GraphQL error format - Step-by-Step Execution

Concept Flow - GraphQL error format

Client sends GraphQL query

↓

Server processes query

↓

Error occurs?

No→Return data response

Yes↓

Create error object

↓

Include error fields: message, locations, path, extensions

↓

Return error response to client

The server processes a GraphQL query and if an error occurs, it creates a structured error object with specific fields and returns it to the client.

Execution Sample

GraphQL

{
  user(id: "123") {
    name
    age
  }
}

// Error response example

A GraphQL query requesting user data, with an example error response returned if user not found.

Execution Table

Step	Action	Evaluation	Result
1	Receive query from client	Query with user(id: "123")	Start processing query
2	Resolve user field	Check if user with id 123 exists	User not found
3	Create error object	Build error with message and location	{"message": "User not found", "locations": [{"line": 2, "column": 3}], "path": ["user"]}
4	Return response	Include errors array in response	{"errors": [{"message": "User not found", "locations": [{"line": 2, "column": 3}], "path": ["user"]}], "data": {"user": null}}
5	End	Response sent to client	Execution complete

💡 Error detected during field resolution, error object created and returned in response

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
query	null	{ user(id: "123") { name age } }	{ user(id: "123") { name age } }	{ user(id: "123") { name age } }
userData	null	null (user not found)	null (user not found)	null (user not found)
errorObject	null	null	{"message": "User not found", "locations": [{"line": 2, "column": 3}], "path": ["user"]}	{"message": "User not found", "locations": [{"line": 2, "column": 3}], "path": ["user"]}
response	null	null	null	{"errors": [{"message": "User not found", "locations": [{"line": 2, "column": 3}], "path": ["user"]}], "data": {"user": null}}

Key Moments - 3 Insights

Why does the error response include both 'errors' and 'data' fields?

The 'errors' field contains details about what went wrong, while the 'data' field shows partial or null data. This allows clients to receive as much valid data as possible even if some parts failed, as seen in step 4 of the execution_table.

What is the purpose of the 'locations' field in the error object?

The 'locations' field shows where in the query the error happened (line and column). This helps developers quickly find and fix the problem, as shown in step 3 of the execution_table.

Why is the 'path' field included in the error object?

The 'path' field indicates which part of the query caused the error, helping clients understand which field failed. This is demonstrated in step 3 where the path is ["user"].

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'userData' after step 2?

A Error object

B User object with id 123

C null (user not found)

D Query string

💡 Hint

Check the variable_tracker row for 'userData' after step 2

At which step is the error object created?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the execution_table action 'Create error object'

If the user was found successfully, how would the response differ at step 4?

A Errors array would contain error message

B Errors array would be empty and data would contain user info

C Data field would be null

D Response would be empty

💡 Hint

Consider the purpose of 'errors' and 'data' fields in the response

Concept Snapshot

GraphQL error format:
- Errors returned in 'errors' array
- Each error has 'message', 'locations', 'path'
- 'locations' shows query line/column
- 'path' shows field causing error
- 'data' may contain partial results
- Enables clients to handle errors gracefully

Full Transcript

When a GraphQL query is sent, the server processes it. If an error occurs, such as a missing user, the server creates an error object with a message, location in the query, and the path to the field that caused the error. This error object is included in an 'errors' array in the response. The 'data' field may still contain partial data or null for the failed field. This structured error format helps clients understand what went wrong and where, allowing better debugging and user experience.