Do One Thing, but Do It Really Well
In life there are people who will consider themselves Jacks of all Trades. But as the saying goes, they are master of none.
Websites will always start out as a codebase that does everything. There will be a couple files that add users, encode video to Flash, pull rss feeds, assemble HTML, update products, charge users, manipulate images, redirect old links, handle file uploads, calculate shipping, delete categories, create rss feeds, search the database, etc. Sometimes the code to do these will be organized into files, sometimes it won’t. The whole site will run on a single server, or more likely, a slice of a single server.
None of the things in the above list will be done well. None. This is mostly because there is too little code and too little hardware focused on doing too much. Also, every piece of code will be tightly coupled. So any one of those features could potentially get a ton of traffic, or hit a bump, and consume a ton of resources. Once that happens, it’s safe to assume the whole thing will go down in flames.
So to avoid the Fail Whale, its really important to build sites as a group of components that work together. Architecture is key, and when carefully thought out, can ensure that the most important parts of the site stay up. Even when your image manipulation script on the backend freaks out, the home page should continue to load flawlessly.
With database-driven apps (almost every major site on the web), there needs to be particular attention paid to a caching layer. Again, since most sites start out with a jumbled codebase, the likelihood that all the code to manage data is in the same place is unlikely. Given the complexities of managing cache objects, making sure that objects are invalidated on update is crucial to making updates look seamless. So there needs to be a set of code that’s good at one thing: managing data and its cache.
Search is another area that commonly relies on database, and can eat a ton of resources. If performing search in SQL, difficult queries can lock tables and keep other queries from being answered. As good as some DBMSes have gotten at handling search ( ie MySQL’s FULLTEXT ), they still can’t fulfill the concurrency demands of a site with heavy traffic. So, again, the solution is a change where a resource intensice feature needs to be isolated from other code. There are a few different ways to do this. One is running replication, which may not be possible in smaller hosting environments. Another is to use Full Text Search (Lucene, Sphinx, etc.) Again, this may not be possible in smaller hosting environments.
Using code that’s already good at managing and retrieving data, an interface can be built to query your data. A second hosting environment that’s suitable for running the search tool of your choice can then query the data code for updates it needs to keep itself updated. In turn, this server will return search results without tying up any resources necessary for doing important stuff, like serving the home page.
So in these two short examples, we’ve created a theoretical architecture that can sustain heavy, site-breaking traffic to the search, and still continue to serve the home page. Of course, until the Apache server becomes so inundated with requests that it can’t do anything. Then it’s time to get that load balancer in place…