In most ways WebSockets will probably scale better than AJAX/HTML requests. However, that doesn’t mean WebSockets is a replacement for all uses of AJAX/HTML.
Each TCP connection in itself consumes very little in terms server resources. Often setting up the connection can be expensive but maintaining an idle connection it is almost free. The first limitation that is usually encountered is the maximum number of file descriptors (sockets consume file descriptors) that can be open simultaneously. This often defaults to 1024 but can easily be configured higher.
Ever tried configuring a web server to support tens of thousands of simultaneous AJAX clients? Change those clients into WebSockets clients and it just might be feasible.
HTTP connections, while they don’t create open files or consume port numbers for a long period, are more expensive in just about every other way:
-
Each HTTP connection carries a lot of baggage that isn’t used most of the time: cookies, content type, conetent length, user-agent, server id, date, last-modified, etc. Once a WebSockets connection is established, only the data required by the application needs to be sent back and forth.
-
Typically, HTTP servers are configured to log the start and completion of every HTTP request taking up disk and CPU time. It will become standard to log the start and completion of WebSockets data, but while the WebSockets connection doing duplex transfer there won’t be any additional logging overhead (except by the application/service if it is designed to do so).
-
Typically, interactive applications that use AJAX either continuously poll or use some sort of long-poll mechanism. WebSockets is a much cleaner (and lower resource) way of doing a more event’d model where the server and client notify each other when they have something to report over the existing connection.
-
Most of the popular web servers in production have a pool of processes (or threads) for handling HTTP requests. As pressure increases the size of the pool will be increased because each process/thread handles one HTTP request at a time. Each additional process/thread uses more memory and creating new processes/threads is quite a bit more expensive than creating new socket connections (which those process/threads still have to do). Most of the popular WebSockets server frameworks are going the event’d route which tends to scale and perform better.
The primary benefit of WebSockets will be lower latency connections for interactive web applications. It will scale better and consume less server resources than HTTP AJAX/long-poll (assuming the application/server is designed properly), but IMO lower latency is the primary benefit of WebSockets because it will enable new classes of web applications that are not possible with the current overhead and latency of AJAX/long-poll.
Once the WebSockets standard becomes more finalized and has broader support, it will make sense to use it for most new interactive web applications that need to communicate frequently with the server. For existing interactive web applications it will really depend on how well the current AJAX/long-poll model is working. The effort to convert will be non-trivial so in many cases the cost just won’t be worth the benefit.
Update:
Useful link: 600k concurrent websocket connections on AWS using Node.js