Map-Reduce Concurrency
Note
Aggregation Pipeline as an Alternative to Map-Reduce
Starting in MongoDB 5.0, map-reduce is deprecated:
Instead of map-reduce, you should use an aggregation pipeline. Aggregation pipelines provide better performance and usability than map-reduce.
You can rewrite map-reduce operations using aggregation pipeline stages, such as
$group,$merge, and others.For map-reduce operations that require custom functionality, you can use the
$accumulatorand$functionaggregation operators, available starting in version 4.4. You can use those operators to define custom aggregation expressions in JavaScript.
For examples of aggregation pipeline alternatives to map-reduce, see:
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.
merge,replace,reduce) take a write lock. This write lock is global, and blocks all operations on themongodinstance.