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Map-Reduce Concurrency
Note
Aggregation Pipeline as an Alternative to Map-Reduce
As of MongoDB 5.0 the map-reduce operation is deprecated.
An aggregation pipeline provides better performance and usability than a map-reduce operation.
Map-reduce operations can be rewritten using aggregation pipeline
operators, such as
$group, $merge, and others.
For map-reduce operations that require custom functionality, MongoDB
provides the $accumulator and $function
aggregation operators starting in version 4.4. Use these operators to
define custom aggregation expressions in JavaScript.
For examples of aggregation pipeline alternatives to map-reduce operations, see Map-Reduce to Aggregation Pipeline and Map-Reduce Examples.
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.
Note
The final write lock during post-processing makes the results appear
atomically. However, output actions merge and reduce may
take minutes to process. For the merge and reduce, the
nonAtomic flag is available, which releases the lock between
writing each output
document. Starting in MongoDB 4.2, explicitly setting nonAtomic:
false is deprecated. See the db.collection.mapReduce()
reference for more information.