async/retry.ts doesn't allow applying jitter to exponential backoff

[lionel-rowe]

Is your feature request related to a problem? Please describe.

It seems like one of the major benefits of exponential backoff as a retry strategy is that it can space out the retries with random jitter to prevent bottlenecks, but async/retry.ts currently doesn't implement this or provide it as an option.

Stack Overflow — Why is random jitter applied to back-off strategies?

Suppose you have multiple clients that send messages that collide. They all decide to back off. If they use the same deterministic algorithm to decide how long to wait, they will all retry at the same time -- resulting in another collision. Adding a random factor separates the retries.

Describe the solution you'd like

Add an option to apply jitter (probably just a boolean, or maybe something more fine-grained).

Possibly have this option enabled by default — would there be any disadvantages to doing that?

Describe alternatives you've considered

N/A

[lionel-rowe]

https://aws.amazon.com/blogs/architecture/exponential-backoff-and-jitter/ gives 5 examples of backoff functions, of which it concludes Decorrelated Jitter and Full Jitter are the optimal ones.

Those 5 functions could be implemented something like this:

export type BackoffFn = (
  cap: number,
  base: number,
  attempt: number,
  prev: number,
) => number;

export const backoffFns = {
  none: () => {
    return 0;
  },
  exponential: (cap, base, attempt) => {
    return Math.min(cap, base * 2 ** attempt);
  },
  fullJitter: (cap, base, attempt) => {
    return randomBetween(0, Math.min(cap, base * 2 ** attempt));
  },
  equalJitter: (cap, base, attempt) => {
    const temp = Math.min(cap, base * 2 ** attempt);
    return temp / 2 + randomBetween(0, temp / 2);
  },
  decorrelatedJitter: (cap, base, _attempt, prev) => {
    return Math.min(cap, randomBetween(base, prev * 3));
  },
} satisfies Record<string, BackoffFn>;

Of these, the current implementation used by retry is functionally equivalent to exponential.

They're all fairly simple, so providing all of them out-of-the-box and allowing consumers to supply a custom BackoffFn would allow a nice degree of flexibility.