Consistency, Availability, Partition tolerance: pick two during a partition.
When a network partition happens, you have to pick: consistency or availability. You can't have both.
Cassandra for the message store is AP: during a partition, writes succeed on the reachable replicas; reads may briefly miss recent messages on a recovering node
Postgres ledger is CP: money writes through a single primary, so partition means no writes (the alternative, divergent ledgers, is worse than downtime)
Caches are typically AP: under partition you'd rather serve a slightly stale value than fail the read