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Hi all. In the 4.4 release, mapReduce was re-implemented to use the aggregation framework. Under the hood, it is implemented using the machinery for $out/$merge. As a result, this project has allowed non-inline mapReduce operations to run on secondaries. As such, I am resolving this ticket as I believe it has been completed.
However, since we want to encourage applications to use aggregation instead of mapReduce, we do not intend to add non-inline mapReduce on secondaries as a supported feature. Drivers and the shell should continue to ignore non-primary read preference for non-inline mapReduce operations. Mongos will always send non-inline mapReduce to shard primaries regardless of the read preference. Also, causal consistency for mapReduce on secondaries remains explicitly banned.
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