SignalR between initial load and updates

Question

I'm trying to build an application with signalR. The application outline will be:

• Load the initial state from the database
• Subscribe to succeeding messages

I was looking for a set of best practices. If I subscribe to the updates after initial load there is a risk of missing updates. Between initial load and subscription there could be and update and that update I would miss.

Secondly I could use some sort of object state id and then first subscribe to the update stream, retrieve the data and then apply the updates if the objects are behind. This would require quite some code.

Are there any best practices to handle this?

Answer 1

There could be a miss depending on when you query for updates. For instance, say your backend queries for some notifications and pushes this down to the client, then the user connects (subscribes) for notifications - the original data set you pushed could be stale if an update was sent during this init phase and missed.

What I have found, as a best practice, is to scrap sending any initial data. Instead, subscribe as soon as you can and query for updates on a successful connect. for example....

var proxy = $.connection.yourHub;

$.connection.hub.start()
    .done(function () { // <--- immediately call once connected

        // define and call server hub method
        // this will query updates and immediately
        // invoke the associated client function

        proxy.server.queryUpdates();
    })
    .fail(function () {
        console.log('Could not connect!');
    });

proxy.client.serveUpdates = function(updates) {
    // here's your fetched updates on initial connect
}

---

// YourHub.cs
public void QueryUpdates()
{
    var updates = this.yourService.yourQueryUpdatesmethod();

    Clients.Caller.serveUpdates(updates) // this calls proxy.client.serveUpdates
}

This gives you the benefits of establishing your connection, listening for updates, and effectively serving the most fresh set of data in a one-time fashion when you connect.

Note this doesn't have to be a SignalR specific implementation. You could make a typical ajax call in this .done() callback to retrieve your results, but I don't see why you can't re-use all the logic you already have on your hub. You can essentially just craft this to call your existing hub method by explicitly invoking it from the client.