Error Handling

Data Fetcher Error Handling

Viaduct provides two extension points for customizing error handling in resolvers. Both are optional for service architects.

ResolverErrorBuilder

When a resolver throws an exception, Viaduct will catch it and return it as a GraphQL error. As a service architect, you can customize resolver exception handling by implementing your own ResolverErrorBuilder . This interface has a single method, exceptionToGraphQLError, which takes the thrown exception and constructs a GraphQLError object.

ResolverErrorBuilder is an interface with a single method, exceptionToGraphQLError. This method produces a list of GraphQLError objects.

import graphql.GraphqlErrorBuilder

class MyResolverErrorBuilder : ResolverErrorBuilder {
  override fun exceptionToGraphQLError(
    exception: Throwable,
    env: DataFetchingEnvironment,
    errorMetadata: ErrorMetadata
  ): List<GraphQLError> {
    return when (exception) {
      is MyCustomException -> listOf(
        GraphqlErrorBuilder.newError(env)
          .message("A custom error occurred: ${exception.customMessage}")
          .build()
      )
      else -> listOf(
        GraphqlErrorBuilder.newError(env)
          .message("An unexpected error occurred")
          .errorType(ErrorType.DataFetchingException)
          .build()
      )
    }
  }
}

ResolverErrorReporter

In addition to returning errors in ExecutionResult, Viaduct also provides allows you to configure an error reporter called from within the engine.

ResolverErrorReporter is an interface with a single method, reportError. This method is called whenever a resolver throws an exception and allows you to log the error or send it to an external monitoring system. This interface does not affect error reporting to clients or handling within the Viaduct engine.

For instance, if you wanted to emit exceptions to Sentry, you could implement the interface like this:

import graphql.schema.DataFetchingEnvironment
import graphql.schema.GraphQLFieldDefinition

class MyResolverErrorReporter : ResolverErrorReporter {
  override fun reportError(
    exception: Throwable,
    fieldDefinition: GraphQLFieldDefinition,
    env: DataFetchingEnvironment,
    errorMessage: String,
    errorMetadata: ErrorMetadata
  ) {
    Sentry.captureException(exception) {
      it.setExtra("fieldName", fieldDefinition.name)
      it.setExtra("parentType", fieldDefinition.type.name)
      it.setExtra("path", env.executionStepInfo.path.toString())
      it.setExtra("errorMessage", errorMessage)
      it.setExtra("errorMetadata", errorMetadata.toString())
    }
  }
}

ResolverErrorReporter provides information about the exception, allowing you to include additional context about the error in your monitoring system. The example above includes the field name, parent type, execution path and error metadata.