utils

package main

import (
	"context"
	"fmt"
	"time"
)

type ExampleItemStatus string

const (
	ExampleStatusNotStarted ExampleItemStatus = "Not-started"
	ExampleStatusStarted    ExampleItemStatus = "Started"
	ExampleStatusSucceeded  ExampleItemStatus = "Completed"
)

type ExampleCacheItem struct {
	status ExampleItemStatus
	id     string
}

func (e *ExampleCacheItem) ID() string {
	return e.id
}

type ExampleService struct {
	jobStatus map[string]ExampleItemStatus
}

func newExampleService() *ExampleService {
	return &ExampleService{jobStatus: make(map[string]ExampleItemStatus)}
}

// advance the status to next, and return
func (f *ExampleService) getStatus(id string) *ExampleCacheItem {
	if _, ok := f.jobStatus[id]; !ok {
		f.jobStatus[id] = ExampleStatusStarted
	}
	f.jobStatus[id] = ExampleStatusSucceeded
	return &ExampleCacheItem{f.jobStatus[id], id}
}

func main() {
	// This auto-refresh cache can be used for cases where keys are created by caller but processed by
	// an external service and we want to asynchronously keep track of its progress.
	exampleService := newExampleService()

	// define a sync method that the cache can use to auto-refresh in background
	syncItemCb := func(ctx context.Context, obj CacheItem) (CacheItem, error) {
		return exampleService.getStatus(obj.(*ExampleCacheItem).ID()), nil
	}

	// define resync period as time duration we want cache to refresh. We can go as low as we want but cache
	// would still be constrained by time it takes to run Sync call for each item.
	resyncPeriod := time.Millisecond

	// Since number of items in the cache is dynamic, rate limiter is our knob to control resources we spend on
	// sync.
	rateLimiter := NewRateLimiter("ExampleRateLimiter", 10000, 1)

	// since cache refreshes itself asynchronously, it may not notice that an object has been deleted immediately,
	// so users of the cache should have the delete logic aware of this shortcoming (eg. not-exists may be a valid
	// error during removal if based on status in cache).
	cache := NewAutoRefreshCache(syncItemCb, rateLimiter, resyncPeriod)

	// start the cache with a context that would be to stop the cache by cancelling the context
	ctx, cancel := context.WithCancel(context.Background())
	cache.Start(ctx)

	// creating objects that go through a couple of state transitions to reach the final state.
	item1 := &ExampleCacheItem{status: ExampleStatusNotStarted, id: "item1"}
	item2 := &ExampleCacheItem{status: ExampleStatusNotStarted, id: "item2"}
	_, err1 := cache.GetOrCreate(item1)
	_, err2 := cache.GetOrCreate(item2)
	if err1 != nil || err2 != nil {
		fmt.Printf("unexpected error in create; err1: %v, err2: %v", err1, err2)
	}

	// wait for the cache to go through a few refresh cycles and then check status
	time.Sleep(resyncPeriod * 10)
	fmt.Printf("Current status for item1 is %v", cache.Get(item1.ID()).(*ExampleCacheItem).status)

	// stop the cache
	cancel()

}

Output:

Current status for item1 is Completed