DP-420 Question #198: Real Exam Question with Answer & Explanation

Question

You have an Azure subscription. The subscription contains an Azure Cosmos DB for NoSQL account named account1 that uses strong consistency and is deployed to the East US, West US, and Central US Azure regions. The round-trip time (RTT) between the regions is shown in the following table. Which write latency should you estimate for write requests against account1?

Options

• A150 ms
• B232 ms
• C282 ms
• D572 ms

Explanation

Calculate two times the maximum round-trip time (RTT) + 10 ms overhead. In this scenario this would be 2 x 136 ms + 10 ms, which is 282 ms. Note: For an Azure Cosmos DB account with strong consistency and multiple regions, write latency is approximately two times the round-trip time (RTT) between the two farthest regions. This is because a write must be committed to all regions to be considered complete, requiring the data to be sent to the farthest region and then confirmed back to the originating region. Calculation and factors Core calculation: The write latency is essentially 2 x RTT + constanst overhead. For example, if the RTT between the two farthest regions is 50 ms, the write latency would be roughly 2 x 50 ms + overhead=100 ms + overhead. Overhead: In practice, you must also add a small constant overhead, which is approximately 10 ms at the 99th percentile for strong consistency, to account for the time it takes for the write to be acknowledged after the data reaches all replicas. https://learn.microsoft.com/en-us/azure/networking/azure-network-latency

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