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Map-Reduce Concurrency

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.

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 the mongod instance.

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