Stale fallback

A stale fallback remembers the last successful value of each key. When the steps added before it fail, it returns that value instead of the error. This pattern is also known as stale-if-error.

import { Tenace } from '@julr/tenace'

const policy = Tenace.policy().withTimeout('1s').withStaleFallback({ maxAge: '1h' })

await policy.execute(() => getUser(id), { key: `user:${id}` })

API

policy.withStaleFallback(stale?: StaleFallback | StaleFallbackOptions)
new StaleFallback(options?: StaleFallbackOptions)

Option	Type	Default	Description
maxAge	Duration	no limit	Oldest value that may be served.
maxEntries	number	1000	Keys remembered. The least recently written key is forgotten first.
key	string	the execution key	The key values are remembered under. A key is required, from this option or from the execution.

Behavior

When neither the stale fallback nor the execution has a key, the execution rejects with a TypeError.

Each success stores its value under the key, with the time on the execution clock. A failure returns the stored value when there is one and it is at most maxAge old. Otherwise the failure goes through. A stale fallback does not serve a value when the signal is aborted.

The values live in the StaleFallback instance, in memory. Share the instance to share the values between policies.

Error channel

Unchanged. A failure with no stored value still fails, so the errors stay in the channel.

Events

Event	Fields	When
fallback.stale	key, ageMs	A stored value replaces an error.