Lock-per-key functionality when using custom loader functions

[lukasbindreiter]

Hi,

I was skimming through the code a little bit, trying to figure out if there is support for a use case for locking just a specific key in the cache while a custom loader (thats slow, like e.g. making an HTTP request) is running.

For example, imaging something like this:

websiteLoader := ttlcache.LoaderFunc[string, string](
	func(c *ttlcache.Cache[string, string], key string) *ttlcache.Item[string, string] {
		// load website content
                content := ...  // make http request to fetch url=key
		item := c.Set(key, content)
		return item
	},
)
websiteCache := ttlcache.New[string, string](
	ttlcache.WithLoader[string, string](loader),
)

Now imagine I have three goroutines called at the exact same time:

// goroutine1
content := websiteCache.Get("https://www.some.site")
// goroutine2
content := websiteCache.Get("https://www.other.site")
// goroutine3
content := websiteCache.Get("https://www.some.site")

Is there a way to add a lock-per-cache-key, e.g. to make goroutine3 block while goroutine1 is still running, but let goroutine2 continue as normal? From what I saw in the code, I think not, right?

There is a lockAndLoad parameter here, but if I understood this correctly it locks the whole cache, so in the above example goroutine2 would block while goroutine1 is running, right?

Do you think this would be a feature worth adding?
I was thinking about implementing a wrapper around ttlcache that supports something like that, but then I thought this could be a potential useful feature in general?

Of course one thing about this is that error handling is maybe a bit unclear then, so I also see an argument that this is already too much magic happening by the cache, instead of letting the user implement it, but I would be interested in opinions in any case.

[swithek]

That's an interesting idea, but I'm not sure how big the scope of this issue would be, because:

• If method calls can block each other for an unspecified amount of time (e.g., the loader might be loading something from a db), each of these calls should be able to be cancelled (a context would have to be used). There's an issue for context-related functionality already here: Add GetContext() method and LoaderContext interface #120
• Should this be implemented at the cache level or perhaps there could be a loader wrapper, like SuppressedLoader, that would handle per-key locking? At the moment, the latter would make more sense.

[lukasbindreiter]

I like the suggestion of the GetContext api in the issue you linked, I think it could make a lot of sense here as well.

GetContext(context.Context, key K, opts... Options) (Item, error)

This way we can pass in a context (for cancellation) and we can also get back potential errors by the loader.

If I adapt my loader a little bit to something like the LoaderContext proposed in #120 it could look like this, right?

websiteLoader := ttlcache.LoaderContext[string, string](
	func(c *ttlcache.Cache[string, string], ctx context.Context, key string) (*ttlcache.Item[string, string], error) {
		// load website content
                content, err := ...  // make http request to fetch url=key
                if err != nil {
                    return nil, err
                }
		item := c.Set(key, content)
		return item
	},
)

But this is now actually a separate from this issue (lock-per-key) but that could become a separate option then, e.g.

websiteCache := ttlcache.New[string, string](
	ttlcache.WithLoaderContext[string, string](websiteLoader),
        ttlcache.WithLockAndLoad(),  // a new option
)

Then the usage could look like this:

ctx := context.Background()

// goroutine1
content, err := websiteCache.GetContext(ctx, "https://www.some.site")
// goroutine2
content, err := websiteCache.GetContext(ctx, "https://www.other.site")
// goroutine3
content, err := websiteCache.GetContext(ctx, "https://www.some.site")

Alternatively, instead of a new option such as ttlcache.WithLockAndLoad a wrapped loader could also be used of course.
What do you think, would an API like this make sense? (It would at the very least solve our exact use-case at hand 😄 )