MongoDB Indexes

http://docs.mongodb.org/manual/core/indexes/

The following discussion refers to the design inside of MongoDB 2.4

Foreground Index Creation

• ensure index holds a write lock on the database until the index is built
• ensure index blocks queries
  • s.a.ensureIndex() blocks s.a.find()
  • s.a.ensureIndex() blocks s.b.find()
• ensure index blocks insertions
  • s.a.ensureIndex() blocks s.a.insert()
  • s.a.ensureIndex() blocks s.b.insert()
• ensure index blocks another ensure index on the same collection
  • s.a.ensureIndex({a:1}) blocks s.a.ensureIndex({b:1})
• ensure index blocks another ensure index on a different collection in the same database
  • s.a.ensureIndex() blocks s.b.ensureIndex()
• ensure index on different databases run in parallel
  • s.a.ensureIndex() runs in parallel with t.a.ensureIndex()

Background Index Creation

• background ensure index does not finish until the index is built
• background ensure index periodically yields the processor by sleeping for awhile
• background ensure index + queries run in parallel
• background ensure index + inserts run in parallel
• multiple background ensure index operations on the same collection run in parallel

Background Index Creation Locking

• The background indexer is run on the client thread.
• A write lock on the database is held while indexing.
• Periodically, the background indexer yields the processor. It releases the database lock before sleeping for awhile and then reacquires it.
• Concurrency of other operations on the database occur when the indexer is sleeping.
• This algorithm is SLOW.