Stephen A. Fuqua (SAF) is a Bahá'í, software developer, and conservation and interfaith advocate in the DFW area of Texas.

Server Side Push Notifications With SignalR

September 7, 2014

Pushing With SignalR

This style of real-time communication requires either a push or a pull mechanism: is the browser going to pull notifications from the server every few seconds (polling)? Or will the server push notifications to the browser (remote procedure call, RPC)? Polling is considered to be more resource intensive of the two: it requires constantly opening and closing connections, and frequently asking "got anything for me?". If there is a message for the browser on one of every 1000 requests, then that's 999 connections that wasted bandwith on the server (the client might not care, but this adds up on the server over time). Better to keep a single connection open between the browser and client, and push messages out just in time.

SignalR is Microsoft's technology for managing that RPC push. It is an open source .Net (server side) and JavaScript (client side) technology that will work with any modern browser. It does this by detecting the browser capability and choosing the most efficient of several RPC options for that browser. While relatively easy to use, it does have some gotchas that can really trip you up.

Server-Side Push

There are two primary interaction styles: one where the server is pushing out messages, and another where the server is just a proxy for client-to-client messaging (almost acting as an enterprise service bus). Think of a multi-player game or a chat room in the latter case. Interesting, but not what I wanted to know. Therefore I'll stick with the server side. Here are some useful references:

• ASP.NET SignalR Hubs API Guide - Server (C#)
• Tutorial: Server Broadcast with SignalR 2 (Stock Ticker)
• Mapping SignalR Users to Connections. In other words, this gives you the critical knowledge for sending a message to the right person, not just everyone.
• These guides come from Learn About ASP.Net SignalR, which has quite a bit of other important documentation around security, troubleshooting, scalability, and client-side scripting.

Dependency Injection

When your SignalR code needs any outside resource, you will need to set up a DependencyResolver to manage Dependency Injection (DI). Ultimately you just need a simple class that implements IDependencyResolver (the System.Web.Http version, not the System.Web.Mvc interface with the same name and purpose). As shown in the DI link above, it is trivial to build an adapter so that IoC container can act as this resolver. Alternately, you could hard-code any simple dependencies into a custom resolver, but I wouldn't recommend it.

Once you have the resolver, it is easy to use it in a HubConfiguration while starting up the SignalR server:

public class Startup
{
    public void Configuration(IAppBuilder app)
    {
        var config = new HubConfiguration();
        config.Resolver = SomeMethodThatBuildsYourResolver();
        app.MapSignalR(config);
    }
}

But there is a problem with doing this. And it eluded me for quite a while, causing me substantial angst. In the StockTicker tutorial sited above, we are introduced to the GlobalHost for classes outside the Hub to interact with that Hub, and thus to send messages to clients. Once you introduce a DependencyResolver into an individual mapping, the GlobalHost needs to know about it too. Otherwise, it cannot instantiate a hub for you properly. There will be no compile-time or run-time errors. But the system just won't work. If you build the StockTicker example and add a DependencyResolver as shown above, the outcome will be rather boring. One small change will fix that though (thank you, StackOverflow):

public class Startup
{
    public void Configuration(IAppBuilder app)
    {
        var config = new HubConfiguration();
        GlobalHost.DependencyResolver  = SomeMethodThatBuildsYourResolver();
        app.MapSignalR(config);
    }
}

Note that the HubConfiguration is no longer in use. I left it there because there as a reminder that one might like to inquire about other useful settings that can be injected via this object.

Sending to a Single User

The stocker ticker example broadcasts to everyone. How do you send a message to a single user? The references above have a link with more detail. There are several ways to solve this problem. The In-Memory Storage method is a good option, except that you might as well use thread-safe collections and avoid having use a lock. Replace Dictionary with ConcurrentDictionary and HashSet with ConcurrentBag. Or, stick with the locking and use the experimental MultiValueDictionary.

Updating the Client Browser

The Stock Ticker example sends a particular message to the browser. For a little notification count as shown in the Twitter screenshot, we can create a new client-side JavaScript function, adding it to the StockTicker.js file. Here's a stub entry for a new "notify" function:

    // ** TUTORIAL CODE **
    // Add a client-side hub method that the server will call
    ticker.client.updateStockPrice = function (stock) {
        var displayStock = formatStock(stock),
            $row = $(rowTemplate.supplant(displayStock));
$stockTableBody.find('tr[data-symbol=' + stock.Symbol + ']')
            .replaceWith($row);
        }

    // ** NEW CODE **
    ticker.client.notify = function() {
    }

On the server, you can add this to the StockTicker class (leaving LookupConnectionIdFromConcurrentDictionary as an exercise for the reader):

public void NotifyUser(string userToNotify)
{
    var connectionId = LookupConnectionIdFromConcurrentDictionary(userToNotify);
    Clients.Client(connectionId).notify();
}

Back in the browser, obviously you need to do something other than just define an empty function. Let's assume the HTML has a <div id="notification">0<div>. Simply grab the current value and increment it with the help of jQuery:

    ticker.client.notify = function() {
        var div = $("#notification");
        var value = parseInt(div.text());
        div.text(++value);
    }