ts-promise-cache

Loading cache for promises. Does not suffer from thundering herds problem (aka cache stampede).

Usage

The constructor takes a loader that loads missing entries. By default rejected Promises are not kept in the cache.

loader: (key: string) => Promise

const cache = new PromiseCache<string>((key: string) => Promise.resolve("value"));
 
const value = await cache.get("key");
 
expect(value).to.eq("value");

Config

The second constructor argument is an optional config (Partial<CacheConfig> config is ok).

export class CacheConfig<T> {
    // how often to check for expired entries
    public readonly checkInterval: number | "NEVER" = "NEVER";
    // time to live (milliseconds)
    public readonly ttl: number | "FOREVER" = "FOREVER";
    // specifies that entries should be removed 'ttl' milliseconds from either when the cache was accessed (read) or when the cache value was created or replaced
    public readonly ttlAfter: "ACCESS" | "WRITE" = "ACCESS";
    // remove rejected promises?
    public readonly removeRejected: boolean = true;
    // fallback for rejected promises
    public readonly onReject: (error: Error, key: string, loader: (key: string) => Promise<T>) => Promise<T>
        = (error) => Promise.reject(error)
    // called before entries are removed because of ttl
    public readonly onRemove: (key: string, p: Promise<T>) => void
        = () => undefined
}

Using Partial<CacheConfig> it looks like:

new PromiseCache<string>(loader, {ttl: 1000, onRemove: (key) => console.log("removing", key)});

Single promise cache

In case you have a single value to cache, use singlePromiseCache factory.

// singlePromiseCache<T>(loader: () => Promise<T>, config?: Partial<CacheConfig<T>>): () => Promise<T>
 
const cache = singlePromiseCache(() => Promise.resolve("value"));
 
const value = await cache();
expect(value).to.eq("value");

TTL Expiry

The default behavior is for values to remain in the cache for 'ttl' milliseconds since the last access. It is sometimes useful for cache expiry to be relative to when the cached value was first created (or last changed).

For this situation, set ttlAfter:"WRITE":

new PromiseCache<string>(loader, {ttl: 1000, ttlAfter:"WRITE"});
 

Statistics

stats() returns statistics:

interface Stats {
   readonly misses: number;
   readonly hits: number;
   readonly entries: number;
   readonly failedLoads: number;
}

Set value

You can set a value directly

cache.set("key", "value");
expect(await cache.get("key")).to.eq("value");

Retry

Retry can by implemented by using ts-retry-promise

const loader = failsOneTime("value");
const cache = new PromiseCache<string>(() => retry(loader));
expect(await cache.get("key")).to.eq("value");