For map-reduce operations that require custom functionality, MongoDB
aggregation operators starting in version 4.4. Use these operators to
The map-reduce operation is composed of many tasks, including reads
from the input collection, executions of the
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
take minutes to process. For the
nonAtomic flag is available, which releases the lock between
writing each output
document. Starting in MongoDB 4.2, explicitly setting
false is deprecated. See the
reference for more information.