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Question 109

A company's operations are supported by an Azure Cosmos DB. The database is configured for single master replication across the North Central US and South Central US regions. The database is configured with Staleness bound consistency as its default consistency.

You need to determine how this impacts your company's recovery objectives in case of a catastrophic regional failure.

What is the guaranteed recovery time objective (RTO) for this configuration?

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Explanation

The RTO for this configuration is < 15 minutes. The same value applies for Session, Consistent Prefix, and Eventual consistency for single master replication over multiple regions. It also applies to Strong consistency with single or multi-master replication.

RTO and recovery point objective (RPO) are key considerations in calculating recovery. RTO refers to maximum time a resource may be down after a catastrophic failure. RPO is related to allowable data loss and is the age of backups that must be recovered to restore data.

Supported consistency levels are:

* Strong - Reads always return the most recent committed version of an item.

* Bounded staleness - Reads might lag behind writes based on configured update versions (K) or time (t).

* Session - Scoped to a client session, and reads honor consistency guarantees including the consistent-prefix, monotonic reads, monotonic writes, read-your-writes, and write-follows-reads guarantees.

* Consistent prefix - Updates that are returned contain some prefix of all the updates, and reads never see out-of-order writes.

* Eventual - There is no ordering guarantee for reads, and replicas eventually converge.

RPO for the scenario configuration is based on the K and t values.

Multi-master replication over multiple regions provides an RTO of 0 minutes for all consistencies except Strong.

When deployed in a single region, the RTO is < 1 week for any consistency.

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