Thursday, December 17th, 2009
Cache, cache, cache! (Part 2: Architectures)
On Tuesday I focused mainly on memcached and PHP, but today I'll take a wider look at cacheing architectures in general. The main question about defining a cache architecture is to decide where to locate the caching component:
 Status quo, the three-tier architecture
In theory, the commonly accepted standard architecture of a software product is divided into three tiers: the presentation tier, the application tier and finally the data tier. In the context of web applications we rediscover this tiers in the trinity of web server, application server and database server.
In the above diagram we find these three tiers with the user (or in technical terms: the browser) on top of this stack.
 Cache on top
One very obvious idea is to place the cache in front of the web server, between user and web server. Usually we find this architecture in a so called reverse proxy configuration. A reverse proxy is quite easy to set up and has a positive impact for web sites with more static content. But for highly dynamic web applications - like most of today's Web 2.0 applications - the caching benefit of a reverse proxy may be not that big.
In general: having a reverse proxy is better than no caching at all. A reverse proxy will give you always a performance benefit.
 Cache in between of web and application server
Let's move the cache one level down the stack in between web server and application server. On the fist sight this may look like a very good idea, because the cache now protects the application server. But on the second sight you'll realize that this configuration is mostly the same as that one from architecture 2, just without the benefit also caching your web server's data.
For exotic scenarios there may be a good reason for this configuration (esp. in combination with load balancing functionality) but in general you should favor architecture 2 over this one.
 Cache in between of application and database
And another level down in the stack. The cache now sits between application server and database. Again this looks good, and seems to be a good idea - on the first sight. But on the second or third sight you may realize that nearly every database system has its own internal query cache and our cache is only a cache for a cache. And caching a cache is basically never a good idea and can lead to unpredictable, bad consequences.
Another difficulty with this approach is that it's hard to decide when the cache gets dirty (cache jargon for obsolete) and when it's time to clear the cache.
 Cache inside of application
And now half a level up again: right into the application tier. This is the most challenging but also the most powerful place to implement caching strategies. Identify time-critical and frequently accessed data during the development process and implement dedicated and customized caching mechanisms. But don't try do build an abstract, unified, common cache for everything.
It's very important to find a specific and suitable solutions for each kind of data you want to cache in your application. Otherwise you'll will probably just end with another row-based cache for your database (like architecture 4) or some kind of reverse proxy (like architecture 2).
Architectures 2, 3 and 4 can be easily setup by system administration without having to involve development in any way. It's mostly a matter of clever configuration which also may add some load balancing features. In general you'll definitely achieve a better performance of your application, but there is always a given limit by the architecture and scaling quality of your core application.
Architecture 5 is probably the best choice, but - to get best results - needs to be started in an early stage and during the whole development and designing process of your web application you should always have caching in mind. What data is most frequently accessed? What data is expensive (hard to retrieve)? What data depends on user sessions? How up to date does the data need to be?
If you are curious about these questions, please stay tuned for part 3.