Why use your application-level cache if database already provides caching?

Question

Modern database provide caching support. Most of the ORM frameworks cache retrieved data too. Why this duplication is necessary?

Answer 1

Because to get the data from the database's cache, you still have to:

1. Generate the SQL from the ORM's "native" query format
2. Do a network round-trip to the database server
3. Parse the SQL
4. Fetch the data from the cache
5. Serialise the data to the database's over-the-wire format
6. Deserialize the data into the database client library's format
7. Convert the database client librarie's format into language-level objects (i.e. a collection of whatevers)

By caching at the application level, you don't have to do any of that. Typically, it's a simple lookup of an in-memory hashtable. Sometimes (if caching with memcache) there's still a network round-trip, but all of the other stuff no longer happens.

Answer 2

Here are a couple of reasons why you may want this:

• An application caches just what it needs so you should get a better cache hit ratio
• Accessing a local cache will probably be a couple of orders of magnitude faster than accessing the database due to network latency - even with a fast network