MongoDB Concurrency

This wiki describes the concurrency seen by MongoDB applications and how it is implemented.

Observed concurrency

MongoDB allows concurrent queries on any collection in any database

• Query database(a).collection() || Query database(a).collection()
• Query database(a).collection() || Query database(b).collection(), a != b

MongoDB does not allow concurrent insertions to the same database

• Insert database(a).collection() blocks Insert database(a).collection()

MongoDB does allow concurrent insertions to different databases

• Insert database(a).collection() || Insert database(b).collection(), a != b

MongoDB queries block insertions to the same database

• Query database(a).collection() blocks Insert database(a).collection()

MongoDB insertions block queries in the same database

• Insert database(a).collection() blocks Query database(a).collection()

MongoDB allows concurrent insertions and queries in different databases

• Insert database(a).collection() || Query database(b).collection(), a != b
• Query database(a).collection() || Insert database(b).collection(), a != b

MongoDB does not allow concurrent queries with dropping the collection

• Query database(a).collection(b) blocks database(a).collection(b).drop

Locking mechanism

What is the dblocks map? The dblocks map is a map from database name to a reader writer lock.

How is the dblocks map used?

What is the qlock?

How is the qlock used?

Locking examples

Inserts

• Find the rwlock associated with the database name in the dblocks map
• Grab a write lock on database's rwlock
• Grab a little write lock on the qlock

Queries

• Find the rwlock associated with the database name in the dblocks map
• Grab a read lock on the database's rwlock
• Grab a little read lock on the qlock