Question

To be able to immediately notify a client that something changed, we need a connection to that client. Since web browsers do not traditionally accept connections and clients are usually behind devices that would block such connections anyway, having the server initiate this connection is not practical.

We can arrange for the client to open the connection and keep it around so that the server can use it to send information when it needs to do so.

But an HTTP request allows only a simple flow of information, where the client sends a request, the server comes back with a single response, and that is it. There is a technology called web sockets, supported by modern browsers, which makes it possible to open connections for arbitrary data exchange. But using them properly is somewhat tricky.

In this chapter, we will use a relatively simple technique, long polling, where clients continuously ask the server for new information using regular HTTP requests, and the server simply stalls its answer when it has nothing new to report.

As long as the client makes sure it constantly has a polling request open, it will receive information from the server immediately. For example, if Alice has our skill-sharing application open in her browser, that browser will have made a request for updates and be waiting for a response to that request. When Bob submits a talk on Extreme Downhill Unicycling, the server will notice that Alice is waiting for updates and send information about the new talk as a response to her pending request. Alice’s browser will receive the data and update the screen to show the talk.

To prevent connections from timing out (being aborted because of a lack of activity), long-polling techniques usually set a maximum time for each request, after which the server will respond anyway, even though it has nothing to report, and the client will start a new request. Periodically restarting the request also makes the technique more robust, allowing clients to recover from temporary connection failures or server problems.

A busy server that is using long polling may have thousands of waiting requests, and thus TCP connections, open. Node, which makes it easy to manage many connections without creating a separate thread of control for each one, is a good fit for such a system.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.