How does the memory cache handle multiple threads entering when the cache is expired?

Summary: What are you wanting to achieve? I’m trying to figure out if my use of a semaphore is even needed.

What code or approach do you have so far?
my policy is basically:

Policy.CacheAsync<T>(cacheProvider: cacheProvider.AsyncFor<T>(),
                    ttl: TimeSpan.FromMinutes(cacheMinutes),
                    onCacheError: onCacheError);

private readonly SemaphoreSlim _usersLock; 
...
_usersLock = new SemaphoreSlim(1, 1); 
...

await _usersLock.WaitAsync(cancellationToken);
try
{                     
    var cacheResult = await _cache.ExecuteAndCaptureAsync(async (_, _) =>
                            await _masterDataApi.GetUserAccountsAsync(new UserAccountLookupDto(), cancellationToken), 
                            new Context(UsersCacheKey), 
                            cancellationToken);
    
    cacheResult.ThrowIfFailure(_masterDataApi.GetType(), nameof(_masterDataApi.GetUserAccountsAsync));
    
    return cacheResult.Result;
}
finally
{
    _usersLock.Release();
}

I did this a while back because I thought if multiple threads tried to read when the cache was expired then I would get multiple http calls. I’m pretty sure I had tested this but it was a few years ago. I was going over my code and got to wondering if this was needed at all anymore (or ever was)?

[joelhulen]

Since you are dealing with potentially multiple instances accessing the same limited resource, I suggest looking into the Distributed Circuit Breaker pattern. This sample project shows how to implement a distributed circuit breaker in Azure Functions (using Durable functions), where you use a circuit breaker id with each call by your instances to share the circuit breaker state. Otherwise, if you don’t share the circuit breaker state, then each instance only knows about its own state and not the state of other instances also trying to access the resource, as you have seen.

My caveat here is that the sample implementation I linked you to is about three years old, so you’ll want to update the SDK versions, but theoretically, it should still work. If you want to go down this path and need some help getting it to work, please let me know.

[github-actions[bot]]

This issue was closed because it has been inactive for 14 days since being marked as stale.