如何将具有很多属性的数据库数据存储到缓存中？

Question

Let's say that I have a table with columns TABLE_ID, CUSTOMER_ID, ACCOUNT_NUMBER, PURCHASE_DATE, PRODUCT_CATEGORY, PRODUCT_PRICE. This table contains all purchases made in some store.

Please don't concentrate on changing the database model (there are obvious improvement possibilities) because this is a made-up example and I can't change the actual database model , which is far from perfect. The only thing I can change is the code which uses the already existing database model.

Now, I don't want to access the database all the time, so I have to store the data into cache and then read it from there. The problem is, my program has to support all sorts of things:

1. What is the total value of purchases made by customer X on date Y?
2. What is the total value of purchases made for products from category X?
3. Give me a list of total amounts spent grouped by customer_id.

etc.

I have to be able to preserve this hierarchy in my cache.
One possible solution is to have a map inside a map inside a map... etc.
However, that gets messy very quickly, because I need an extra nesting level for every attribute in the table.

Is there a smarter way to do this?

Answer 1

Have you already established that you need a cache? Are you sure the performance of your application requires it? The database itself can optimize queries, have things in memory, etc.

If you're sure you need a cache, you also need to think about cache invalidation: is the data changing from beneath your feet, ie is another process changing the data in the database, or is the database data immutable, or is your application the only process modifying your data.

What do you want your cache to do? Just keep track of queries and results that have been requested so the second time a query is run, you can return the result from the cache? Or do you want to aggressively pre calculate some aggregates? Can the cache data fit into your app memory or do you want to use ReferenceMaps for example that shrink when memory gets tight?

For your actual question, why do you need maps inside maps? You probably should design something that's closer to your business model, and store objects that represent the data in a meaningful way. You could have each query (PurchasesByCustomer, PurchasesByCategory) represented as an object and store them in different maps so you get some type safety. Similarly don't use maps for the result but the actual objects you want.

Sorry, your question is quite vague, but hopefully I've given you some food for thoughts.