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rate-limiter.js
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rate-limiter.js
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function Deferred() {
this.promise = new Promise((resolve, reject) => {
this.resolve = resolve;
this.reject = reject;
});
}
module.exports = function configureRateLimiter(options) {
if (typeof options === 'number') {
options = { concurrencyLimit: options };
}
const { concurrencyLimit, queueLimit, queueTimeout } = options;
let maxConcurrency = 0;
let concurrency = 0;
const queue = [];
function setQueueTimeout(deferred) {
if (queueTimeout !== undefined) {
setTimeout(
() => {
const index = queue.indexOf(deferred);
if (index !== -1) {
queue.splice(index, 1);
}
deferred.reject(new Error(`Request timed-out in queue after ${queueTimeout} ms.`));
},
queueTimeout
);
}
}
function tryRunNext() {
if (concurrency >= concurrencyLimit) {
return;
}
const next = queue.shift();
if (next) {
concurrency++;
if (concurrency > maxConcurrency) {
maxConcurrency = concurrency;
}
next.resolve();
}
}
function rateLimiter(asyncFunction) {
return async (...args) => {
if (queueLimit !== undefined && queue.length >= queueLimit) {
throw new Error(`Rate limiting queue has reached limit of ${queueLimit}.`);
}
const deferred = new Deferred();
setQueueTimeout(deferred);
queue.push(deferred);
const completionPromise = deferred.promise.then(async () => {
try {
return await asyncFunction(...args);
} finally {
concurrency--;
tryRunNext();
}
});
tryRunNext();
return completionPromise;
};
};
// Syntactic sugar to run a one-off task on a rate limiter.
rateLimiter.run = async (asyncFunction, ...args) => rateLimiter(asyncFunction)(...args);
// Expose read-only diagnostic info.
Object.defineProperty(rateLimiter, 'diagnostics', { get: () => ({
concurrencyLimit,
maxConcurrency,
queueLimit,
queueTimeout,
queueLength: queue.length,
})});
return rateLimiter;
};