Module 4Growing Past One Machine
Most systems never need more than one node. Sharding splits a collection’s points across nodes, so each node holds a slice. Replication keeps a copy of each shard on more than one node, so search survives losing one.
Use them when one node cannot hold the collection, or when search must continue after a node fails. The two differ in what they cost to add: a replica is usually a live change, while resharding an existing collection moves data. If search is slow, measure and tune the index before adding nodes. Adding nodes costs more, and it will not make an unindexed filter faster. Distributed Deployment covers both.