.net core InMemoryCache is throwing error

Question

I am using InMemoryCache like below. I did not configure through startup.cs. The problem here is it is throwing the following error in the hosted docker image, not facing this error locally.

[Error] ConnectionId:0 RequestPath:/BuildSpec/RetrieveFor RequestId:1:0, SpanId:|aaaaa-ccccccc., a-415ea81b37b05aaa4f3, ParentId: Controller.RetrieveFor (core.web) => Microsoft.EntityFrameworkCore.Query: An exception occurred while iterating over the results of a query for context type 'act.core.data.ActDbContext'.
System.InvalidOperationException: A second operation started on this context before a previous operation completed. This is usually caused by different threads using the same instance of DbContext. For more information on how to avoid threading issues with DbContext, see https://go.microsoft.com/fwlink/?linkid=2097913.
   at Microsoft.EntityFrameworkCore.Internal.ConcurrencyDetector.EnterCriticalSection()

But we inject DB context into the constructor. Pasting the following code snippet I am using here.

 private static readonly MemoryCache _cache = new MemoryCache(new MemoryCacheOptions
        {
            SizeLimit = 10
        });

        private async Task<IQueryable<SoftwareComponent>> GetOrCreateSoftwareComponent()
        {
            const string key = "xyz";
            if (!_cache.TryGetValue(key, out IQueryable<xyz> xyz))
            {
                softwareComponents = await Task.Run(() => _ctx.xyztable.AsNoTracking()
                    .Include(a => a.SoftwareComponentEnvironments).AsNoTracking());

                var cacheEntryOptions = new MemoryCacheEntryOptions()
                    .SetPriority(CacheItemPriority.High)
                    .SetSize(1)
                    .SetSlidingExpiration(TimeSpan.FromMinutes(30))
                    .SetAbsoluteExpiration(TimeSpan.FromMinutes(60));

                _cache.Set(key, xyz, cacheEntryOptions);
            }
            return softwareComponents;
        }

The problem here is, I don't get the error here. Why it is MySQL error. Note: If I redeploy the same code the issue resolves for someday. But error comes back again and again.

Answer 1

You are getting the error at this line here:

 softwareComponents = await Task.Run(() => _ctx.xyztable.AsNoTracking()
                    .Include(a => a.SoftwareComponentEnvironments).AsNoTracking());

Task.Run > Queues the specified work to run on the ThreadPool and returns a task or Task handle for that work.

That means that the delegate in there, will run on a separate thread.

So if you request some other cache data (or even the same, before the first one is done), then yet another thread will start and use the same context. This is not allowed in EF, thus you get the error.

The simple workaround is to use a lock

        private readonly object lockObject = new object();
        private async Task<IQueryable<SoftwareComponent>> GetOrCreateSoftwareComponent()
        {
          const string key = "xyz";
          lock (lockObject)
          {
            if (!_cache.TryGetValue(key, out IQueryable<xyz> xyz))
            {
                softwareComponents = await Task.Run(() => _ctx.xyztable.AsNoTracking()
                    .Include(a => a.SoftwareComponentEnvironments).AsNoTracking());

                var cacheEntryOptions = new MemoryCacheEntryOptions()
                    .SetPriority(CacheItemPriority.High)
                    .SetSize(1)
                    .SetSlidingExpiration(TimeSpan.FromMinutes(30))
                    .SetAbsoluteExpiration(TimeSpan.FromMinutes(60));

                _cache.Set(key, xyz, cacheEntryOptions);
            }
          }
          return softwareComponents;
        }

You could provide a new context every time, but without some kind of synchronization, you'd probably make multiple queries for the same value.