Validation error details in Rest API - Step-by-Step Execution

Concept Flow - Validation error details

Receive API Request

↓

Validate Input Data

↓

Process Request

↓

Send Success

↓

End

The API receives a request, checks the input data, and either processes it or returns detailed validation errors.

Execution Sample

Rest API

POST /api/users
{
  "email": "bademail",
  "age": -5
}

An API receives a POST request with invalid email and age fields to demonstrate validation error details.

Execution Table

Step	Validation Check	Condition	Result	Error Detail Collected
1	Check email format	"bademail" contains '@'	False	"email": "Invalid email format"
2	Check age >= 0	-5 >= 0	False	"age": "Must be non-negative"
3	All validations passed?	No	No	Return errors collected
4	Send response	Errors exist	Send 400 Bad Request	{"email": "Invalid email format", "age": "Must be non-negative"}

💡 Validation failed due to invalid email and negative age, so error details are returned.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
errors	{}	{"email": "Invalid email format"}	{"email": "Invalid email format", "age": "Must be non-negative"}	{"email": "Invalid email format", "age": "Must be non-negative"}

Key Moments - 2 Insights

Why do we collect all errors instead of stopping at the first one?

Collecting all errors helps the user fix all issues at once, as shown in steps 1 and 2 where both email and age errors are collected before responding.

What happens if all validations pass?

If all validations pass, the API processes the request and sends a success response instead of error details, which is not shown here but implied after step 3.

Visual Quiz - 3 Questions

Look at the execution table, what error detail is collected at step 1?

A "age": "Must be non-negative"

B "email": "Invalid email format"

C No error collected

D "email": "Missing field"

💡 Hint

Check the 'Error Detail Collected' column at step 1 in the execution table.

At which step does the API decide to send an error response?

A Step 4

B Step 2

C Step 3

D Step 1

💡 Hint

Look for the step where the response is sent with error details in the execution table.

If the age was 10 instead of -5, how would the errors variable change after step 2?

A {"age": "Must be non-negative"}

B {}

C {"email": "Invalid email format"}

D {"email": "Invalid email format", "age": "Must be non-negative"}

💡 Hint

Refer to variable_tracker and consider that only the email error remains if age is valid.

Concept Snapshot

Validation error details in REST API:
- Receive input data
- Check each field for correctness
- Collect all errors found
- Return errors in response with 400 status
- Helps client fix all issues at once

Full Transcript

This visual trace shows how a REST API handles validation error details. The API receives a request with input data. It checks each field, like email format and age value. If a field is invalid, it adds an error message to a collection. After all checks, if any errors exist, the API sends a 400 Bad Request response with all error details. This approach helps users fix all input problems together instead of one by one.

Practice

1. What is the main purpose of validation error details in a REST API response?

easy

A. To explain which input fields are wrong and why

B. To speed up the server response time

C. To encrypt the data sent to the client

D. To log user activity for analytics

Solution

1. Step 1: Understand validation errors

   Validation errors tell us what input data is incorrect or missing.

2. Step 2: Purpose of error details

   Error details explain which fields caused the problem and why, helping fix input.

3. Final Answer:

   To explain which input fields are wrong and why -> Option A

4. Quick Check:

   Validation error details = Explain input errors [OK]

Hint: Validation errors show what and why input is wrong [OK]

Common Mistakes:

• Thinking error details speed up server
• Confusing error details with encryption
• Assuming error details are for logging only

2. Which of the following is the correct JSON structure for validation error details in a REST API?

easy

A. {"data": {"email": "[e-mail]"}}

B. ["email", "password"]

C. {"status": 200, "message": "Success"}

D. {"errors": {"email": "Invalid format", "password": "Too short"}}

Solution

1. Step 1: Identify error details format

   Error details usually use a JSON object with field names as keys and error messages as values.

2. Step 2: Check options

   {"errors": {"email": "Invalid format", "password": "Too short"}} shows a JSON object with "errors" key and field-specific messages, which is correct.

3. Final Answer:

   {"errors": {"email": "Invalid format", "password": "Too short"}} -> Option D

4. Quick Check:

   Validation errors = JSON object with field messages [OK]

Hint: Look for JSON object with field names and error messages [OK]

Common Mistakes:

• Using arrays instead of objects for errors
• Confusing success response with error details
• Missing field names in error messages

3. Given this JSON error response:

{"errors": {"username": "Required field", "age": "Must be a number"}}

What message should the client show for the 'age' field?

medium

A. Must be a number

B. Invalid email format

C. Required field

D. Password too short

Solution

1. Step 1: Read the JSON error response

   The error for "age" is "Must be a number".

2. Step 2: Match the message to the field

   The client should show the message exactly as given for the "age" field.

3. Final Answer:

   Must be a number -> Option A

4. Quick Check:

   Field 'age' error = Must be a number [OK]

Hint: Match field name to its error message in JSON [OK]

Common Mistakes:

• Mixing error messages between fields
• Showing unrelated error messages
• Ignoring the JSON structure

4. You receive this validation error JSON:

{"error": "Invalid input", "fields": ["email", "password"]}

What is wrong with this error detail format?

medium

A. It correctly shows error details

B. It uses a list instead of mapping fields to messages

C. It uses wrong HTTP status code

D. It encrypts error messages

Solution

1. Step 1: Analyze error detail format

   The "fields" key has a list of field names but no messages explaining the errors.

2. Step 2: Identify correct format

   Validation error details should map each field to a specific error message, not just list fields.

3. Final Answer:

   It uses a list instead of mapping fields to messages -> Option B

4. Quick Check:

   Error details need field-message pairs [OK]

Hint: Error details need field names paired with messages [OK]

Common Mistakes:

• Listing fields without error explanations
• Assuming any error JSON is correct
• Ignoring the need for descriptive messages

5. You want to design a REST API validation error response that shows multiple errors per field. Which JSON structure correctly supports this?

hard

A. {"errors": ["email: Required", "password: Too short"]}

B. {"error": "Multiple errors found"}

C. {"errors": {"email": ["Required", "Invalid format"], "password": ["Too short"]}}

D. {"fields": ["email", "password"]}

Solution

1. Step 1: Understand multiple errors per field

   To show many errors for one field, use a list (array) of messages for that field.

2. Step 2: Check JSON structures

   {"errors": {"email": ["Required", "Invalid format"], "password": ["Too short"]}} uses a dictionary with field keys and lists of error messages, which fits the need.

3. Final Answer:

   {"errors": {"email": ["Required", "Invalid format"], "password": ["Too short"]}} -> Option C

4. Quick Check:

   Multiple errors per field = list of messages [OK]

Hint: Use lists inside error fields to show multiple messages [OK]

Common Mistakes:

• Using flat lists without field keys
• Not supporting multiple messages per field
• Using vague error messages without details