Index builds in MongoDB can have an adverse impact on the availability of your MongoDB cluster. If you trigger a foreground index build on a large collection on your production server, you may find that your cluster is unresponsive until the index build is complete. On a large collection, this could take several hours or days, as described in the perils of index building in MongoDB.
The recommended best practice is to trigger index builds in the background. However, on large collection indexes, we've seen multiple problems with this approach. In the case of a three-node cluster, both secondaries start building the index and stop responding to any requests. Consequently, the primary does not have quorum and moves to the secondary state, taking your cluster down. Also, the default index builds triggered from the command line are foreground index builds — making this a now widespread problem. In future releases, we're hopeful that this becomes background by default.
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