Thanks! I'm glad you enjoyed it.
The behavioral difference between Caching KeyValue Store and KeyValueStore is that it doesn't fetch from the underlying data source when called a second time. So that's what I would test. Given the signature you provided, I would either instrument KeyValueStore so I could tell if it had been run twice, or (if KeyValueStore has multiple implementations) I might create a ExampleKeyValueStore in my tests that had that instrumentation.
I solved this problem a bit differently in my code, though. Instead of making a CachingKeyValueStore, I made a MemoryCache. It has a get() method that takes a key and a function. The first time it's called, it runs the function; the second time, it returns the stored result. Here's one of my actual tests for that code (JavaScript):
it("caches result of function, so it's only run once", async function() {
const cache = createIndependentCache();
let runCount = 0;
const getFn = () => runCount++;
await cache.getAsync(NULL_LOG, "key", getFn);
await cache.getAsync(NULL_LOG, "key", getFn);
assert.equal(runCount, 1);
});
And in case you’re curious, here’s the implementation, excluding a bunch of instrumentation and logging:
async getAsync(log, key, getFnAsync) {
if (this._cache[key] === undefined) {
this._cache[key] = getFnAsync();
}
try {
return await this._cache[key];
}
catch (err) {
delete this._cache[key]; // don't cache errors
throw err;
}
}