Real-time systems - web
As described in the previous article the web was built over the Internet infrastructure and protocols by adding new technologies: in particular the web browser on the client, the HTTP protocol and web server (HTTP Server) technology. HTTP is a stateless request/reponse protocol which was designed around the idea of requesting and serving web pages. The client browser requests a resource identified by a URL and the web server processes the request and responds with an HTTP response. While this works for simple web pages, it is not a good basis for large, real-time data transfers, or even the paradigm where the server pushes real-time data to the browser. Some technologies were created that addressed this need for updated data in primitive ways - for example the Atom protocol allows a client to periodically check for updated data to update the content of a news feed. This works for relatively infrequently updated information such as blog posts, but it not designed for continual real-time data streams.
Before the real-time web
Going back to 2003 there were real-time applications to be found on the web. Excite Chat was a very popular chat program. There were also messenger applications such as Yahoo Messenger, MSN Messenger, ICQ, and others. Skype was an exciting voice and textual communications program - and the new kid on the block. There were also social media sites such as Friendster, Multiply, and ICQ Lovematch.
These applications were typically implemented using traditional Internet technologies. They were often native client programs implemented in C, with C-based server code. The chat protocols were often proprietary, while using traditional Internet protocols on the lower layers of the stack. Excite Chat used TCP for a persistent connection between the chat client, which could be a stand alone desktop app or a Java applet running within the web browser. To scale the chat servers, multiplexers whose sole task was to manage the socket connections between the client and server were used. The multiplexers would concentrate the chat messages from multiple clients into a large packet which would then be passed on to the master chat server via a single TCP socket connection. Login and authentication was managed by additional servers connected to SQL databases. This architecture enabled the Exite Chat master servers to scale to 100,000 client connections. While amazing at the time it falls short of the scale of chat today, where Viber has 400 million users and Line has over 200 million active users.
Smart phones at this time were not widely available and iOS and Android did not yet exist.
So, while real-time applications were common, the technologies used for implementation tended to based around traditional programming technologies such as TCP/IP sockets.
Introducing the real-time web
The introduction of web chat applications (which evolved into web apps such as Slack), and social media sites such as FaceBook and Twitter heralded the beginning of the real-time web. In early implementations it was necessary to reload a web page in the browser to see your updates. The user would therefore have to periodically poll the web server to check for status updates. This is not user friendly - it is far more efficient for the user's activity feed to update in real-time (as and when events occur) without the user having to reload a page.
Users quickly became used to doing everything in the web browser, and became reluctant to install additional clients for certain applications. Also for web application developers, it was far better to create applications that could present information using the browser, but also now incorporate real-time updates.
For example, you might have a web application where the user checked the value of an investment portfolio and received real-time updates of key prices while viewing the value of the assets. Another application might be where you create a document on a server and are able to collaborate with several other team members, all editing the document in real-time. The status/activity feeds of social media sites also represents another key use-case of the real-time web. A Twitter feed will automatically update showing the user that there are new tweets available to read without the user having to reload the page. Rather than a request-reponse paradigm, the server would now push out data in real-time to a multitude of client browsers and in recent years native apps running on smartphones.
Real-time web technologies
There are many real-time web technologies out there today. One key part of the technology is the use of a protocol between client and server that moves away from the HTTP protocol and its inherent problems for real-time applications. The solution is to use a protocol such as web sockets. With web sockets HTTP is used to open a full-duplex TCP stream between the client and the server. There are then many tools and libraries for accessing this stream from the web app and these can leverage several communication models such as publish-subscribe (pubsub).
This article will not go into the details of using these technologies as they are often language dependent. An example is the use of socket.io in the node.js world. It is typical to find the use of an event driven model in building real-time web applications.
In addition to language libraries and communication models there are also database systems targeting real-time web applications. These databases eschew the more traditional SQL query request-response model and instead adopt a push model where data, usually in JSON format, is broadcast to clients. Firebase for example allows data to be synched out to clients on a global scale. RethinkDB is another example where based on a query, data is continually pushed to clients in real-time.
Some examples of real-time web applications include:
- Collaborative web and mobile apps
- Slack and similar web chat apps
- Streaming analytics apps
- Multiplayer games
- Real-time marketplaces
- Connected devices
- Trading platforms
- Real-time information displays (e.g. train times on a web site)
- Web applications that represent real-time data form the Internet of Things (IoT)
A personal favourite is Flight Radar 24.
This article has touched on aspects of the real-time web. Typically a connection-oriented full-duplex stream model using TCP is established via a web sockets interface, the stateless request-response HTTP protocol being unsuitable for real-time web apps.
In the next article I will take a brief look at the Internet of Things (IoT), with potential application of real-time web technologies, and some of the scaling challenges inherent is such a vast network.