Resolver Serial Execution

This project demonstrates N+1 query issues when the resolver execution strategy is set to Serial.

About the Project

Types

There are two types defined. These types are User and Post. A user has many posts and a post belongs to an author (User).

Flipping between Serial and Parallel

In Program.cs we can toggle between serial and parallel execution strategies.

Recreate N+1

Run the project with the debugger attached and a breakpoint within the UserDataLoader.GetUsersAsync method.

Now run the following query:

query {
    posts {
        author {
            id
        }
    }
}

We are asking for all posts and for each post, the author id. The expected behavior is that rather than executing a query for each post to fetch the author, we should fetch all authors in a batch.

You can observe actual behavior when the breakpoint is hit. If method is executed multiple times in one request, then there is an N+1 issue. In this event, you'll notice that when the method is called, it is passing a single id at a time.

Fix N+1

Now within Program.cs, change ExecutionStrategy.Serial to ExecutionStrategy.Parallel. Now repeat the Recreation step except that this time, you should only observe the dataloader being executed once.

Helpful Links

• HotChocolate DataLoader Documentation
• DataLoader LoadAsync Implementation