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Multi-Threaded Entity Framework Core Samples

Entity Framework DB contexts are not thread safe. If an ASP.NET Core app wishes to process requests in a multi-threaded way while using Entity Framework Core, it's important to carefully manage DbContexts so that a single context isn't used on multiple threads simultaneously.

This simple sample app demonstrates three techniques for safely using DbContexts in multi-threaded environments.

About the sample

The sample uses Entity Framework Core to add and remove 'book' entities to to and from a local SQL database. Because many books may be added/removed at once, the work is done in parallel by a configurable number of tasks (16, by default).

Multi-threaded EF Core approaches

The first API in BooksController (ModifyBooks) will, by design, fail because it incorrectly uses a DbContext on multiple threads simultaneously.

The three other action methods in the controller demonstrate three options for fixing such a problem. Which option is best will depend on the architecture of the app being worked on. All of the suggested fixes have some necessary drawbacks that may make them less suitable for some environments, though I have tried to list the options from most-to-least likely to be useful.

Fix 1: Use a transient DbContext

The first option (demonstrated in ModifyBooksFix1) is often the simplest. By making the DbContext's lifetime transient (in Startup.cs), individual parallel workers can create their own instances of the DbContext by simply retrieving it from the app's IServiceProvider.

Drawbacks: One potential challenge with this solution is that it means DbContexts used by an API's dependencies may not be the same instance as used by that API unless the context is explicitly passed as a parameter (since every instance of the context retrieved from DI will be unique).

Fix 2: Create DbContexts, as needed, from DI-delivered DbContextOptions

A second option (which is similar to the first one) is to create DbContexts directly (using the DbContext constructor) instead of retrieving them via dependency injection. This is possible because the DbContextOptions needed to create a DbContext can be retrieved from the dependency injection container and will cause no threading issues if reused in parallel.

Depending on the needs of the project, other (non-parallel) code paths can still retrieve DbContexts from DI or create them from options, as necessary.

Drawbacks: If DbContexts are sometimes retrieved from DI and sometimes created from options, there could be issues with dependencies sometimes sharing a context with a caller and other times not. If the DbContexts are always shared, this becomes similar to fix #1 except with explicit DbContext creation (based on DI-retrieved options).

Fix 3: Use nested service provider scopes

If it is desirable for DbContexts to continue having scoped lifetimes, it is still possible to retrieve unique DbContexts for different parallel workers by creating new service scopes for each of the workers. This will maintain scoped behavior for the DbContext (so that dependencies retrieving from the DI container will get the same instance, for example).

Drawbacks: The drawback of this approach is that creating new scopes for the parallel workers means that all scoped services (not just DbContexts) resolved by them or their dependencies will be distinct from those used by other workers or from those in the parent request scope. Depending on the project's behavior this may by undesirable.

Running the sample

To run the sample, simply launch the app (via dotnet run or f5 from VS) and use the Swagger UI to run either the broken API or any of the three fixed APIs by providing a count for the number of items to add and remove (the default value of 100 tends to be a good value in my experience).

The three fixed APIs should return 200. The broken API will fail with exceptions similar to this:

An exception occurred in the database while saving changes for context type 'SampleWebApi.Data.BookContext'.
    System.InvalidOperationException: A second operation started on this context before a previous operation completed. Any instance members are not guaranteed to be thread safe.
        at Microsoft.EntityFrameworkCore.Internal.ConcurrencyDetector.EnterCriticalSection()
        at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChangesAsync(IReadOnlyList`1 entriesToSave, CancellationToken cancellationToken)
        at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChangesAsync(Boolean acceptAllChangesOnSuccess, CancellationToken cancellationToken)
        at Microsoft.EntityFrameworkCore.DbContext.SaveChangesAsync(Boolean acceptAllChangesOnSuccess, CancellationToken cancellationToken)