python bottle server sent events - one way to handle client disconnects

2015-08-24

A Lot of people including me wonder how to handle client disconnections when using Server Sent Events (SSE). This example shows one way of doing it in bottle.

There is a stack overflow question I asked about this here

Note that for my purposes, I don't care if events are 'lost' when there is no client present, but you may care. If so, then take advantage of the SSE ID: record as discussed on Simon Baynes's blog post

How it works:

There is a main event queue event_queue
There is a queue crated each time a SSE request comes in
The SSE request handler (def event_stream() ) registers it's new
queue with the EventDispatcher.
The event dispatcher copies items from the main event_queue to each of the clients individual queues
The individual event queues are destroyed when the WSCGI server fails to write to the client, and hence reliqusihes it's hold on the local event queue. The EventDispatcher only has a week reference to the local event queues threfore it is automatically removed from the client list.

This seems like a merry dance, and it is. But I have not found a way as yet to be notified when a client has 'gone away' in bottle. I suspect it is possible by attaching to the internal bottle event handler in some way. However, for my particular case the code below works well. With a little tidy up and testing it can be put into proper service.

    class EventDispatcher:
        def __init__(self, main_queue):
            self.main_queue = main_queue
            self.clients = weakref.WeakSet()

        def register_client(self, queue):
            logging.debug("Registering client queue %s", queue)
            self.clients.add(queue)

        def start(self):

            class EnqueueThread:
                def __init__(self, main_queue, clients):
                    self.main_queue = main_queue
                    self.clients = clients

                def __call__(self):
                    while True:
                        msg = self.main_queue.get()
                        logging.debug("Got message. There are %d clients", len(self.clients))
                        for x in self.clients:
                            logging.debug("Delivering message to client %s", x)
                            try:
                                x.put_nowait(msg)
                            except Queue.Full:
                                logging.error("Client %s queue is full, message dropped", x)  

            self.enqueue_thread = EnqueueThread(self.main_queue, self.clients)
            threading.Thread(None, self.enqueue_thread).start()

python bottle server sent events - one way to handle client disconnects

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