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
$accumulator
and$function
aggregation 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 themongod
instance.
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.