DataLoader used by `suspend` function fails to complete

Library Version

1.4.2

Describe the bug

When I use a BatchLoader to resolve a property defined as a suspended function the request will complete maybe once or twice, and then subsequently fail to complete. The Dataloader body

If I define the property as a function that returns a CompletableFuture it returns successfully every time.

This may be expected behavior. If so I’d like to update the example code.

To Reproduce

1. Check out the following branch https://github.com/rharriso/graphql-kotlin/tree/demostrate-batch-issue
2. Run the spark example
3. Request the suspended property multiple times:

query {
  searchCourses(params: { ids: [1,2,3] }) {
    university {
      id
      name
    }
  }
}

1. See complete once or twice and fail afterwards.
2. Query the property supplied by CompletableFuture

query {
  searchCourses(params: { ids: [1,2,3] }) {
    universityFuture {
      id
      name
    }
  }
}

1. This query should always succeed.

Expected behavior

Not sure. If this is a bad usage of a batch loader I’d like to know.

[DrMetallius]

I don’t see the benefit of declaring using suspend functions with DataLoader as this just caches the response to be used at a later time.

The example provided by the original poster is contrived indeed, but the benefit here is simple: I get some result from another service through a loader, then I use it differently in different resolvers. Obviously, I don’t want to call said external server multiple times in the same query, that’s why I need some mechanism for batching and caching.

I can’t just move the logic from the resolvers into a data loader. I have one external service, but the result is used differently by different resolvers, so that would result in different data loaders thus defeating the purpose. The only way of making this work that I can see is by abandoning coroutines altogether and transforming the CompletableFuture from the data loader via thenApplyAsync or something similar. Obviously, this doesn’t exactly thrill me as it’s neither consistent, nor convenient.

[rharriso]

if DataLoader expects a CompletableFuture, you will have to return that. I don’t think there is a way for them to support co-routines since the library is implemented with Java/

My use-case makes a network request to another service over gRPC.

I’ll update the spark example to include asynchronous example.