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in sharded setup each shard contains subset of data, each node doesn't contain all data: https://docs.mongodb.com/manual/core/sharded-cluster-shards/ https://do

• user5994461 8y ago
  In a sharded setup, every shard must be on a 3 nodes cluster for redundancy (a replica set), the 3 nodes keep the exact same data with only one node accepting reads and writes. You have to add capacity (shards) 3 nodes at a time, two third of which sit unused. It's not scalable at all.
  • riku_iki 8y ago
    Its scalable in terms that if your total data doesn't fit into single machine, you can distribute it between shards in mongo, but can't in pgsql. replica set is for redundancy and availability, but you can use replicas for reads, so scale your reads traffic. pgsql works absolutely the same way, you have one master which accepts writes and read-only slaves/replicas.