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Map-Reduce Concurrency

Note

Aggregation Pipeline as an Alternative to Map-Reduce

Starting in MongoDB 5.0, map-reduce is deprecated:

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 the mongod instance.

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