked
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// at least twice before sending another)
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//
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// This design is based largely on RabbitMQ's heartbeating:
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// https://github.com/rabbitmq/rabbitmq-common/blob/master/src/rabbit_heartbeat.erl
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// %% Yes, I could apply the same 'actually passage of time' thing to
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// %% send as well as to recv.
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'use strict';
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var EventEmitter = require('events');
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// Exported so that we can mess with it in tests
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module.exports.UNITS_TO_MS = 1000;
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class Heart extends EventEmitter {
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constructor (interval, checkSend, checkRecv) {
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super();
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this.interval = interval;
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var intervalMs = interval * module.exports.UNITS_TO_MS;
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// Function#bind is my new best friend
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var beat = this.emit.bind(this, 'beat');
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var timeout = this.emit.bind(this, 'timeout');
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this.sendTimer = setInterval(
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this.runHeartbeat.bind(this, checkSend, beat), intervalMs / 2);
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// A timeout occurs if I see nothing for *two consecutive* intervals
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var recvMissed = 0;
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function missedTwo () {
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if (!checkRecv())
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return (++recvMissed < 2);
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else { recvMissed = 0; return true; }
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}
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this.recvTimer = setInterval(
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this.runHeartbeat.bind(this, missedTwo, timeout), intervalMs);
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}
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clear () {
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clearInterval(this.sendTimer);
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clearInterval(this.recvTimer);
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}
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runHeartbeat (check, fail) {
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// Have we seen activity?
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if (!check())
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fail();
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}
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}
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module.exports.Heart = Heart;
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