Composite operations (multi-resource) in Rest API - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this composite REST API call?

Consider a REST API that supports batch requests to create multiple resources in one call. The batch request payload is:

{"operations": [{"method": "POST", "path": "/users", "body": {"name": "Alice"}}, {"method": "POST", "path": "/orders", "body": {"userId": 1, "product": "Book"}}]}

If the API returns the following response, what is the value of response.operations[1].status?

{"operations": [{"status": 201, "body": {"id": 1, "name": "Alice"}}, {"status": 201, "body": {"id": 101, "userId": 1, "product": "Book"}}]}

A 400

B 201

C 404

D 500

💡 Hint

Look at the status code for the second operation in the response.

The second operation in the batch creates an order and returns status 201, which means 'Created'.

🧠 Conceptual

intermediate

Which statement best describes composite operations in REST APIs?

Composite operations allow clients to:

A Cache responses locally to avoid repeated API calls.

B Execute only one resource operation per HTTP request to ensure atomicity.

C Send multiple related requests in a single HTTP call to reduce network overhead.

D Use WebSockets instead of HTTP for faster communication.

💡 Hint

Think about how composite operations improve efficiency.

Composite operations bundle multiple related resource operations into one HTTP request, reducing network overhead and latency.

🔧 Debug

advanced

Identify the error in this composite operation request payload

Given this JSON payload for a composite REST API call:

{"operations": [{"method": "POST", "path": "/users", "body": {"name": "Bob"}}, {"method": "POST", "path": "/orders", "body": {"userId": "abc", "product": "Pen"}}]}

What error will the API most likely return?

A 400 Bad Request due to invalid userId type

B 404 Not Found because /orders path is incorrect

C 500 Internal Server Error due to server crash

D 201 Created for both operations

💡 Hint

Check the data type of userId in the second operation.

The userId should be a numeric ID, but "abc" is a string, causing a validation error and 400 Bad Request.

🚀 Application

advanced

How to ensure atomicity in composite REST API operations?

You want to create a composite operation that either completes all resource creations or none (all-or-nothing). Which approach is best?

A Use a transaction mechanism on the server that rolls back all changes if any operation fails.

B Send operations sequentially and ignore failures in intermediate steps.

C Send all operations in parallel without rollback support.

D Use client-side retries to fix failed operations after the batch completes.

💡 Hint

Atomicity means all succeed or all fail together.

Server-side transactions ensure that if any operation in the batch fails, all changes are undone, preserving atomicity.

❓ Predict Output

expert

What is the final state after this composite operation with dependencies?

Consider a composite REST API call with two operations:

{"operations": [{"id": "op1", "method": "POST", "path": "/projects", "body": {"name": "ProjectX"}}, {"id": "op2", "method": "POST", "path": "/tasks", "body": {"projectId": "${op1.body.id}", "title": "Task1"}}]}

If the server processes these operations and returns:

{"operations": [{"id": "op1", "status": 201, "body": {"id": 42, "name": "ProjectX"}}, {"id": "op2", "status": 201, "body": {"id": 101, "projectId": 42, "title": "Task1"}}]}

What is the value of response.operations[1].body.projectId?

A null

B "${op1.body.id}"

C 101

D 42

💡 Hint

Check how the second operation uses the first operation's result.

The second operation references the first operation's created project ID (42) dynamically, so the projectId is 42.

Practice

1. What is the main benefit of using composite operations in REST APIs?

easy

A. They combine multiple API calls into a single request to save time and keep data consistent.

B. They allow you to write SQL queries inside REST endpoints.

C. They automatically generate API documentation.

D. They encrypt all API responses for security.

Solution

1. Step 1: Understand what composite operations do

   Composite operations bundle several API calls into one request to reduce network overhead.

2. Step 2: Identify the main benefit

   This bundling saves time and ensures related data changes happen together, keeping consistency.

3. Final Answer:

   They combine multiple API calls into a single request to save time and keep data consistent. -> Option A

4. Quick Check:

   Composite operations = combine calls + consistency [OK]

Hint: Composite means combining multiple calls into one [OK]

Common Mistakes:

• Confusing composite operations with documentation tools
• Thinking composite operations encrypt data
• Assuming composite operations run SQL queries

2. Which of the following is the correct way to structure a composite operation request in JSON?

easy

A. [{"method": "DELETE", "path": "/items/1"}]

B. {"operation": "GET", "url": "/users"}

C. {"requests": {"method": "POST", "endpoint": "/orders"}}

D. {"operations": [{"method": "POST", "path": "/users"}, {"method": "PUT", "path": "/accounts/123"}]}

Solution

1. Step 1: Identify the correct JSON structure for composite operations

   Composite operations usually use an "operations" array with each operation having "method" and "path" keys.

2. Step 2: Compare options

   {"operations": [{"method": "POST", "path": "/users"}, {"method": "PUT", "path": "/accounts/123"}]} matches this structure exactly, while others use incorrect keys or formats.

3. Final Answer:

   {"operations": [{"method": "POST", "path": "/users"}, {"method": "PUT", "path": "/accounts/123"}]} -> Option D

4. Quick Check:

   Composite request = "operations" array with method and path [OK]

Hint: Look for "operations" array with method and path keys [OK]

Common Mistakes:

• Using singular "operation" instead of plural "operations"
• Using wrong keys like "url" or "endpoint"
• Not wrapping operations inside an array

3. Given this composite operation request:

{
  "operations": [
    {"method": "POST", "path": "/users", "body": {"name": "Alice"}},
    {"method": "POST", "path": "/orders", "body": {"userId": "${1.id}", "item": "Book"}}
  ]
}

What is the expected behavior?

medium

A. Create a user named Alice, then create an order linked to that user using the new user ID.

B. Create an order first, then create a user named Alice.

C. Create a user named Alice only; the order creation will fail due to syntax error.

D. The request will fail because referencing previous operation results is not allowed.

Solution

1. Step 1: Understand operation order and referencing

   The first operation creates a user named Alice. The second operation uses "${1.id}" to reference the ID from the first operation's result.

2. Step 2: Determine expected behavior

   This means the order is created linked to the new user. This chaining is a key feature of composite operations.

3. Final Answer:

   Create a user named Alice, then create an order linked to that user using the new user ID. -> Option A

4. Quick Check:

   Reference previous result with ${1.id} = chained operations [OK]

Hint: Look for ${1.id} to see chaining between operations [OK]

Common Mistakes:

• Assuming operations run in parallel without order
• Thinking referencing previous results is invalid
• Confusing operation indexes starting at 0 instead of 1

4. You have this composite operation request:

{
  "operations": [
    {"method": "POST", "path": "/products", "body": {"name": "Pen"}},
    {"method": "PUT", "path": "/inventory/${1.id}", "body": {"stock": 100}}
  ]
}

But the API returns an error saying "Invalid reference ${1.id}". What is the likely cause?

medium

A. The syntax for referencing previous results should be ${0.id}, not ${1.id}.

B. The first operation did not return an ID, so ${1.id} is invalid.

C. PUT method cannot be used in composite operations.

D. The inventory path should not include dynamic IDs.

Solution

1. Step 1: Check the reference usage

   The error "Invalid reference ${1.id}" means the first operation's response does not contain an "id" field.

2. Step 2: Understand why this causes error

   If the first operation's response lacks "id", the reference ${1.id} is invalid and causes failure.

3. Final Answer:

   The first operation did not return an ID, so ${1.id} is invalid. -> Option B

4. Quick Check:

   Reference error = missing id in first operation response [OK]

Hint: Check if referenced operation returns the expected field [OK]

Common Mistakes:

• Assuming index starts at 0 instead of 1
• Thinking PUT is disallowed in composite operations
• Ignoring that referenced fields must exist in response

5. You want to create a composite operation that creates a customer, then creates two orders for that customer, and finally updates the customer's status to "active". Which JSON structure correctly represents this multi-resource composite operation?

hard

A. {
"operations": [
{"method": "POST", "path": "/customers", "body": {"name": "Bob"}},
{"method": "POST", "path": "/orders", "body": {"customerId": "1", "item": "Laptop"}},
{"method": "POST", "path": "/orders", "body": {"customerId": "1", "item": "Mouse"}},
{"method": "PUT", "path": "/customers/1", "body": {"status": "active"}}
]
}

B. {
"operations": [
{"method": "POST", "path": "/orders", "body": {"item": "Laptop"}},
{"method": "POST", "path": "/customers", "body": {"name": "Bob"}},
{"method": "PUT", "path": "/customers/1", "body": {"status": "active"}}
]
}

C. {
"operations": [
{"method": "POST", "path": "/customers", "body": {"name": "Bob"}},
{"method": "POST", "path": "/orders", "body": {"customerId": "${1.id}", "item": "Laptop"}},
{"method": "POST", "path": "/orders", "body": {"customerId": "${1.id}", "item": "Mouse"}},
{"method": "PUT", "path": "/customers/${1.id}", "body": {"status": "active"}}
]
}

D. {
"operations": [
{"method": "POST", "path": "/customers", "body": {"name": "Bob"}},
{"method": "POST", "path": "/orders", "body": {"customerId": "${2.id}", "item": "Laptop"}},
{"method": "POST", "path": "/orders", "body": {"customerId": "${2.id}", "item": "Mouse"}},
{"method": "PUT", "path": "/customers/${2.id}", "body": {"status": "active"}}
]
}

Solution

1. Step 1: Identify correct operation order and referencing

   The customer creation is first (operation 1), so its ID is referenced as ${1.id} in subsequent operations.

2. Step 2: Verify all references use ${1.id} and order is logical

   Orders and update use ${1.id} correctly. { "operations": [ {"method": "POST", "path": "/customers", "body": {"name": "Bob"}}, {"method": "POST", "path": "/orders", "body": {"customerId": "${1.id}", "item": "Laptop"}}, {"method": "POST", "path": "/orders", "body": {"customerId": "${1.id}", "item": "Mouse"}}, {"method": "PUT", "path": "/customers/${1.id}", "body": {"status": "active"}} ] } matches this perfectly. { "operations": [ {"method": "POST", "path": "/customers", "body": {"name": "Bob"}}, {"method": "POST", "path": "/orders", "body": {"customerId": "${2.id}", "item": "Laptop"}}, {"method": "POST", "path": "/orders", "body": {"customerId": "${2.id}", "item": "Mouse"}}, {"method": "PUT", "path": "/customers/${2.id}", "body": {"status": "active"}} ] } incorrectly uses ${2.id} which refers to the first order, not the customer.

3. Final Answer:

   { "operations": [ {"method": "POST", "path": "/customers", "body": {"name": "Bob"}}, {"method": "POST", "path": "/orders", "body": {"customerId": "${1.id}", "item": "Laptop"}}, {"method": "POST", "path": "/orders", "body": {"customerId": "${1.id}", "item": "Mouse"}}, {"method": "PUT", "path": "/customers/${1.id}", "body": {"status": "active"}} ] } -> Option C

4. Quick Check:

   Use ${1.id} to reference first operation's customer ID [OK]

Hint: Reference first created resource as ${1.id} in later operations [OK]

Common Mistakes:

• Referencing wrong operation index for customer ID
• Hardcoding IDs instead of referencing dynamically
• Incorrect operation order breaking dependencies