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89 lines
3.4 KiB
89 lines
3.4 KiB
// |
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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-server/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 inherits = require('util').inherits; |
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var EventEmitter = require('events').EventEmitter; |
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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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function Heart(interval, checkSend, checkRecv) { |
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EventEmitter.call(this); |
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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()) 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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inherits(Heart, EventEmitter); |
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module.exports.Heart = Heart; |
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Heart.prototype.clear = function() { |
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clearInterval(this.sendTimer); |
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clearInterval(this.recvTimer); |
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}; |
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Heart.prototype.runHeartbeat = function(check, fail) { |
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// Have we seen activity? |
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if (!check()) fail(); |
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};
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