Add exponential backoff utility
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Kuldeep
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import {
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ExponentialBackoffStoppedRetriesError,
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ExponentialBackoffMaxRetriesHitError,
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ExponentialBackoffNonIntegerError,
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ExponentialBackoffNumberTooSmallError,
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ExponentialBackoffNumberOutOfBoundsError,
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ExponentialBackoffNumberNotFiniteError,
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} from './errors.ts';
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import { isWithinBounds } from '../misc.ts';
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export type ExponentialBackoffOptions = {
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/**
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* The maximum delay between attempts in milliseconds
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*/
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maxDelay: number;
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/**
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* The maximum number of attempts. Passing 0 will result in infinite attempts.
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*/
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maxAttempts: number;
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/**
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* The base delay between attempts in milliseconds
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*/
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baseDelay: number;
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/**
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* The growth rate of the delay
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*/
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growthRate: number;
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/**
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* The jitter of the delay as a percentage of growthRate. The jitter is subtracted from the delay.
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*/
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jitter: number;
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};
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/**
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* The function to call to stop the retries.
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* This mimics the AbortSignal.abort function by taking in a reason for stopping
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*
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* @param reason - The reason for stopping the retries.
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*/
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export type ExponentialBackoffStopRetriesFunction = (reason: unknown) => void;
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/**
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* The parameters for the task function
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*
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* @param stopRetries - The function to call to stop the retries
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*/
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export type ExponentialBackoffCallbackParameters = {
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stopRetries: ExponentialBackoffStopRetriesFunction;
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};
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/**
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* Options that control a single exponential-backoff run.
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*/
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export type ExponentialBackoffRunOptions = {
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/**
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* Called after each failed task attempt.
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*/
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onError?: ((error: Error) => void) | undefined;
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/**
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* Stops pending delays and prevents future attempts when aborted.
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*/
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signal?: AbortSignal | undefined;
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};
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/**
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* Options accepted by the static exponential-backoff run helper.
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*/
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export type ExponentialBackoffStaticRunOptions = Partial<ExponentialBackoffOptions> & ExponentialBackoffRunOptions;
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/**
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* Exponential backoff is a technique used to retry a function after a delay.
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*
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* The delay increases exponentially with each attempt, up to a maximum delay.
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*
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* The jitter is a random amount of time subtracted from the delay to prevent thundering herd problems.
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*
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* The growth rate is the factor by which the delay increases with each attempt.
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*/
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export class ExponentialBackoff {
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readonly #options: ExponentialBackoffOptions;
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/**
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* Creates a new exponential-backoff instance.
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*
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* Unspecified options use the defaults listed below.
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*
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* @param options - Exponential-backoff configuration overrides.
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* @param options.maxDelay - Maximum delay between retries. Default: `10_000` ms.
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* @param options.maxAttempts - Maximum number of attempts; `0` retries indefinitely. Default: `10`.
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* @param options.baseDelay - Delay used as the basis for the first retry. Default: `1_000` ms.
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* @param options.growthRate - Multiplier applied to the delay after each attempt. Default: `2`.
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* @param options.jitter - Maximum proportional reduction subtracted from each delay (0–1). Default: `0.1`.
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*
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* @throws An {@link ExponentialBackoffNumberNotFiniteError} if a provided option is not a finite number
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* @throws An {@link ExponentialBackoffNumberOutOfBoundsError} if a provided option is out of bounds
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* @throws An {@link ExponentialBackoffNumberTooSmallError} if a provided option is too small
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* @throws An {@link ExponentialBackoffNonIntegerError} if a provided option is not an integer
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*/
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constructor(options: Partial<ExponentialBackoffOptions> = {}) {
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this.#options = {
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maxDelay: 10_000,
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maxAttempts: 10,
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baseDelay: 1_000,
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growthRate: 2,
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jitter: 0.1,
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...options,
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};
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ExponentialBackoff.validateOptions(this.#options);
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}
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/**
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* Create a new ExponentialBackoff instance
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*
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* @param config - The configuration for the exponential backoff
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*
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* @throws An {@link ExponentialBackoffNumberNotFiniteError} if a provided option is not a finite number
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* @throws An {@link ExponentialBackoffNumberOutOfBoundsError} if a provided option is out of bounds
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* @throws An {@link ExponentialBackoffNumberTooSmallError} if a provided option is too small
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* @throws An {@link ExponentialBackoffNonIntegerError} if a provided option is not an integer
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*
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* @returns The ExponentialBackoff instance
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*/
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public static from(config?: Partial<ExponentialBackoffOptions>): ExponentialBackoff {
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const backoff = new ExponentialBackoff(config);
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return backoff;
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}
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/**
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* Run the function with exponential backoff
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*
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* @param taskFn - The function to run
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* @param options - Backoff configuration and options for this run
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*
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* @throws An {@link ExponentialBackoffMaxRetriesHitError} with all the errors that were thrown by the task function
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* @throws An {@link ExponentialBackoffStoppedRetriesError} if the abort signal is activated
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*
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* @returns The result of the function
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*/
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public static run<T>(
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taskFn: (callbackParameters: ExponentialBackoffCallbackParameters) => Promise<T>,
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options: Partial<ExponentialBackoffStaticRunOptions> = {},
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): Promise<T> {
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// Grab onError and signal from the options
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const { onError, signal, ...backoffOptions } = options;
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const backoff = ExponentialBackoff.from(backoffOptions);
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return backoff.run(taskFn, { onError, signal });
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}
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/**
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* Validate the options for the exponential backoff
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*
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* @param options - The options to validate
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*
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* @throws {@link ExponentialBackoffNumberNotFiniteError} if a provided option is not a finite number
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* @throws {@link ExponentialBackoffNonIntegerError} if a provided option is not an integer
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* @throws {@link ExponentialBackoffNumberOutOfBoundsError} if a provided option is out of bounds
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* @throws {@link ExponentialBackoffNumberTooSmallError} if a provided option is too small
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*/
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public static validateOptions(options: ExponentialBackoffOptions): void {
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/** Validate the value is finite, throwing an {@link ExponentialBackoffInvalidInfiniteIntegerError} if the value is infinite */
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const assertIsFinite = (key: string, value: number): void => {
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if (!Number.isFinite(value)) {
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throw new ExponentialBackoffNumberNotFiniteError(key, value);
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}
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};
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/** Validate the value is an integer, throwing a {@link ExponentialBackoffNonIntegerError} if it is not an integer */
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const assertIsInteger = (key: string, value: number): void => {
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if (!Number.isInteger(value)) {
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throw new ExponentialBackoffNonIntegerError(key, value);
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}
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};
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/** Validate the value is greater than the minimum, throwing a {@link ExponentialBackoffNumberTooSmallError} if it is not */
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const assertIsHigherThan = (key: string, value: number, min: number): void => {
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if (value < min) {
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throw new ExponentialBackoffNumberTooSmallError(key, value, min);
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}
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};
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/** Validate the value is within the bounds, throwing a {@link ExponentialBackoffNumberOutOfBoundsError} if it is not within the bounds */
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const assertIsWithinBounds = (key: string, value: number, min: number, max: number): void => {
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if (!isWithinBounds(value, min, max)) {
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throw new ExponentialBackoffNumberOutOfBoundsError(key, value, min, max);
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}
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};
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// Validate the max delay
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assertIsFinite('maxDelay', options.maxDelay);
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assertIsHigherThan('maxDelay', options.maxDelay, 0);
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// Validate the max attempts
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assertIsFinite('maxAttempts', options.maxAttempts);
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assertIsInteger('maxAttempts', options.maxAttempts);
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assertIsHigherThan('maxAttempts', options.maxAttempts, 0);
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// Validate the base delay
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assertIsFinite('baseDelay', options.baseDelay);
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assertIsHigherThan('baseDelay', options.baseDelay, 0);
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// Validate the growth rate
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assertIsFinite('growthRate', options.growthRate);
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assertIsHigherThan('growthRate', options.growthRate, 0);
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// Validate the jitter
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assertIsFinite('jitter', options.jitter);
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assertIsWithinBounds('jitter', options.jitter, 0, 1);
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}
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/**
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* Run the function with exponential backoff
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*
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* If the function fails but we have not hit the max attempts, the error will be passed to the onError callback
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* and the function will be retried with an exponential delay
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*
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* If every attempt fails, an ExponentialBackoffMaxRetriesHitError will be thrown
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* with all errors from the task function.
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*
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* @param taskFn - The function to run
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* @param options - Options that control this run
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*
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* @throws An {@link ExponentialBackoffMaxRetriesHitError} with all the errors that were thrown by the task function
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* @throws An {@link ExponentialBackoffStoppedRetriesError} if the abort signal is activated
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*
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* @returns The result of the function
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*/
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public async run<T>(
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taskFn: (callbackParameters: ExponentialBackoffCallbackParameters) => Promise<T>,
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options: ExponentialBackoffRunOptions = {},
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): Promise<T> {
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const { onError, signal: suppliedSignal } = options;
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// The task can stop its own retries without owning the signal supplied by the caller.
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const abortController = new AbortController();
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const stopRetries = abortController.abort.bind(abortController);
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// Start with the abort controller signal
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const signals = [ abortController.signal ];
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// If we received a signal from the caller, add it to the signals array
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if (suppliedSignal !== undefined) {
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signals.push(suppliedSignal);
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}
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// Compose the signals together
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const signal = AbortSignal.any(signals);
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// If the composed signal is already aborted, throw an error
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if (signal.aborted) {
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throw new ExponentialBackoffStoppedRetriesError(signal.reason);
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}
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// Initialize an empty array to store the errors
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const errors: Error[] = [];
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// Initialize the attempt counter
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let attempt = 0;
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// If the max attempts is 0, we should continue indefinitely.
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const unlimitedAttempts = this.#options.maxAttempts === 0;
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// Loop until we succeed, hit the max attempts, or the abort signal is activated
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while (true) {
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try {
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// Await the promise before returning so its execution context remains in the try-catch
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// If we didn't await, this `run` function would successfully return and any errors would not be caught here.
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return await taskFn({ stopRetries });
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} catch (error) {
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// Store the error in case we fail every attempt
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const errorInstance = error instanceof Error ? error : new Error(`${error}`);
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onError?.(errorInstance);
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// If we have unlimited attempts, don't append this to the errors array to prevent a memory leak.
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if (!unlimitedAttempts) {
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errors.push(errorInstance);
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}
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}
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// Check if the abort signal has been activated
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if (signal.aborted) {
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// Throw an error if the abort signal has been activated
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throw new ExponentialBackoffStoppedRetriesError(signal.reason);
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}
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// Calculate the count for next attempt. Do this now so we can exit before waiting and before running the next attempt.
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const nextAttemptCount = attempt + 1;
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const nextAttemptExceedsMaxAttempts = nextAttemptCount >= this.#options.maxAttempts;
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// If the next attempt exceeds the max attempts, break out of the loop
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if (!unlimitedAttempts && nextAttemptExceedsMaxAttempts) {
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break;
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}
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// Wait before going to the next attempt
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const delay = this.#calculateDelay(this.#options, attempt);
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// Wait for the delay or the abort signal
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await new Promise<void>((resolve, reject) => {
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// eslint-disable-next-line prefer-const
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let timeout: ReturnType<typeof setTimeout>;
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// Handle the abort signal
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const abortHandler = (): void => {
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clearTimeout(timeout);
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reject(new ExponentialBackoffStoppedRetriesError(signal.reason));
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};
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// Handle the timeout
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const timeoutHandler = (): void => {
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signal.removeEventListener('abort', abortHandler);
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resolve(undefined);
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};
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// Set the timeout
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timeout = setTimeout(timeoutHandler, delay);
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// Add the abort handler to the abort signal
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signal.addEventListener('abort', abortHandler, { once: true });
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});
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attempt++;
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}
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// We completed the loop without ever succeeding. Throw an ExponentialBackoffMaxRetriesHitError with all the errors we got
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throw new ExponentialBackoffMaxRetriesHitError(errors);
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}
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/**
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* Calculate the delay before we should attempt to retry
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*
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* @param options - The configuration for the exponential backoff
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* @param attempt - The current attempt number
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* @returns The time in milliseconds before another attempt should be made
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*/
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#calculateDelay(options: ExponentialBackoffOptions, attempt: number): number {
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// Get the power of the growth rate
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const power = options.growthRate ** attempt;
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// Get the delay before jitter or limit
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const rawDelay = options.baseDelay * power;
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// Cap the delay to the maximum. Do this before the jitter so jitter does not become larger than delay
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const cappedDelay = Math.min(rawDelay, options.maxDelay);
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// Get a random number for the amount to "jitter" the delay by
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const jitterAmount = Math.random();
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// Calculate the jitter
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const jitter = jitterAmount * options.jitter * cappedDelay;
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// Subtract the jitter from the delay
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return cappedDelay - jitter;
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}
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}
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