Build Admin feature that requests heartbeat from all connected clients.

[r002]

Problem

SSE doesn't automatically detect if a client has died. This means over time our clientMap will be polluted with zombie clients. Cycling the server will request an auto-reconnect (supported by SSE out of the box) but we need another way to prune dead clients from clientMap.

Solution

Add a REST endpoint that, when hit, requests a heartbeat from all connected clients that are still alive. Any client that doesn't pong back within X seconds will then be removed from clientMap. [Should this REST endpoint be a GET or POST?]

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Tasks:

• Put the clientMap into a beautiful table.
• Add the Prune zombie clients! button & function to the admin GUI.

[r002]

Today's Progress

• "Broadcast Flash Message" function now works. From the Admin GUI, I can blast flash messages to all connected clients.
• "Request heartbeat from all clients" function now works. From the Admin GUI, I can request all connected clients heartbeat back to the server. Here's an example:

Here, I just clicked on the "Request heartbeat..." link and Connections No 3 & 4 updated their "Last Heartbeat" timestamps. Conn No 1 & 2 didn't update though because those clients have been closed (either due to connectivity loss of the Chrome tab was force-closed, etc).

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Next Steps

• I need to implement a loop on the server that periodically runs when clientMap.size() > 0 that evicts stale connections. Update the status field for each client in the server's clientMap. Do the "stale" vs "fresh" ms delta calculation on the server.

[r002]

Next

• Fix: clientMap should not be broadcasted on every heartbeat! If two clients are connected, then clientMap will be broadcasted to all endpoints every REFRESH_INTERVAL.

Not a bug: clientMap on the Admin GUI is 10 seconds behind the ts that's being broadcast to all client endpoints. Investigate and fix.

clientMap should only be broadcasted to all endpoints in the following cases:

1. The "refresh_map" loop is firing.
2. Client is added? (Or wait until the loop fires?)
3. Client is removed? (Or wait until the loop fires?)