Long-running operations (async responses) in Rest API - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a long-running operation in the context of REST APIs?

A long-running operation is a task that takes a significant amount of time to complete, such as processing large data or complex computations, which cannot be handled instantly in a single API request-response cycle.

beginner

Why do REST APIs use asynchronous responses for long-running operations?

Asynchronous responses allow the server to accept the request and immediately return a response without waiting for the operation to finish, improving user experience and server efficiency by avoiding timeouts and blocking.

intermediate

What HTTP status code is commonly used to indicate that a long-running operation has started and is processing asynchronously?

HTTP status code 202 Accepted is used to indicate that the request has been accepted for processing, but the processing is not yet complete.

intermediate

How does a client typically check the status of a long-running operation in an async REST API?

The server usually provides a URL in the response (often in the Location header) where the client can poll or check the status of the operation until it is complete.

intermediate

What is a common pattern to handle long-running operations asynchronously in REST APIs?

The client sends a request, the server responds with 202 Accepted and a status URL, the client polls the status URL until the operation completes, then the server returns the final result or status.

Which HTTP status code indicates a long-running operation has been accepted but not completed?

A 202 Accepted

B 200 OK

C 404 Not Found

D 500 Internal Server Error

202 Accepted means the request is accepted for processing but not finished yet.

What does a REST API usually provide to track a long-running operation?

A A direct result in the response

B A URL to check status

C An error message

D A redirect to homepage

The server gives a URL where the client can check the operation status.

Why should clients avoid waiting synchronously for long-running operations?

A It makes the server faster

B It is required by HTTP standards

C It reduces network traffic

D It can cause timeouts and poor user experience

Waiting synchronously can cause timeouts and block resources.

What is the main benefit of asynchronous responses in REST APIs?

A Immediate final results

B Simpler code for clients

C Improved responsiveness and scalability

D No need for status checking

Async responses help servers handle many requests without blocking.

Which method is commonly used by clients to get updates on a long-running operation?

A Polling the status URL

B Sending repeated POST requests

C Opening a WebSocket connection

D Refreshing the homepage

Clients poll the provided status URL to check progress.

Explain how asynchronous responses help manage long-running operations in REST APIs.

Think about how the server and client communicate without waiting.

Describe the typical workflow for a client interacting with a REST API that handles long-running operations asynchronously.

Focus on the steps from request to final result.

Practice

1. What is the main purpose of using long-running operations with async responses in REST APIs?

easy

A. To make the server process all requests instantly.

B. To let clients start a task and check its progress later without waiting.

C. To block the client until the task finishes.

D. To avoid using URLs for task status.

Solution

1. Step 1: Understand client-server interaction in async operations

   Clients start a task and receive a URL to check progress later, avoiding waiting.

2. Step 2: Identify the benefit of async responses

   This approach keeps apps responsive and user-friendly by not blocking clients.

3. Final Answer:

   To let clients start a task and check its progress later without waiting. -> Option B

4. Quick Check:

   Async responses = clients check progress later [OK]

Hint: Async means start task, check later URL [OK]

Common Mistakes:

• Thinking server processes all requests instantly
• Assuming client waits for task completion
• Ignoring the use of status URLs

2. Which HTTP status code is typically returned immediately by a REST API to indicate a long-running operation has started asynchronously?

easy

A. 200 OK

B. 404 Not Found

C. 202 Accepted

D. 500 Internal Server Error

Solution

1. Step 1: Recall HTTP status codes for async start

   202 Accepted means the request is accepted but processing is not complete yet.

2. Step 2: Compare with other codes

   200 OK means completed, 404 means missing resource, 500 means server error.

3. Final Answer:

   202 Accepted -> Option C

4. Quick Check:

   Async start = 202 Accepted [OK]

Hint: 202 means accepted but not done yet [OK]

Common Mistakes:

• Using 200 OK for async start
• Confusing 404 with async status
• Assuming 500 means task started

3. Given this REST API response after starting a long-running task:

{"status": "accepted", "status_url": "/tasks/123/status"}

What should the client do next to get the task result?

medium

A. Send a DELETE request to /tasks/123/status to cancel the task.

B. Send a POST request to /tasks/123/status to get the result.

C. Wait for the server to push the result automatically.

D. Send a GET request to /tasks/123/status to check progress.

Solution

1. Step 1: Identify the purpose of status_url

   The status_url is provided for the client to check the task progress or result.

2. Step 2: Determine correct HTTP method to check status

   GET requests are used to retrieve information, so client should GET the status URL.

3. Final Answer:

   Send a GET request to /tasks/123/status to check progress. -> Option D

4. Quick Check:

   Check progress = GET status URL [OK]

Hint: Use GET on status URL to check progress [OK]

Common Mistakes:

• Using POST instead of GET to check status
• Expecting server to push result without polling
• Using DELETE to check or cancel unintentionally

4. A developer wrote this code to start a long-running task and check its status:

response = start_task()
if response.status_code == 202:
    status_url = response.json()["status_url"]
    result = requests.post(status_url)
    print(result.json())

What is the error in this code?

medium

A. Using POST instead of GET to check the status URL.

B. Not checking if status_url exists in the response.

C. Not handling the case when status_code is not 202.

D. Printing result without parsing JSON.

Solution

1. Step 1: Review HTTP method used to check status

   The code uses POST to check status, but status URLs require GET requests.

2. Step 2: Confirm other parts of code

   Checking status_url existence and status_code handling are good practices but not the main error here.

3. Final Answer:

   Using POST instead of GET to check the status URL. -> Option A

4. Quick Check:

   Status check = GET, not POST [OK]

Hint: Status check always uses GET method [OK]

Common Mistakes:

• Using POST instead of GET for status check
• Ignoring missing status_url key
• Not handling unexpected status codes

5. You want to design a REST API endpoint for a long-running image processing task. Which approach best follows async response principles?

hard

A. Client POSTs image, server returns 202 Accepted with a status URL; client polls status URL until done.

B. Client POSTs image, server processes image immediately and returns 200 OK with result.

C. Client POSTs image, server returns 404 Not Found if task is slow.

D. Client POSTs image, server holds connection open until processing finishes.

Solution

1. Step 1: Understand async response design

   Server accepts task, returns 202 with status URL for client to poll progress.

2. Step 2: Evaluate other options

   Immediate processing (200 OK) blocks client; 404 is wrong status; holding connection open blocks client.

3. Final Answer:

   Client POSTs image, server returns 202 Accepted with a status URL; client polls status URL until done. -> Option A

4. Quick Check:

   Async design = 202 + status URL + polling [OK]

Hint: Async = 202 + status URL + client polls [OK]

Common Mistakes:

• Returning 200 OK immediately for slow tasks
• Using 404 for slow processing
• Keeping connection open until done