A Simple Example: Before Cache

­ Caching is a technology based on the memory subsystem of your computer. The main purpose of a cache is to accelerate your computer while keeping the price of ­the computer low. Caching allows you to do your computer tasks more rapidly.

To understand the basic idea behind a cache system, let's start with a super-simple example that uses a librarian to demonstrate caching concepts. Let's imagine a librarian behind his desk. He is there to give you the books you ask for. For the sake of simplicity, let's say you can't get the books yourself -- you have to ask the librarian for any book you want to read, and he fetches it for you from a set of stacks in a storeroom (the library of congress in Washington, D.C., is set up this way). First, let's start with a librarian without cache.

­The first customer arrives. He asks for the book Moby Dick. The librarian goes into the storeroom, gets the book, returns to the counter and gives the book to the customer. Later, the client comes back to return the book. The librarian takes the book and returns it to the storeroom. He then returns to his counter waiting for another customer. Let's say the next customer asks for Moby Dick (you saw it coming...). The librarian then has to return to the storeroom to get the book he recently handled and give it to the client. Under this model, the librarian has to make a complete round trip to fetch every book -- even very popular ones that are requested frequently. Is there a way to improve the performance of the librarian?

Yes, there's a way -- we can put a cache on the librarian. In the next section, we'll look at this same example but this time, the librarian will use a caching system.