Recipe:

Create a global Cassandra cluster with regional datacenters.

Use one keyspace per region

Use per-keyspace replication to only replicate that region's data locally, and to one or more additional datacenters

Have stateless app servers colocated with Cassandra in each DC handling all local traffic

Run spark on top of Cassandra to do analytics, or to do the etl to a dedicated analytics system

Optionally have a single "master" DC, with replicas of all data from all locations, that doesn't serve end user traffic, but is to allow efficient cross region analytics.

Profit (optional step)

And yes, the company I work for (Datastax) has a product and services to help make it simple.