Callback for cache access

I’m trying to add Prometheus metrics around my cache use. The metrics I’d like are:

• count of accesses
• count of hits
• count of evictions

Hit rate can then be hits / accesses, and eviction rate evictions / accesses.

This is what I have now;

// ... init metrics counters

return memoizee(myFunction, {
  max: 250,
  dispose() {
    evictionCounter.inc()
  },
})

Having another callback, in addition to dispose which gets called after every access and has (hit: boolean) as the parameter would be really useful.

For example:

function onAccess(hit: boolean) {
  accessCounter.inc()
  if (hit) {
    hitCounter.inc()
  }
}

[ThomWright]

That solution seems to work, thanks. For reference, here is what I did:

const memoizeeExtensions = require("memoizee/lib/registered-extensions")
memoizeeExtensions.metrics = function(
  _: unknown,
  conf: EventEmitter,
  options: memoizee.Options<Parse>,
) {
  const postfix =
    (options.async && memoizeeExtensions.async) ||
    (options.promise && memoizeeExtensions.promise)
      ? "async"
      : ""

  conf.on("set" + postfix, (id: string) => {
    queryCounter.inc()
  })
  conf.on("get" + postfix, (id: string) => {
    queryCounter.inc()
    hitsCounter.inc()
  })
  conf.on("delete" + postfix, (id: string) => {
    evictionCounter.inc()
  })
}

return memoizee(parse, {
  primitive: true,
  max: 250,
  metrics: true,
})

To get the types to work I also did this:

// typings/memoizee/index.d.ts
import "memoizee"

declare module "memoizee" {
  export interface Options<F extends (...args: any[]) => any> {
    // To turn on our own extension
    metrics?: boolean
  }
}

[ThomWright]

Great, thank you. I’ll have a look at the extensions.