Exam DP-420 topic 1 question 41 discussion

Throughput Optimization: Session consistency offers a balance between strong consistency and high throughput, which makes it a good option for applications that require predictable performance without significantly increasing the Request Unit (RU) cost. Highest Consistency Guarantees: Among the weaker consistency levels (below Strong and Bounded Staleness), Session consistency provides strong consistency guarantees within the session context of a single user. It ensures monotonic reads, monotonic writes, and read-your-own-writes for requests within the same session. While Strong (A) and Bounded Staleness (B) offer higher consistency guarantees than Session, they tend to use more RUs, potentially reducing throughput. Eventual (default) and Consistent Prefix (D) have higher throughput but offer weaker consistency guarantees compared to Session.

Consistent Prefix: Consistent prefix guarantees that all reads will see writes in the order they were made. It's the strongest consistency level that still allows for high throughput. It's stronger than session consistency because it guarantees global ordering of writes, not just within a session.

Consistent prefix will not increase the RU consumption whilst providing better consistency guarantee than session and eventual. the other two increase RU consumption and therefore incorrect.

For strong and bounded staleness, reads are done against two replicas in a four replica set (minority quorum) to provide consistency guarantees. Session, consistent prefix and eventual do single replica reads. The result is that, for the same number of request units, read throughput for strong and bounded staleness is half of the other consistency levels. https://learn.microsoft.com/en-us/azure/cosmos-db/consistency-levels#consistency-levels-and-throughput

Answer is D - Consistent preffix. Why ? We need to provide the highest consistency guarantees without decreasing throughput. Strong and bounded staleness provide half the throughput of Session, Consistent Prefix and Eventual, so by choosing them you decrease the throughput.

https://learn.microsoft.com/en-us/azure/cosmos-db/consistency-levels

highest consistency guarantees -> strong

"Strong" is the answer as it needs guaranteed consistency