Following reading describes implementation details of query execution process. It may clarify some internals of GraphQL runtime but is not required to use it.
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There are 3 types of errors in GraphQL:
- Syntax: query has invalid syntax and could not be parsed;
- Validation: query is incompatible with type system (e.g. unknown field is requested);
- Execution: occurs when some field resolver throws (or returns unexpected value).
Obviously, when Syntax or Validation error is detected - the process is interrupted and the query is not executed.
Execution process never throws exceptions. Instead, all errors are caught and collected in execution result.
GraphQL is forgiving to Execution errors which occur in resolvers of nullable fields. If such field throws or returns unexpected value the value of the field in response will be simply replaced with null and error entry will be registered.
If an exception is thrown in the non-null field - error bubbles up to the first nullable field.
This nullable field is replaced with null and error entry is added to the result.
If all fields up to the root are non-null - data entry will be removed from the result
and only errors key will be presented.