Refer to https://twitter.com/tvii/status/1231867362967683072

app-svc-db-cache-prometheus.go

// pkg/svc
type SettingsService struct {
	db DB // DB is an interface
}

func (svc *SettingsService) IsProCustomer(token Token) bool {
	v := svc.db.GetProCustomer(makeProPKFromToken(token))
	return v == "1"
}


// pkg/db/kv
type KV struct {
	remoteDB RemoteDBaaSClient
	cache 	 Cache
	pubsub   PubSub // pubsub invalidates local cache on set operations
}

// KV implements DB interface
var _ DB = (*KV)(nil)

func (db *KV) GetProCustomer(pk) string {
	if v, ok := db.cache.Get(···); ok {
		return v
	}
	db.remoteDB.Get(···)
	/* get foo via DBaaS client, put value into cache, return value */
}


// pkg/pubsub
type PubSub struct {···}


// vendor/cache
type Cache struct {
	getOpDuration prometheus.Histogram // cache_get_duration_seconds
}

func (cache *Cache) Get(k string) (string, bool) {
	defer func(t time.Time) {
		getOpDuration.WithLabelValues(···).Observe(time.Since(t))
	}(time.Now())
	/* ··· */
}

// How to re-implement it so we could filter cache's metrics by "svc" or by "db implementation".
// I.e. to have the "<host>.<application>.settings-svc.kv.cache.get.duration-seconds" metrics.