The map-reduce operation is composed of many tasks, including reads from the input collection, executions of the
map function, executions of the
reduce function, writes to a temporary collection during processing, and writes to the output collection.
During the operation, map-reduce takes the following locks:
- The read phase takes a read lock. It yields every 100 documents.
- The insert into the temporary collection takes a write lock for a single write.
- If the output collection does not exist, the creation of the output collection takes a write lock.
- If the output collection exists, then the output actions (i.e.
reduce) take a write lock. This write lock is global, and blocks all operations on the
The final write lock during post-processing makes the results appear atomically. However, output actions
reduce may take minutes to process. For the
nonAtomic flag is available, which releases the lock between writing each output document. See the
db.collection.mapReduce() reference for more information.