IdentityServer's ICache<T> interface has superfluous functionality, which makes it hard to implement.

[eugene-rebedailo]

I was trying to implement MS's hybrid cache for identity server, because I have an API attached to it that manages server's configuration in an environment with a lot of distributed services. I am trying to implement resource cache invalidation upon updates to said resources, so way to do this this would be to replace IdentityServer's stock implementation of ICache<T> for certain resources.

The best fit for this in my scenario would be to use Microsoft's Hybrid Cache because it supports cache invalidation by tags (which is desirable in my case), but the issue is that it doesn't expose Get by itself, only GetOrCreate, which is arguably a more semantically correct way to use the cache. Looking at the sources, it doesn't seem too daunting to remove this method in favour of GetOrCreate.

Is the Get method really necessary in this interface?

[wcabus]

Removing ICache<T>.GetAsync(string key) from our interface would be a breaking change, so in that sense, it is indeed necessary to keep it around. This method is also being used in the CachingResourceStore in IdentityServer.

You can implement GetAsync using hybrid cache by calling GetOrCreateAsync, but disabling a write to both local/distributed cache on a miss:

public async Task<T?> GetAsync(string key)
{
    return await cache.GetOrCreateAsync(key, _ => ValueTask.FromResult(default(T?)),
        new HybridCacheEntryOptions
        {
            Flags = HybridCacheEntryFlags.DisableLocalCacheWrite |
                    HybridCacheEntryFlags.DisableDistributedCacheWrite
        });
}

[eugene-rebedailo]

Thanks, will try that.

[damianh]

We've moved to HybridCache for Duende.AccessTokenManagement and will do for Duende.AspNetCore.Authentication.OAuth2Introspection. It's likely we'll consider same for a future version of IdentityServer (but that could be a breaking change etc).