Using HTTP 202 for Asynchronous Processing

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Okay, here’s a comprehensive article on using HTTP 202 (Accepted) for asynchronous processing, spanning approximately 5000 words:

HTTP 202 (Accepted) for Asynchronous Processing: A Deep Dive

In the world of web development, responsiveness and efficiency are paramount. Users expect immediate feedback, even when they trigger operations that might take a significant amount of time to complete. Waiting for a lengthy server-side process to finish before receiving any response is a recipe for a poor user experience and potentially, lost users. This is where asynchronous processing and the HTTP 202 (Accepted) status code come into play.

This article provides a deep dive into the concept of asynchronous processing and how the HTTP 202 status code is crucial for building robust and user-friendly web applications that handle long-running tasks. We’ll cover the following aspects:

  1. Understanding Synchronous vs. Asynchronous Processing:

    • The fundamental differences.
    • The limitations of synchronous processing.
    • The benefits of asynchronous processing.
    • Real-world examples.

  2. Introduction to HTTP Status Codes:

    • Brief overview of HTTP and its role.
    • The 1xx, 2xx, 3xx, 4xx, and 5xx categories.
    • Focus on the 2xx (Successful) category.

  3. Deep Dive into HTTP 202 (Accepted):

    • The formal definition (RFC specifications).
    • The core meaning and implications.
    • Essential headers associated with 202.
    • Proper usage scenarios.
    • Comparison with other relevant status codes (200 OK, 201 Created, 204 No Content).

  4. Implementing Asynchronous Processing with 202:

    • Architectural patterns:
      • Message Queues (RabbitMQ, Kafka, SQS, etc.).
      • Task Queues (Celery, RQ, etc.).
      • Event-Driven Architectures.
      • Webhooks.
      • Server-Sent Events (SSE).
      • WebSockets.
    • Client-side handling:
      • Polling.
      • Long Polling.
      • Handling the Location header.
    • Error handling and retries.
    • Security considerations.

  5. Practical Examples (Code Snippets in Multiple Languages):

    • Python (Flask/FastAPI) with Celery.
    • Node.js (Express) with Bull.
    • Java (Spring Boot) with JMS.
    • Ruby (Rails) with Sidekiq.
    • PHP (Laravel) with Queues.

  6. Advanced Considerations:

    • Idempotency.
    • Monitoring and observability.
    • Handling failures and rollbacks.
    • Scaling asynchronous workers.
    • Dealing with long-running tasks exceeding typical timeout limits.
    • Combining 202 with other asynchronous patterns.

  7. Best Practices and Common Pitfalls:

    • Clear and consistent API design.
    • Proper documentation.
    • Avoiding race conditions.
    • Choosing the right tools for the job.
    • Testing asynchronous workflows.

  8. Conclusion

1. Understanding Synchronous vs. Asynchronous Processing:

At the heart of this discussion lies the fundamental difference between synchronous and asynchronous processing models.

  • Synchronous Processing:

    In a synchronous operation, tasks are executed sequentially, one after the other. The client (e.g., a web browser or another application) sends a request to the server and waits for the server to fully complete the request before receiving a response. This “waiting” is the key characteristic. The client is blocked and cannot perform other actions until the server responds.

    • Analogy: Imagine standing in line at a coffee shop. You place your order, and you must wait until your coffee is prepared before you can do anything else (like check your phone or talk to a friend). You are blocked until the barista hands you your drink.

    • Limitations:

      • Poor User Experience: For long-running operations, the user interface might freeze or become unresponsive, leading to frustration.
      • Resource Inefficiency: The client’s resources (e.g., a thread) are tied up while waiting, even if they could be used for other tasks.
      • Scalability Challenges: Handling many concurrent synchronous requests can quickly overwhelm the server, as each request consumes resources for the entire duration.

  • Asynchronous Processing:

    In asynchronous processing, the client sends a request to the server, and the server acknowledges receipt of the request immediately, even if the actual processing hasn’t started or finished. The server then processes the request in the background, potentially using a separate thread, process, or even a different server. The client is not blocked and can continue performing other tasks. Once the server has finished processing the request, it can notify the client (or the client can periodically check for completion).

    • Analogy: Imagine ordering food at a restaurant with a buzzer system. You place your order, receive a buzzer, and then you are free to sit down, chat with friends, or use your phone. You’re not stuck at the counter. When your food is ready, the buzzer alerts you.

    • Benefits:

      • Improved User Experience: The application remains responsive, even during lengthy operations. Users can continue interacting with the application.
      • Increased Resource Efficiency: Client and server resources are not needlessly tied up waiting.
      • Enhanced Scalability: The server can handle a larger number of concurrent requests, as it can quickly acknowledge them and process them in the background.

  • Real-World Examples:

    • Synchronous:

      • Fetching a simple web page (usually).
      • Making a database query that returns results quickly.
      • Performing a simple calculation.

    • Asynchronous:

      • Processing a large file upload (e.g., video transcoding).
      • Sending a large batch of emails.
      • Generating a complex report.
      • Training a machine learning model.
      • Performing image processing (e.g., resizing, watermarking).
      • Running complex simulations.

2. Introduction to HTTP Status Codes:

HTTP (Hypertext Transfer Protocol) is the foundation of data communication on the World Wide Web. It defines how clients (like web browsers) and servers interact, including how requests are made and how responses are formatted. A crucial part of this interaction is the HTTP status code.

  • What are HTTP Status Codes?

    HTTP status codes are three-digit numbers returned by a server in response to a client’s request. They indicate the outcome of the request – whether it was successful, encountered an error, or requires further action. These codes are standardized by the Internet Engineering Task Force (IETF) in various RFC (Request for Comments) documents.

  • The Five Categories of Status Codes:

    Status codes are grouped into five categories based on their first digit:

    • 1xx (Informational): The request was received, and the server is continuing the process. Examples:

      • 100 Continue: The server has received the request headers and the client should proceed to send the request body.
      • 101 Switching Protocols: The server is switching protocols as requested by the client.
      • 102 Processing: (WebDAV) The server is processing the request, but no response is available yet.

    • 2xx (Successful): The request was successfully received, understood, and accepted. Examples:

      • 200 OK: The standard success code for most requests.
      • 201 Created: The request has been fulfilled, and a new resource has been created.
      • 202 Accepted: (This is our focus!) The request has been accepted for processing, but the processing has not been completed.
      • 204 No Content: The server successfully processed the request, but there is no content to return.

    • 3xx (Redirection): The client must take additional action to complete the request (usually by following a redirection URL). Examples:

      • 301 Moved Permanently: The requested resource has been permanently moved to a new URL.
      • 302 Found: The requested resource has been temporarily moved to a new URL.
      • 304 Not Modified: The client’s cached version of the resource is still valid.

    • 4xx (Client Error): The request contains bad syntax or cannot be fulfilled. Examples:

      • 400 Bad Request: The server cannot understand the request due to invalid syntax.
      • 401 Unauthorized: Authentication is required to access the resource.
      • 403 Forbidden: The server understands the request, but refuses to authorize it.
      • 404 Not Found: The requested resource could not be found on the server.
      • 429 Too Many Requests: The user has sent too many requests in a given amount of time (“rate limiting”).

    • 5xx (Server Error): The server failed to fulfill a valid request. Examples:

      • 500 Internal Server Error: A generic error message indicating a server-side problem.
      • 502 Bad Gateway: The server, acting as a gateway or proxy, received an invalid response from an upstream server.
      • 503 Service Unavailable: The server is temporarily unavailable (e.g., due to maintenance or overload).
      • 504 Gateway Timeout: The server, acting as a gateway or proxy, did not receive a timely response from an upstream server.

  • Focus on the 2xx (Successful) Category:

    The 2xx category indicates success. However, the specific meaning of “success” varies depending on the exact code. 200 OK is the most common, signifying that the request was processed and the response contains the requested data. 201 Created is used when a new resource is created as a result of the request (e.g., a new user account). 204 No Content is used when the request is successful, but there’s no data to send back (e.g., a successful DELETE request). And then there’s 202 Accepted, which we’ll explore in detail next.

3. Deep Dive into HTTP 202 (Accepted):

The HTTP 202 (Accepted) status code is specifically designed for handling asynchronous operations. It provides a clear and standardized way for a server to communicate to a client that a request has been received and will be processed, but that the processing is not yet complete.

  • Formal Definition (RFC Specifications):

    The 202 status code is defined in RFC 7231, Section 6.3.3:

    “The 202 (Accepted) status code indicates that the request has been accepted for processing, but the processing has not been completed. The request might or might not eventually be acted upon, as it might be disallowed when processing actually takes place. There is no facility in HTTP for re-sending a status code from an asynchronous operation.”

    This definition highlights several key points:

    • Acknowledgement: The server has received and understood the request.
    • Asynchronous Nature: Processing is not immediate; it will happen at some later point.
    • No Guarantee of Completion: The request might be processed, but it’s also possible it could be rejected later. This is important for error handling.
    • No Retransmission: HTTP itself doesn’t provide a built-in mechanism for the server to send updates about the asynchronous process’s status. This is where other mechanisms (like polling or WebSockets) come in.

  • Core Meaning and Implications:

    • Immediate Response: The client receives a 202 response quickly, avoiding long wait times.
    • Deferred Processing: The actual work is done later, often in a separate process or thread.
    • Client Responsibility: The client needs a mechanism to determine when (or if) the processing is complete.

  • Essential Headers Associated with 202:

    While a 202 response can be minimal, certain HTTP headers are crucial for providing the client with the necessary information to manage the asynchronous operation:

    • Location (Highly Recommended): This header is essential for most 202 use cases. It provides a URL that the client can use to check the status of the asynchronous operation. This URL often points to a resource that represents the status of the task or the eventual result.

      http
      HTTP/1.1 202 Accepted
      Location: /api/tasks/12345

    • Retry-After (Optional): This header can suggest a time (in seconds or as an HTTP date) after which the client should check the status again. This is useful for simple polling scenarios.

      http
      HTTP/1.1 202 Accepted
      Location: /api/tasks/12345
      Retry-After: 60

    • Content-Type (Optional): While the 202 response itself typically doesn’t contain a body, if you do include a body (e.g., for providing a task ID or other metadata), you should set the Content-Type header appropriately (e.g., application/json).

    • Content-Length (Optional): If a body is present, Content-Length indicates the size of the body in bytes.

  • Proper Usage Scenarios:

    The 202 status code is appropriate for any situation where a server-side operation takes a significant amount of time to complete, and you want to provide an immediate response to the client. Examples include:

    • Long-running computations: Scientific simulations, data analysis, report generation.
    • Resource-intensive operations: Video encoding, image processing, large file uploads.
    • Batch processing: Sending bulk emails, processing large datasets.
    • Interactions with external services: Tasks that involve calling slow or unreliable third-party APIs.
    • Workflow orchestration: Complex processes that involve multiple steps and dependencies.

  • Comparison with Other Relevant Status Codes:

    It’s important to understand how 202 differs from other 2xx status codes:

    • 200 OK vs. 202 Accepted: 200 OK means the request is complete, and the response contains the result. 202 Accepted means the request is accepted, but the result is not yet available. Use 200 OK for synchronous operations, and 202 Accepted for asynchronous operations.

    • 201 Created vs. 202 Accepted: 201 Created is used when a request immediately creates a new resource. 202 Accepted can be used even if a resource is created, but the creation process is asynchronous. For example, if creating a user account involves a lengthy verification process, you might use 202. If the account is created instantly, use 201.

    • 204 No Content vs. 202 Accepted: 204 No Content implies that the operation was successful and there’s nothing to return. 202 Accepted implies that the operation is in progress and a result might be available later. Use 204 No Content for operations that don’t produce a result (e.g., deleting a resource), and 202 Accepted for operations that will eventually produce a result.

4. Implementing Asynchronous Processing with 202:

Implementing a robust asynchronous processing system with 202 involves several key components, both on the server-side and the client-side.

  • Architectural Patterns:

    Several architectural patterns are commonly used to implement asynchronous processing. The choice of pattern depends on the specific requirements of your application, including scalability, reliability, and complexity.

    • Message Queues (RabbitMQ, Kafka, SQS, etc.):

      Message queues are a fundamental component of many asynchronous systems. They provide a reliable mechanism for decoupling the request submission from the actual processing.

      1. Client Request: The client sends a request to the server.
      2. 202 Response: The server immediately returns a 202 (Accepted) response, often with a Location header pointing to a status resource.
      3. Message Enqueueing: The server places a message (containing the details of the request) onto a message queue.
      4. Worker Processes: Separate worker processes (which can be on the same server or distributed across multiple servers) listen to the message queue.
      5. Message Dequeueing and Processing: A worker process picks up a message from the queue, processes the request, and potentially updates the status resource.

      6. Client Status Check: The client periodically polls the status resource (using the Location header) to check for completion.

      7. Advantages:

        • High Reliability: Message queues typically offer persistence, ensuring that messages are not lost even if a worker process crashes.
        • Scalability: You can easily add more worker processes to handle increased load.
        • Decoupling: The request-handling part of the server is decoupled from the processing, improving resilience.
      8. Examples:
        • RabbitMQ: A popular open-source message broker.
        • Apache Kafka: A distributed streaming platform often used for high-throughput, real-time data pipelines.
        • Amazon SQS (Simple Queue Service): A fully managed message queuing service from AWS.
        • Azure Queue Storage: A similar service from Microsoft Azure.
        • Google Cloud Pub/Sub: A globally distributed messaging service from Google Cloud.

    • Task Queues (Celery, RQ, etc.):

      Task queues are built on top of message queues and provide a higher-level abstraction for managing asynchronous tasks. They often include features like scheduling, retries, and monitoring.

      • Advantages:
        • Simplified Development: Task queues provide a more convenient API for defining and managing tasks.
        • Built-in Features: Features like retries and scheduling are often included out of the box.
      • Examples:
        • Celery (Python): A very popular and feature-rich task queue, often used with frameworks like Django and Flask.
        • RQ (Redis Queue) (Python): A simpler task queue that uses Redis as a message broker.
        • Bull (Node.js): A robust Redis-based queue for Node.js.
        • Sidekiq (Ruby): A popular background processing library for Ruby on Rails.
        • Resque (Ruby): Another Redis-backed background job processing library.

    • Event-Driven Architectures:

      Event-driven architectures rely on the concept of events – significant occurrences within the system. Components publish events, and other components subscribe to those events to react accordingly.

      1. Client Request: The client sends a request.
      2. 202 Response: The server returns a 202 response.
      3. Event Publication: The server publishes an event (e.g., “TaskCreated”) to an event bus or message broker.
      4. Event Subscription: Worker processes (or other components) subscribe to the relevant event.
      5. Event Handling: When an event is received, the subscriber processes the corresponding task.

      6. Status Updates (Optional): The worker might publish further events (e.g., “TaskProgressUpdated,” “TaskCompleted,” “TaskFailed”) to update the status.

      7. Advantages:

        • Loose Coupling: Components are highly decoupled, making the system more flexible and maintainable.
        • Scalability: Event-driven systems can be highly scalable.
        • Real-time Updates: Events can be used to provide real-time updates to clients (e.g., using WebSockets).
      8. Examples:
        • Apache Kafka (again): Kafka is often used as the backbone of event-driven architectures.
        • AWS EventBridge: A serverless event bus service from AWS.
        • Azure Event Grid: A similar service from Microsoft Azure.
        • Google Cloud Pub/Sub (again): Can also be used for event-driven architectures.

    • Webhooks:

      Webhooks provide a mechanism for a server to push notifications to a client (or another server) when an event occurs. Instead of the client polling for updates, the server actively sends data to a pre-registered URL.

      1. Client Request: The client sends a request and registers a webhook URL with the server. This URL is where the server will send notifications.
      2. 202 Response: The server returns a 202 response.
      3. Asynchronous Processing: The server processes the request asynchronously.
      4. Webhook Notification: When the processing is complete (or at intermediate stages), the server sends an HTTP request (usually a POST) to the registered webhook URL. This request contains data about the event (e.g., the result of the task).

      5. Client handles notification.

      6. Advantages:

        • Real-time Updates: Clients receive updates as soon as they are available.
        • Reduced Polling: Eliminates the need for constant polling.
        • Efficiency: Server resources are not wasted on unnecessary polls
      7. Disadvantages:
        • Client-Side Requirements: The client needs to have a publicly accessible endpoint to receive the webhook notifications.
        • Error Handling: The server needs to handle cases where the webhook endpoint is unavailable. This can be done by implementing a retry mechanism, or by logging the error.
        • Security: Webhooks need to be secured to prevent unauthorized access. This can be done by using HTTPS, and by verifying the signature of the webhook request.

    • Server-Sent Events (SSE):
      Server-Sent Events (SSE) is a technology that allows a server to push updates to a client over a single HTTP connection. It’s a simpler alternative to WebSockets, particularly well-suited for unidirectional communication (server to client).

      1. Client makes and initial request.
      2. The server responds with Content-Type: text/event-stream
      3. Connection stays open: The server keeps the HTTP connection open.
      4. Server sends events: The server sends events to the client as they occur. Each event is a text-based message with a specific format.

      5. Client handles events: The client uses JavaScript to listen for events and update the UI accordingly.

      6. Advantages:

        • Simpler than WebSockets: Easier to implement for unidirectional communication.
        • Built-in Reconnection: Browsers automatically handle reconnection if the connection is lost.
        • HTTP-based: Works well with existing HTTP infrastructure.
      7. Disadvantages:
        • Unidirectional: Only supports server-to-client communication.
        • Limited Browser Support (Edge): While widely supported, older versions of Microsoft Edge did not support SSE natively (though polyfills are available).

    • WebSockets:

      WebSockets provide a full-duplex communication channel between a client and a server. Unlike HTTP, which is request-response based, WebSockets allow for bidirectional communication over a single, persistent connection.

      1. Handshake: The client initiates a WebSocket connection with a special HTTP handshake.
      2. Persistent Connection: Once the handshake is successful, a persistent connection is established.
      3. Bidirectional Communication: Both the client and the server can send messages to each other at any time.
      4. Client Sends Initial request to server for long-running process.

      5. Server sends 202 response, opens a websocket and sends updates on progress.

      6. Advantages:

        • Real-time, Bidirectional Communication: Ideal for applications that require real-time updates in both directions (e.g., chat applications, online games).
        • Low Latency: Avoids the overhead of repeated HTTP requests.
      7. Disadvantages:
        • More Complex: Requires more complex server-side and client-side implementation than simple polling.
        • Resource Intensive: Maintaining persistent connections can be more resource-intensive for the server.

  • Client-Side Handling:

    The client plays a crucial role in handling asynchronous responses. The primary methods are:

    • Polling:

      The simplest approach is for the client to periodically send requests to the status URL provided in the Location header. This is known as polling.

      1. Initial Request: The client sends the initial request.
      2. 202 Response: The server returns a 202 response with a Location header.
      3. Periodic Requests: The client sends GET requests to the Location URL at regular intervals (e.g., every 5 seconds).
      4. Status Check: The server responds to each poll with the current status of the task. This might be:
        • 200 OK: The task is complete, and the response body contains the result.
        • 202 Accepted: The task is still in progress.
        • 404 Not Found: The task ID is invalid or the task has been deleted.
        • 500 Internal Server Error: An error occurred during processing.

      5. Result Handling: Once the client receives a 200 OK response, it processes the result.

      6. Advantages:

        • Simple to Implement: Relatively easy to implement on both the client and server.
        • Works with Any HTTP Client: Doesn’t require special libraries or protocols.
      7. Disadvantages:
        • Inefficient: Can lead to a large number of unnecessary requests, especially if the task takes a long time to complete.
        • Not Real-time: Updates are not immediate; there’s a delay between status changes.

    • Long Polling:

      Long polling is a variation of polling that reduces the number of unnecessary requests. Instead of immediately returning a response, the server holds the connection open until either the task is complete or a timeout is reached.

      1. Initial Request: The client sends the initial request.
      2. 202 Response: The server returns a 202 response with a Location header.
      3. Long Poll Request: The client sends a GET request to the Location URL.
      4. Server Waits: The server does not immediately respond. It waits until either:
        • The task is complete.
        • A predefined timeout is reached (e.g., 30 seconds).
      5. Response:
        • If the task completes before the timeout, the server responds with 200 OK and the result.
        • If the timeout is reached before the task completes, the server responds with 202 Accepted (or potentially a custom response indicating that the task is still in progress).

      6. Immediate Re-poll: If the client receives a 202 Accepted response (or a timeout indication), it immediately sends another long poll request.

      7. Advantages:

        • More Efficient than Regular Polling: Reduces the number of requests, as the server only responds when there’s an update or a timeout.
        • Closer to Real-time: Provides updates more quickly than regular polling.
      8. Disadvantages:
        • Still Not Truly Real-time: There’s still a potential delay (up to the timeout value).
        • Can Be Resource-Intensive: Holding connections open for extended periods can consume server resources.

    • Handling the Location Header:

      Regardless of the polling method, the client must correctly handle the Location header. This involves:

      • Extracting the URL: The client needs to parse the Location header value to extract the status URL.
      • Using an Absolute URL: The Location header should ideally contain an absolute URL (including the scheme, hostname, and path). If it’s a relative URL, the client needs to resolve it relative to the original request URL.
      • Error Handling: The client should handle cases where the Location header is missing or invalid.

  • Error Handling and Retries:

    Asynchronous operations introduce new challenges for error handling. The client needs to be prepared for various scenarios:

    • Task Failures: The asynchronous task might fail due to various reasons (e.g., invalid input, network errors, resource exhaustion).
      • Status Codes: The status URL should return an appropriate error code (e.g., 500 Internal Server Error) if the task fails.
      • Error Messages: The response body should include a detailed error message explaining the cause of the failure.
      • Retry Mechanisms: The client might implement a retry mechanism, attempting to resubmit the task (possibly with a backoff strategy to avoid overwhelming the server). The server should ideally support idempotent operations to prevent duplicate processing.
    • Network Errors: The client might encounter network errors when communicating with the server (either for the initial request or when polling the status URL).
      • Timeouts: The client should implement appropriate timeouts to avoid waiting indefinitely for a response.
      • Retries (with Backoff): The client should retry failed requests, but with an exponential backoff strategy to avoid overwhelming the server.
    • Status URL Errors: The status URL itself might be unavailable or return errors.
      • Fallback Mechanisms: The client might have a fallback mechanism (e.g., displaying a generic error message to the user) if it cannot determine the status of the task.

  • Security Considerations:

    Security is crucial for any web application, and asynchronous processing introduces some specific considerations:

    • Authentication and Authorization: Both the initial request and the status URL should be protected by appropriate authentication and authorization mechanisms. The client needs to provide valid credentials (e.g., API keys, session tokens) to access these resources.
    • Input Validation: The server must carefully validate all input received from the client to prevent security vulnerabilities (e.g., SQL injection, cross-site scripting).
    • Data Protection: Sensitive data transmitted between the client and server (including task parameters and results) should be encrypted using HTTPS.
    • Webhook Security: If using webhooks, the server should verify the authenticity of the webhook requests (e.g., using signatures or shared secrets) to prevent attackers from sending malicious notifications.
    • Rate Limiting: To prevent abuse, apply rate limiting to both the endpoint that initiates the asynchronous task and the status endpoint. This helps protect your server from being overwhelmed by too many requests.

5. Practical Examples (Code Snippets in Multiple Languages):

Let’s illustrate the concepts with code examples in several popular programming languages. These examples will demonstrate a simple scenario: a client requests the server to perform a long-running task (simulated with a delay), and the server uses a task queue to process the task asynchronously.

  • Python (Flask/FastAPI) with Celery:

    “`python

    app.py (Flask)

    from flask import Flask, jsonify, request, url_for
    from celery import Celery
    import time

    app = Flask(name)
    app.config[‘CELERY_BROKER_URL’] = ‘redis://localhost:6379/0’ # Replace with your broker URL
    app.config[‘CELERY_RESULT_BACKEND’] = ‘redis://localhost:6379/0’ # Replace with your backend URL

    celery = Celery(app.name, broker=app.config[‘CELERY_BROKER_URL’])
    celery.conf.update(app.config)

    @celery.task(bind=True)
    def long_task(self, data):
    “””Simulates a long-running task.”””
    total = 5
    for i in range(total):
    time.sleep(1)
    self.update_state(state=’PROGRESS’,
    meta={‘current’: i + 1, ‘total’: total})
    return {‘current’: total, ‘total’: total, ‘status’: ‘Task completed!’, ‘result’: 42}
    #In a real scenario, you can put the actual result of computation in the return statement, instead of 42.

    @app.route(‘/longtask’, methods=[‘POST’])
    def start_long_task():
    “””Starts the long-running task.”””
    data = request.get_json() # Get data from the request body
    task = long_task.apply_async(args=[data])
    return jsonify({}), 202, {‘Location’: url_for(‘task_status’, task_id=task.id)}

    @app.route(‘/status/‘)
    def task_status(task_id):
    “””Checks the status of the task.”””
    task = long_task.AsyncResult(task_id)
    if task.state == ‘PENDING’:
    response = {
    ‘state’: task.state,
    ‘current’: 0,
    ‘total’: 1,
    ‘status’: ‘Pending…’
    }
    elif task.state != ‘FAILURE’:
    response = {
    ‘state’: task.state,
    ‘current’: task.info.get(‘current’, 0),
    ‘total’: task.info.get(‘total’, 1),
    ‘status’: task.info.get(‘status’, ”)
    }
    if ‘result’ in task.info:
    response[‘result’] = task.info[‘result’]
    else:
    # something went wrong in the background job
    response = {
    ‘state’: task.state,
    ‘current’: 1,
    ‘total’: 1,
    ‘status’: str(task.info), # this is the exception raised
    }
    return jsonify(response)

    if name == ‘main‘:
    app.run(debug=True) # Turn off debug mode in production.

    To run:

    1. Start Redis server

    2. Start Celery worker: celery -A app.celery worker -l info

    3. Run the Flask app: python app.py

    Client-side (example using requests):

    import requests

    response = requests.post(‘http://localhost:5000/longtask’, json={“some_data”: “value”})

    if response.status_code == 202:

    status_url = response.headers[‘Location’]

    print(f”Task started. Status URL: {status_url}”)

    while True:

    status_response = requests.get(status_url)

    status_data = status_response.json()

    print(f”Task Status: {status_data}”)

    if status_data[‘state’] == ‘SUCCESS’:

    print(f”Task Result: {status_data[‘result’]}”)

    break

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