Composite operations (multi-resource) in Rest API - Time & Space Complexity

Time Complexity: Composite operations (multi-resource)

O(n)

Understanding Time Complexity

When a REST API performs multiple resource calls in one operation, the total time depends on all those calls combined.

We want to know how the total work grows as we add more resources to handle.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

async function compositeOperation(resources) {
  const results = [];
  for (const resource of resources) {
    const response = await fetch(`/api/${resource}`);
    const data = await response.json();
    results.push(data);
  }
  return results;
}

This code fetches data from multiple API endpoints one after another and collects the results.

Identify Repeating Operations

• Primary operation: Fetching each resource from the API.
• How many times: Once for each resource in the input list.

How Execution Grows With Input

Each additional resource adds one more fetch call, so the total work grows directly with the number of resources.

Input Size (n)	Approx. Operations
10	10 fetch calls
100	100 fetch calls
1000	1000 fetch calls

Pattern observation: The total work increases in a straight line as more resources are added.

Final Time Complexity

Time Complexity: O(n)

This means the total time grows proportionally with the number of resources we fetch.

Common Mistake

[X] Wrong: "Fetching multiple resources at once is always faster and takes constant time."

[OK] Correct: Each resource still requires a separate fetch call, so total time grows with how many resources there are, even if done sequentially.

Interview Connect

Understanding how multiple API calls add up helps you design efficient systems and explain your reasoning clearly in interviews.

Self-Check

"What if we changed the code to fetch all resources in parallel instead of one by one? How would the time complexity change?"

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