We are storing lots of data in mongodb let's say 30M docs. And these documents does not get modified very often. There are high number of read queries(~15k qps). And many of these queries(by _id field) will result in empty search result because of the nature of our use case.
I want to understand if mongodb does some sort of optimisation for detecting if a doc is not available in the db,index or not. Are there any plugin to enable this? Other option that I see is to use application level bloom filter but that would be another piece to maintain. AFAIK HBASE has support for bloom filter to see if a document is present or not.
Finding a non-existent document is the worst case of finding a document. Same as in real life, if what you're looking for doesn't exist you'll need more time to check all the places than if the thing existed at some point.
All of the find optimizations apply equally to finding documents that end up not existing (indexes, shard keys, etc.).