CODEWITHSUNDEEP
async-awaitblazor-server-sidememorycache
David Thielen
David Thielen

Reputation: 33006

How can I call the DB once, and have the other calls wait on the cache

I have my Blazor server app set up where I cache the AppUser object for the logged in user. It's a pretty heavy read from the DB with 11 queries (using SplitQueries). But once I have it, it then gives me 90% of what I need for every page. I figure that's a decent trade-off.

The problem is, for most pages I have several components that need this object. So all of them, in their OnInitializedAsync(), call the service that caches this object. So 4 calls come in for the object. All 4 find it is not in the cache and go read it from the DB, and then all 4 place it in the cache. And a ½ second delay to show the first page becomes a 2 second delay.

How can I get around this? The best I've come up with is I have a dictionary of pending queries and a mutex or event. If there's a request for an AppUser in the pending dictionary, then the request blocks on the mutex/event and when free-ed, then starts over looking in the cache, then reading if required (it shouldn't be).

Is there another approach to this? And if the pending - mutex is the best way to do this, is there anything I have to watch out for?

Upvotes: 1

Views: 39

Answers (2)

David Thielen
David Thielen

Reputation: 33006

I discovered a very easy solution to this problem - LazyCache. Designed to solve this specific issue. (Not an endorsement! Just pointing out this solves the issue.)

Upvotes: 0

MrC aka Shaun Curtis
MrC aka Shaun Curtis

Reputation: 30330

You should be able to do something like this with Scoped services:

public class MyService
{
    private Task? _loadingTask;

    // Can kick off the load in the Ctor if you wish
    public MyService()
    {
        _loadingTask = this.Initialize();
    }

    public Task LoadData()
    {
        // If _loadingTask is null then we need to start the loading process
        if (_loadingTask == null)
        {
            _loadingTask = this.Initialize();
            return _loadingTask;
        }

        if (_loadingTask.IsFaulted)
        {
            // Add code to deal with exceptions
        }

        return _loadingTask;
    }

    public async Task GetData()
    {
        await LoadData();
        // return whatever data you want;
    }

    private async Task Initialize()
    {
        // Fake an async operation
        await Task.Delay(1000);
    }
}

Upvotes: 0

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