concurrent fetch with retry policy

fetch-all.ts

export async function fetchAll(urls: string[], concurrency = 5, retryPolicy = defaultRetryPolicy): Promise<Response[]> {
    const controller = new AbortController();
    const semaphore = new Semaphore(concurrency);
    const promises = urls.map(async url => {
        await semaphore.acquire();
        try {
            return await fetchWithRetry(url, retryPolicy, controller.signal);
        } catch (error) {
            controller.abort(); // abort all requests on fatal error
            throw error;
        } finally {
            semaphore.release();
        }
    });
    return Promise.all(promises);
};

async function fetchWithRetry(url: string, retryPolicy: RetryPolicy, signal: AbortSignal): Promise<Response> {
    let attempt = 0;
    const startTime = Date.now();
    while (true) {
        try {
            const response = await fetch(url, { signal });
            return response;
        } catch (error) {
            if (!(error instanceof Error)) throw new Error(`unexpected error when fetching ${url}`);
            attempt++;
            const elapsedTime = Date.now() - startTime;
            if (!retryPolicy.shouldRetry({ attempt, elapsedTime, error })) throw error;
            const delay = retryPolicy.calculateDelay({ attempt, elapsedTime, error });
            await new Promise(resolve => setTimeout(resolve, delay));
        }
    }
}

const defaultRetryPolicy: RetryPolicy = {
    shouldRetry({ elapsedTime, error }) {
        return isNetworkError(error) && elapsedTime < 5000;
    },
    calculateDelay({ attempt }) {
        const delay = Math.pow(2, attempt) * 1000; // exponential backoff
        const jitter = Math.random() * 1000; // jitter
        return delay + jitter;
    }
}

function isNetworkError(error: unknown): boolean {
    return error instanceof TypeError || error instanceof DOMException
        || (error instanceof Error && error.name !== "AbortError" && error.name !== "TypeError")
}

class Semaphore {
    private tasks: CallableFunction[] = [];
    private count: number;

    constructor(count: number) {
        this.count = count;
    }

    async acquire() {
        if (this.count > 0) {
            this.count--;
            return Promise.resolve();
        } else {
            return new Promise(resolve => {
                this.tasks.push(resolve);
            });
        }
    }

    release() {
        if (this.tasks.length > 0) {
            const next = this.tasks.shift();

            if (typeof next === "function") {
                next();
            }

            return;
        }

        this.count++;
    }
}

type RetryPolicyParams = {
    elapsedTime: number;
    error: Error;
    attempt: number;
}

interface RetryPolicy {
    shouldRetry(args: RetryPolicyParams): boolean;
    calculateDelay(args: RetryPolicyParams): number;
}