Index creation is a :ref:`multistage process <index-build-process>`.
The index creation process uses the commit quorum to minimize replication
lag on secondary nodes.
When a secondary node receives a commitIndexBuild oplog entry, the
node stops further oplog applications until the local index build can be
committed. Index builds can take anywhere from moments to days to
complete, so the replication lag can be significant if the secondary
node builds more slowly than the primary.
To manage the replication lag, the commit quorum delays committing the index build on the primary node until a minimum number of secondaries are also ready to commit the index build.
The commit quorum does not guarantee that indexes on secondaries are ready for use when the command completes. To ensure that a specific number of secondaries are ready for use, set an appropriate :ref:`write concern <write-concern>`.
If a secondary node that is not included in the commit quorum receives
a commitIndexBuild oplog entry, the node may block replication until
its index build is complete.