Map-Reduce and Sharded Collections
Note
Aggregation Pipeline as an Alternative to Map-Reduce
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.
Map-reduce supports operations on sharded collections, both as an input
and as an output. This section describes the behaviors of
mapReduce specific to sharded collections.
However, starting in version 4.2, MongoDB deprecates the map-reduce
option to create a new sharded collection as well as the use of the
sharded option for map-reduce. To output to a sharded collection,
create the sharded collection first. MongoDB 4.2 also deprecates the
replacement of an existing sharded collection.
Sharded Collection as Input
When using sharded collection as the input for a map-reduce operation,
mongos will automatically dispatch the map-reduce job to
each shard in parallel. There is no special option
required. mongos will wait for jobs on all shards to
finish.
Sharded Collection as Output
If the out field for mapReduce has the sharded
value, MongoDB shards the output collection using the _id field as
the shard key.
Note
Starting in version 4.2, MongoDB deprecates the use of the
sharded option for
mapReduce/db.collection.mapReduce().
To output to a sharded collection:
If the output collection does not exist, create the sharded collection first.
Starting in version 4.2, MongoDB deprecates the map-reduce option to create a new sharded collection and the use of the
shardedoption for map-reduce. As such, to output to a sharded collection, create the sharded collection first.If you did not create the sharded collection first, MongoDB creates and shards the collection on the
_idfield. However, it is recommended that you create the sharded collection first.Starting in version 4.2, MongoDB deprecates the replacement of an existing sharded collection.
Starting in version 4.0, if the output collection already exists but is not sharded, map-reduce fails.
For a new or an empty sharded collection, MongoDB uses the results of the first stage of the map-reduce operation to create the initial chunks distributed among the shards.
mongosdispatches, in parallel, a map-reduce post-processing job to every shard that owns a chunk. During the post-processing, each shard will pull the results for its own chunks from the other shards, run the final reduce/finalize, and write locally to the output collection.
Note
During later map-reduce jobs, MongoDB splits chunks as needed.
Balancing of chunks for the output collection is automatically prevented during post-processing to avoid concurrency issues.