Question

What are the different ways to cache a web application data, developed using Java and NoSQL database? Databases also provide caching, are they, the only & always the best option to go with, for caching?

How else can I cache my data of users on the application. Application contains very user specific data like in a social network. Are there some simple thumb rules of what type of things should be cached?

Can I also cache my data on the application server using Java ?

Answer 1

I’ll assume you’re building a relatively typical web application that:

1. has a single server used for persistence
2. multiple web servers
3. ties authenticated users to a single server via sticky sessions through a load balancer

Now, with that stated to answer so of your questions. Most persistence, database or NoSQL, likely have some sort of caching built in such that if you execute the same simple query repeatedly (e.g. retrieval by primary key) it’s able to cache the result. However, the more complex the query, the less likely persistence can perform caching on it. In addition, if there’s only one server for persistence (i.e. no sharding, or write master/read slaves) it quickly becomes the bottleneck. So the application level caching you want to do usually should occur on the web servers to reduce load on the database.

As far as what should be cached, the heuristic is items frequently accessed and/or expensive to generate (in terms of database/web server processing/memory). Typical candidates are the home page and any other landing page of a site – often the best approach for these is generating a static file and serving that. The next pieces depend on your application, but typically the most effective strategy is caching as close to the final result as possible – often the HTML being served. For your social network this might be a list of featured updates or some such.

As far as user sessions are concerned, these are definitely a good candidate for caching. In this case you can probably get a lot of mileage out of judicious use of the web server’s session scope (assuming a JSP server). This data lives in memory and is a good place to keep of user specific information shown once a user authenticates on every page (e.g. first and last name).

Now the final thing to consider is dealing with cache invalidation and really is the hard part of all this (naming stuff is the other hard thing in computer science). In this case using something like memcached or ehcache as others have mentioned is the right approach. ehcache can easily run in process with your java application and does a good job of expiring things, with policies for least recently used and least frequently used, and allowing you to use both memory and disk for caching. What you’ll need to think about is the situations where you need to expire something form the cache ahead of this schedule because data’s changed. In this case you need to work through those dependencies in your application’s architecture so that it read/writes to the cache as appropriate.