Error Handling and Nullability in GraphQL Quiz Quiz

Explore key concepts of error handling and nullability in GraphQL, understanding how errors are managed and how nullable fields influence API responses. This quiz helps reinforce best practices and common scenarios for building reliable GraphQL APIs.

1. Identifying Error Propagation Rules

   When a non-nullable field in a nested GraphQL query encounters an error during resolution, what typically happens to the response structure?

   1. Only the erroneous field is set to null, but the rest of the data is returned.
   2. All parent fields up to the nearest nullable field are set to null.
   3. The entire query response is replaced with null.
   4. The error is ignored and the field is omitted from the response.

   Explanation: In GraphQL, when a non-nullable field errors, the parent objects up to the nearest nullable field are replaced with null, preserving the contract of non-nullability. The entire query is not null unless the top-level type is non-nullable and fails. Setting only the erroneous field to null violates the non-nullable guarantee. Ignoring the error or omitting the field from the response does not align with GraphQL's specifications.

2. Understanding Nullability in Schema Design

   What does marking a field as nullable in a GraphQL schema communicate to clients consuming the API?

   1. The field is deprecated and should not be used.
   2. The field may return null under certain circumstances.
   3. The field can never return null.
   4. The field is always required in queries.

   Explanation: A nullable field in GraphQL indicates that it might return null, such as in cases of errors or missing data. Marking a field nullable does not mean it can never return null—this is only true for non-nullable fields. It does not imply deprecation or necessity in queries. Nullable simply signals clients to handle cases where the value might be absent.

3. Recognizing Error Locations in Responses

   Given a GraphQL response with both 'data' and 'errors' fields present, how does this reflect GraphQL's approach to partial data delivery?

   1. GraphQL delivers partial data in 'data' and details any issues in 'errors'.
   2. GraphQL stops query execution upon the first error and returns only 'errors'.
   3. GraphQL hides all errors if any data is returned.
   4. GraphQL only includes 'errors' when no data was retrievable.

   Explanation: In GraphQL, even when errors occur, the 'data' field can include partial results alongside an 'errors' array describing issues encountered. This design enables clients to receive as much information as possible. Execution does not halt at the first error, nor are errors hidden when data exists. The 'errors' field is present even if some data could be retrieved.

4. Handling Optional Arguments in Queries

   In a schema, if a field has an optional argument with no default value, how is its absence in the query handled by GraphQL?

   1. GraphQL throws a validation error.
   2. GraphQL interprets the argument as null.
   3. GraphQL skips execution of the field.
   4. GraphQL automatically supplies a value of zero.

   Explanation: If an optional argument has no default and is omitted, GraphQL treats its value as null, allowing resolver logic to handle the missing argument. There's no automatic assignment of zero, nor is a validation error thrown unless the argument is required. Skipping field execution due to a missing optional argument is not the expected behavior.

5. Differentiating Between Error Types

   Which scenario leads to a field being null in a GraphQL response without an entry in the 'errors' array?

   1. A field threw a runtime exception during resolution.
   2. A required scalar argument was missing from the query.
   3. A field was omitted from the query.
   4. A field returns null due to business logic and no error occurred.

   Explanation: If a resolver deliberately returns null as part of normal logic and no error condition is triggered, the 'errors' array remains empty and the field's value is null. If an exception occurs, or if required arguments are missing, errors will be noted in the 'errors' array. Fields omitted from the query are not present in the response at all.