Python Flask, how to detect a SSE client disconnect from front end Javascript

Python Flask, how to detect a SSE client disconnect from front end Javascript

Detecting a Server-Sent Events (SSE) client disconnect in a Python Flask application can be a bit tricky because SSE connections are designed to be long-lived, and the server does not get immediate feedback when a client disconnects. However, you can implement a workaround by periodically sending "ping" messages from the server and monitoring the client's responses. If the server stops receiving responses, it can assume the client has disconnected. Here's how you can achieve this:

  1. Server-Side Implementation (Python Flask):

    First, you need to set up your Flask application to handle SSE connections and periodically send ping messages. Here's an example:

    from flask import Flask, request, Response import time app = Flask(__name__) def event_stream(): while True: # Send a ping message to the client yield "data: ping\n\n" time.sleep(5) # Adjust the sleep interval as needed @app.route('/sse') def sse(): return Response(event_stream(), content_type='text/event-stream') if __name__ == '__main__': app.run(debug=True)

    In this example, we have a /sse route that establishes the SSE connection and sends a "ping" message every 5 seconds.

  2. Client-Side Implementation (JavaScript):

    On the client-side, you can set up an event listener to detect disconnects. When the client receives a ping message, it responds to the server with an acknowledgment. If the client does not acknowledge a ping message, the server can assume a disconnect.

    const eventSource = new EventSource('/sse'); eventSource.onmessage = (event) => { if (event.data === 'ping') { // Send an acknowledgment to the server eventSource.send('ack'); } else { // Handle other events received from the server console.log('Received event:', event.data); } }; eventSource.onerror = (error) => { if (error.type === 'error') { // Client disconnect detected console.log('Client disconnected'); eventSource.close(); } };

    In this JavaScript code, we listen for messages from the server and send an acknowledgment ('ack') in response to a ping. If an error event occurs, such as a disconnect, we log it as a client disconnect.

By periodically sending ping messages from the server and monitoring acknowledgments on the client, you can infer that a disconnect has occurred when the client stops responding to the pings. Adjust the ping interval and other parameters as needed for your specific use case.

Examples

  1. Query: How to set up a basic SSE endpoint in Flask and handle client connections?

    • Description: Establish an SSE endpoint in Flask and configure the response for server-sent events.

    • Code:

      from flask import Flask, Response import time app = Flask(__name__) def event_stream(): while True: yield "data: Hello, client!\n\n" time.sleep(1) # Send a message every second @app.route('/sse') def sse(): return Response(event_stream(), content_type='text/event-stream') if __name__ == '__main__': app.run(debug=True)

    • JavaScript:

      const source = new EventSource('/sse'); source.onmessage = function(event) { console.log(event.data); // Output: Hello, client! };

  2. Query: How to handle SSE client disconnects in Flask using a context manager?

    • Description: Use a custom context manager to manage client connections and detect disconnects.
    • Code:
      from flask import Flask, Response, stream_with_context import time app = Flask(__name__) def sse_event_stream(): while True: try: yield "data: Hello, client!\n\n" time.sleep(1) except GeneratorExit: print("Client disconnected.") break @app.route('/sse') def sse(): return Response(stream_with_context(sse_event_stream()), content_type='text/event-stream') if __name__ == '__main__': app.run(debug=True)

  3. Query: How to manually track client connections in Flask with SSE?

    • Description: Maintain a list of connected clients and remove them when they disconnect.
    • Code:
      from flask import Flask, Response, stream_with_context import time app = Flask(__name__) clients = set() def sse_event_stream(): clients.add(id(stream_with_context)) while True: try: yield "data: Hello, client!\n\n" time.sleep(1) except GeneratorExit: clients.remove(id(stream_with_context)) print("Client disconnected.") break @app.route('/sse') def sse(): return Response(stream_with_context(sse_event_stream()), content_type='text/event-stream') if __name__ == '__main__': app.run(debug=True)

  4. Query: How to handle SSE client disconnects with a background thread in Flask?

    • Description: Use a background thread to detect client disconnects and log them.
    • Code:
      from flask import Flask, Response, stream_with_context import threading import time app = Flask(__name__) clients = set() def event_stream(): clients.add(threading.current_thread().ident) while True: try: yield "data: Hello, client!\n\n" time.sleep(1) except GeneratorExit: clients.remove(threading.current_thread().ident) print("Client disconnected.") break @app.route('/sse') def sse(): return Response(stream_with_context(event_stream()), content_type='text/event-stream') if __name__ == '__main__': app.run(debug=True)

  5. Query: How to log SSE client connections and disconnections in Flask?

    • Description: Log client connections and disconnections using a logger in Flask.
    • Code:
      import logging from flask import Flask, Response, stream_with_context import time app = Flask(__name__) logger = logging.getLogger(__name__) clients = set() def event_stream(): clients.add(id(stream_with_context)) logger.info("Client connected.") while True: try: yield "data: Hello, client!\n\n" time.sleep(1) except GeneratorExit: clients.remove(id(stream_with_context)) logger.info("Client disconnected.") break @app.route('/sse') def sse(): return Response(stream_with_context(event_stream()), content_type='text/event-stream') if __name__ == '__main__': app.run(debug=True)

  6. Query: How to handle SSE client disconnects in Flask with timeout-based monitoring?

    • Description: Use a timeout to monitor and detect SSE client disconnects.
    • Code:
      import time from flask import Flask, Response, stream_with_context app = Flask(__name__) clients = set() def event_stream(): clients.add(id(stream_with_context)) last_connection_time = time.time() while True: if time.time() - last_connection_time > 10: clients.remove(id(stream_with_context)) print("Client disconnected due to timeout.") break try: yield "data: Hello, client!\n\n" time.sleep(1) except GeneratorExit: clients.remove(id(stream_with_context)) print("Client disconnected.") break @app.route('/sse') def sse(): return Response(stream_with_context(event_stream()), content_type='text/event-stream') if __name__ == '__main__': app.run(debug=True)

  7. Query: How to use a heartbeat mechanism to detect SSE client disconnects in Flask?

    • Description: Implement a heartbeat mechanism to check if clients are still connected.

    • Code:

      from flask import Flask, Response, stream_with_context import time app = Flask(__name__) def heartbeat(): yield "event: heartbeat\ndata: {}\n\n".format(time.time()) def event_stream(): while True: try: yield from heartbeat() yield "data: Hello, client!\n\n" time.sleep(1) except GeneratorExit: print("Client disconnected.") break @app.route('/sse') def sse(): return Response(stream_with_context(event_stream()), content_type='text/event-stream') if __name__ == '__main__': app.run(debug=True)

    • JavaScript:

      const source = new EventSource('/sse'); source.addEventListener('heartbeat', (event) => { console.log(`Heartbeat received at: ${event.data}`); }); source.onmessage = (event) => { console.log(event.data); };

  8. Query: How to gracefully handle SSE client disconnects with context managers in Flask?

    • Description: Use context managers to ensure proper cleanup when SSE clients disconnect.
    • Code:
      from flask import Flask, Response, stream_with_context import time app = Flask(__name__) class SSEClient: def __enter__(self): print("Client connected") return self def __exit__(self, exc_type, exc_val, exc_tb): print("Client disconnected") def event_stream(): with SSEClient(): while True: try: yield "data: Hello, client!\n\n" time.sleep(1) except GeneratorExit: break @app.route('/sse') def sse(): return Response(stream_with_context(event_stream()), content_type='text/event-stream') if __name__ == '__main__': app.run(debug=True)

  9. Query: How to automatically restart SSE connections in JavaScript after a disconnect?

    • Description: Handle client-side reconnections to SSE endpoints when a connection is lost.

    • JavaScript:

      function setupSSE() { const source = new EventSource('/sse'); source.onopen = () => { console.log("SSE connection opened"); }; source.onerror = () => { console.error("SSE connection error"); source.close(); setTimeout(setupSSE, 5000); // Retry after 5 seconds }; source.onmessage = (event) => { console.log(event.data); }; } setupSSE();

    • This JavaScript snippet automatically attempts to reconnect if the SSE connection is lost, providing resilience on the client-side.

  10. Query: How to use a background task to detect SSE client disconnects in Flask?

    • Description: Utilize background tasks or threads to monitor client disconnects.
    • Code:
      import time from flask import Flask, Response, stream_with_context import threading app = Flask(__name__) clients = set() def event_stream(): client_id = id(stream_with_context) clients.add(client_id) try: while True: yield "data: Hello, client!\n\n" time.sleep(1) except GeneratorExit: clients.remove(client_id) print(f"Client {client_id} disconnected") @app.route('/sse') def sse(): return Response(stream_with_context(event_stream()), content_type='text/event-stream') def monitor_clients(): while True: time.sleep(5) print(f"Connected clients: {len(clients)}") if __name__ == '__main__': threading.Thread(target=monitor_clients).start() app.run(debug=True)

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