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Copy pathconnection-loop.ts
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269 lines (229 loc) · 7.39 KB
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import type {Logger} from './logger.js';
import {resolver} from './resolver.js';
import {sleep} from './sleep.js';
export const DEBOUNCE_DELAY_MS = 10;
export const MIN_DELAY_MS = 30;
export const MAX_DELAY_MS = 60_000;
// Max numbers of outstanding connections. This is pretty arbitrary but assuming
// a request takes 100ms then we get a response every 33ms which is 30 FPS.
export const MAX_CONNECTIONS = 3;
type SendRecord = {duration: number; ok: boolean};
export interface ConnectionLoopDelegate extends Logger {
invokeSend(): Promise<boolean>;
debounceDelay: number;
// If null, no watchdog timer is used.
watchdogTimer: number | null;
maxConnections: number;
maxDelayMs: number;
minDelayMs: number;
}
export class ConnectionLoop {
// ConnectionLoop runs a loop sending network requests (either pushes or
// pulls) to the server. Our goal, generally, is to send requests as fast as
// we can, but to adjust in case of slowness, network errors, etc. We will
// send requests in parallel if the server supports it. We also debounce
// pushes since they frequently happen in series very near to one another
// (e.g., during drag'n drops).
//
// The loop flows through the following states forever, until it is closed:
//
// Pending: Wait for event or watchdog
// |
// v
// Debounce: Wait for more events (we debounce pushes)
// |
// v
// Wait for available connection (we limit number of parallel requests
// allowed)
// |
// v
// Wait to send (if requests are taking too long, we will slow down)
// |
// v
// Send (asynchronously, wrt the loop)
// |
// v
// Back to the pending!
// Controls whether the next iteration of the loop will wait at the pending
// state.
private _pendingResolver = resolver<void>();
private readonly _delegate: ConnectionLoopDelegate;
private _closed = false;
constructor(delegate: ConnectionLoopDelegate) {
this._delegate = delegate;
this.run();
}
close(): void {
this._closed = true;
}
send(): void {
this._delegate.debug?.('send');
this._pendingResolver.resolve();
}
async run(): Promise<void> {
const sendRecords: SendRecord[] = [];
let recoverResolver = resolver();
let lastSendTime;
// The number of active connections.
let counter = 0;
const delegate = this._delegate;
const {debug} = delegate;
let delay = 0;
debug?.('Starting connection loop');
while (!this._closed) {
debug?.(
didLastSendRequestFail(sendRecords)
? 'Last request failed. Trying again'
: 'Waiting for a send',
);
// Wait until send is called or until the watchdog timer fires.
const races = [this._pendingResolver.promise];
const t = delegate.watchdogTimer;
if (t !== null) {
races.push(sleep(t));
}
await Promise.race(races);
if (this._closed) break;
debug?.('Waiting for debounce');
await sleep(delegate.debounceDelay);
if (this._closed) break;
debug?.('debounced');
// This resolver is used to wait for incoming push calls.
this._pendingResolver = resolver();
if (counter >= delegate.maxConnections) {
debug?.('Too many request in flight. Waiting until one finishes...');
await this._waitUntilAvailableConnection();
if (this._closed) break;
debug?.('...finished');
}
// We need to delay the next request even if there are no active requests
// in case of error.
if (counter > 0 || didLastSendRequestFail(sendRecords)) {
delay = computeDelayAndUpdateDurations(delay, delegate, sendRecords);
debug?.(
didLastSendRequestFail(sendRecords)
? 'Last connection errored. Sleeping for'
: 'More than one outstanding connection (' +
counter +
'). Sleeping for',
delay,
'ms',
);
}
const clampedDelay = Math.min(
delegate.maxDelayMs,
Math.max(delegate.minDelayMs, delay),
);
if (lastSendTime !== undefined) {
const timeSinceLastSend = Date.now() - lastSendTime;
if (clampedDelay > timeSinceLastSend) {
await Promise.race([
sleep(clampedDelay - timeSinceLastSend),
recoverResolver.promise,
]);
if (this._closed) break;
}
}
counter++;
(async () => {
const start = Date.now();
let ok: boolean;
try {
lastSendTime = start;
debug?.('Sending request');
ok = await delegate.invokeSend();
debug?.('Send returned', ok);
} catch (e) {
debug?.('Send failed', e);
ok = false;
}
if (this._closed) {
debug?.('Closed after invokeSend');
return;
}
debug?.('Request done', {duration: Date.now() - start, ok});
sendRecords.push({duration: Date.now() - start, ok});
if (recovered(sendRecords)) {
recoverResolver.resolve();
recoverResolver = resolver();
}
counter--;
this._connectionAvailable();
if (!ok) {
// Keep trying
this._pendingResolver.resolve();
}
})();
}
}
private _waitingConnectionResolve: (() => void) | undefined = undefined;
private _connectionAvailable() {
if (this._waitingConnectionResolve) {
const resolve = this._waitingConnectionResolve;
this._waitingConnectionResolve = undefined;
resolve();
}
}
private _waitUntilAvailableConnection() {
const {promise, resolve} = resolver();
this._waitingConnectionResolve = resolve;
return promise;
}
}
// Number of connections to remember when computing the new delay.
const CONNECTION_MEMORY_COUNT = 9;
// Computes a new delay based on the previous requests. We use the median of the
// previous successfull request divided by `maxConnections`. When we get errors
// we do exponential backoff. As soon as we recover from an error we reset back
// to MIN_DELAY_MS.
function computeDelayAndUpdateDurations(
delay: number,
delegate: ConnectionLoopDelegate,
sendRecords: SendRecord[],
): number {
const {length} = sendRecords;
if (length === 0) {
return delay;
}
const {duration, ok} = sendRecords[sendRecords.length - 1];
if (!ok) {
return delay * 2;
}
const {maxConnections, minDelayMs} = delegate;
if (length === 1) {
return (duration / maxConnections) | 0;
}
// length > 1
const previous: SendRecord = sendRecords[sendRecords.length - 2];
// Prune
while (sendRecords.length > CONNECTION_MEMORY_COUNT) {
sendRecords.shift();
}
if (ok && !previous.ok) {
// Recovered
return minDelayMs;
}
const med = median(
sendRecords.filter(({ok}) => ok).map(({duration}) => duration),
);
return (med / maxConnections) | 0;
}
function median(values: number[]) {
values.sort();
const {length} = values;
const half = length >> 1;
if (length % 2 === 1) {
return values[half];
}
return (values[half - 1] + values[half]) / 2;
}
function didLastSendRequestFail(sendRecords: SendRecord[]) {
return sendRecords.length > 0 && !sendRecords[sendRecords.length - 1].ok;
}
function recovered(sendRecords: SendRecord[]) {
return (
sendRecords.length > 1 &&
!sendRecords[sendRecords.length - 2].ok &&
sendRecords[sendRecords.length - 1].ok
);
}