error handling a custom ClientException from a Dataloader

[tatianajiselle]

It is my understanding that dataloaders run after the datafetchers return and outside of the normal GQL execution pipeline. This is causing our service's error handling to be skipped, an implementation of DgsDataFetcherExceptionHandler.

sudo code:

DataLoader {

load() {
   return completedFuture( service.apiCall(key) );
}

When the API call fails for say some 4xx, I am struggling to throw a custom GraphQLError type from the datafetcher as i cannot access the error from inside the DataFetcherExceptionHandlerParameters (it doesnt exist there of course).

There is no documentation about this cause and effect in apollo or DGS, and im wondering what the recommended way to solve this is.

Thanks in advance.

[tatianajiselle]

i figured it out and wanted to document an example for future people who might need support:

Overall:

• i had to create a DataLoaderResult which has a success/error, set this using a try catch in your dataloader.load function
• implement a mapping of my throwable errors to a graphql error
• return the success mapped into data or error map into error inside a DataFetcherResult from within the child resolver, checking if error then return error DataFetcherResult

Note that you will need to implement your own createGraphQLError() from your throwable, there are examples of this in the apollo docs

public class DataLoaderResult<T> {
    private final T data;
    private final Throwable error;

    /***
     * Set your data loader result.data
     * @param data the data you want to set returned from the dataloader
     * @return a data loader result object containing resolved data
     * @param <T> any type of object returned from your dataloader
     */
    public static <T> DataLoaderResult<T> setData(T data) {
        return DataLoaderResult.<T>builder().data(data).build();
    }

    /***
     * Set your data loader result.error
     * @param error the throwable error you got from making your api call
     * @return a data loader result object containing resolved error
     * @param <T> any type of object returned from your dataloader
     */
    public static <T> DataLoaderResult<T> setError(Throwable error) {
        return DataLoaderResult.<T>builder().error(error).build();
    }

    public boolean hasError() {
        return error != null;
    }

    /***
     * Use this function to return an error result on a datafetcher when an error occurred
     * @param result pass in the result of the dataloader call from your resolver
     * @return the dataloader error result to be returned in the resolver to the graphql client
     * @param <T> takes in any type to be reusable across any code base
     */
    public static <T> DataFetcherResult<T> createErrorResult(DataLoaderResult<?> result) {
        return DataFetcherResult.<T>newResult()
                .error(createGraphQLError(result.getError()))
                .build();
    }

    /***
     * Use this function to return a data result on a datafetcher when it successfully retrieved data
     * @param data pass in the result of the dataloader call from your resolver
     * @return the dataloader response data result to be returned in the resolver to the graphql client
     * @param <T> takes in any type to be reusable across any code base
     */
    public static <T> DataFetcherResult<T> createSuccessResult(T data) {
        return DataFetcherResult.<T>newResult().data(data).build();
    }
}