.NET Core 3.1 BackgroundService - Consume RabbitMQ and insert using EntityFrameCore

Question

I'm currently writing a background service which reads messages from a queue and inserts these values into a mssql database. I'm using entityframecore to insert the values into the context.

Now I'm having an issue with the context. I keep getting the error:

InvalidOperationException: A second operation started on this context before a previous operation completed. Any instance members are not guaranteed to be thread safe. Microsoft.EntityFrameworkCore.Internal.ConcurrencyDetector.EnterCriticalSection()

Now I know this is an issue with async, but still...

I fixed this by defining a new scope at the beginning. But I'm not sure if this is the way to go... Here is my code currently:

consumer.Received += async (ch, ea) =>
            {
                using (var scope = _scope.BeginLifetimeScope())
                {
                    var context = scope.Resolve<DatabaseContext>();
                    var content = Encoding.UTF8.GetString(ea.Body.ToArray());

                    var message = JsonConvert.DeserializeObject<RabbitMQMessageType>(content);

                    switch (message.Type)
                    {
                        case "config":
                            await HandleConfigMessage(message.Body, context);
                            break;
                        case "measurement":
                            await HandleMeasurementMessage(message.Body, context);
                            break;
                        case "log":
                            await HandleLogMessage(message.Body);
                            break;
                    }

                    _channel.BasicAck(ea.DeliveryTag, false);
                }
            };

You can see that I just pass the context to the other methods and.Add() entities. Is this the way to go?

Thanks for the help!

Kind regards

Answer 1

The Entity Framework does not support concurrent operation on a single DbContext . If you want to use multiple threads to communicate with your database then you have to create multiple DbContext instance. See MSDN
So, in short the DbContext is not thread-safe, that's why the suggested way to register your DbContext in your DI container is to use Transient lifecycle:

services.AddDbContext<YourDbContext>(options =>  
  options.UseSqlServer(Configuration.GetConnectionString(Constants.YourConnectionString)),
  ServiceLifetime.Transient);