How to Implement gRPC Server-to-Client Broadcasting with Long-Polling?

gRPC Server-to-Client Broadcasting

In gRPC, a common challenge arises when broadcasting events from server to multiple connected clients. The question presented seeks guidance on how to achieve this using streams, particularly with the server knowing who is connected and how to address each client.

Utilize a Long-Polling Approach

An alternative solution to streams is to implement a long-polling approach. This technique involves clients sending continuous requests to the server, which keeps the connection alive. When an event occurs on the server, it notifies all waiting clients, prompting them to end and create a new request.

Code Sample for Long-Polling

The following code sample illustrates how long-polling can be implemented in Python:

.PROTO Definitions

<code class=".PROTO">service Updater {
    rpc GetUpdates(GetUpdatesRequest) returns (GetUpdatesResponse);
}

message GetUpdatesRequest {
    int64 last_received_update = 1;
}

message GetUpdatesResponse {
    repeated Update updates = 1;
    int64 update_index = 2;
}

message Update {
    // Your update structure
}</code>

Server Code

<code class="python">class UpdaterServer(UpdaterServicer):
    def __init__(self):
        self.condition = threading.Condition()
        self.updates = []

    def post_update(self, update):
        with self.condition:
            # Remove old updates after some time
            self.updates.append(updates)
            self.condition.notify_all()

    def GetUpdates(self, req, context):
        with self.condition:
            while self.updates[req.last_received_update + 1:] == []:
                self.condition.wait()
            new_updates = self.updates[req.last_received_update + 1:]
            response = GetUpdatesResponse()
            for update in new_updates:
                response.updates.add().CopyFrom(update)
            response.update_index = req.last_received_update + len(new_updates)
            return response</code>

Separate Thread in the Client

<code class="python">request = GetUpdatesRequest()
request.last_received_update = -1
while True:
    stub = UpdaterStub(channel)
    try:
        response = stub.GetUpdates(request, timeout=60*10)
        handle_updates(response.updates)
        request.last_received_update = response.update_index
    except grpc.FutureTimeoutError:
        pass</code>

This example demonstrates how clients can use long-polling to receive updates without server-initiated streams.

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