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resources/noat.cards/14-2 Database caching, what to cache.md

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+# Database caching, what to cache
+
+## Introduction
+
+There are multiple levels you can cache that fall into two general categories: database queries and objects:
+
+- Row level
+- Query-level
+- Fully-formed serializable objects
+- Fully-rendered HTML
+
+Generaly, you should try to avoid file-based caching, as it makes cloning and auto-scaling more difficult.
+
+## Caching at the database query level
+
+Whenever you query the database, hash the query as a key and store the result to the cache. This approach suffers from expiration issues:
+
+- Hard to delete a cached result with complex queries
+- If one piece of data changes such as a table cell, you need to delete all cached queries that might include the changed cell
+
+## Caching at the object level
+
+See your data as an object, similar to what you do with your application code. Have your application assemble the dataset from the database into a class instance or a data structure(s) :
+
+- Remove the object from cache if its underlying data has changed
+- Allows for asynchronous processing: workers assemble objects by consuming the latest cached object
+
+Suggestions of what to cache:
+
+- User sessions
+- Fully rendered web pages
+- Activity streams
+- User graph data