xConnect Scalable Reads
The xConnect Scalable Reads feature lets you offload the primary database from read-only workloads using one of the read-only/secondary replicas.
It is a configured replication for the collection database shard cluster. The read scale-out allows a load balance of complex read workloads without affecting the primary OLTP workload.
xConnect read scale-out is available for both SQL On-Prem and SQL Azure collection database.
The MongoDB provider does not support scalable reads.