DOCS-580 cache FAQ

Author: Bob Grabar

source/faq/fundamentals.txt

@@ -135,6 +135,23 @@ in RAM, MongoDB serves all queries from memory.
 MongoDB does not implement a query cache: MongoDB serves all queries
 directly from the indexes and/or data files.
 
+Does MongoDB require a separate object-caching layer?
+-----------------------------------------------------
+
+No. MongoDB integrates databases with object caches such that an
+object's representation in the database is very close to its
+representation in memory. Retrieval from RAM is a far less expensive
+operation for MongoDB than it is for a traditional RDMS. For an RDMS, a
+high expense is incurred when data must be transformed into an object
+representation when transferred to RAM.
+
+Additionally, MongoDB's integration of databases with memory means there
+is no risk of retrieving stale data from the cache.
+
+Keep in mind, though, that in MongoDB retrieving from RAM is always
+faster than from disk, and it is best to have enough memory to fit all
+of your working data set.
+
 What language is MongoDB written in?
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