graphql-php/docs/error-handling.md

Errors in GraphQL

Query execution process never throws exceptions. Instead all errors that occur during query execution are caught, collected and included in response.

There are 3 types of errors in GraphQL (Syntax, Validation and Execution errors):

Syntax errors are returned in response when query has invalid syntax and could not be parsed. Example output for invalid query {hello (missing bracket):

[
    'errors' => [
        [
            'message' => "Syntax Error GraphQL request (1:7) Expected Name, found <EOF>\n\n1: {hello\n         ^\n",
            'locations' => [
                ['line' => 1, 'column' => 7]
            ]
        ]
    ]
]

Validation errors - returned in response when query has semantic errors. Example output for invalid query {unknownField}:

[
    'errors' => [
        [
            'message' => 'Cannot query field "unknownField" on type "Query".',
            'locations' => [
                ['line' => 1, 'column' => 2]
            ]
        ]
    ]
]

Execution errors - included in response when some field resolver throws (or returns unexpected value). Example output for query with exception thrown in field resolver {fieldWithException}:

[
    'data' => [
        'fieldWithException' => null
    ],
    'errors' => [
        [
            'message' => 'Exception message thrown in field resolver',
            'locations' => [
                ['line' => 1, 'column' => 2]
            ],
            'path': [
                'fieldWithException'
            ]
        ]
    ]
]

Obviously when Syntax or Validation error is detected - process is interrupted and query is not executed. In such scenarios response only contains errors, but not data.

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 added to response.

If exception is thrown in non-null field - it will be bubbled up to first nullable field which will be replaced with null (and error entry added to response). If all fields up to the root are non-null - data entry will be removed from n response and only errors key will be presented.

Debugging tools

Each error entry contains pointer to line and column in original query string which caused the error:

'locations' => [
    ['line' => 1, 'column' => 2]
]

GraphQL clients like Relay or GraphiQL leverage this information to highlight actual piece of query containing error.

In some cases (like deep fragment fields) locations will include several entries to track down the path to field with error in query.

Execution errors also contain path from the very root field to actual field value producing an error (including indexes for array types and fieldNames for object types). So in complex situation this path could look like this:

'path' => [
    'lastStoryPosted',
    'author',
    'friends',
    3
    'fieldWithException'
]

Custom Error Formatting

If you want to apply custom formatting to errors - use GraphQL::executeAndReturnResult() instead of GraphQL::execute().

It has exactly the same signature, but instead of array it returns GraphQL\Executor\ExecutionResult instance which holds errors in public $errors property and data in $data property.

Each entry of $errors array contains instance of GraphQL\Error\Error which wraps original exceptions thrown by resolvers. To access original exceptions use $error->getPrevious() method. But note that previous exception is only available for Execution errors and will be null for Syntax or Validation errors.

Schema Errors

So far we only covered errors which occur during query execution process. But schema definition can also throw if there is an error in one of type definitions.

Usually such errors mean that there is some logical error in your schema and it is the only case when it makes sense to return 500 error code for GraphQL endpoint:

try {
    $schema = new Schema([
        // ...
    ]);
    
    $body = GraphQL::execute($schema, $query);
    $status = 200;
} catch(\Exception $e) {
    $body = json_encode([
        'message' => 'Unexpected error'
    ]);
    $status = 500;
}

header('Content-Type: application/json', true, $status);
echo json_encode($body);