ExpertMedium1 markMultiple Choice
AZ-305 · Question 27 · Domain 2.4: Non-Relational Data Storage
You are designing an Azure Cosmos DB (NoSQL API) database for an IoT fleet management system.
The system receives telemetry from 100,000 vehicles. Each vehicle sends a JSON document every minute. The most common query run by the application is: SELECT * FROM c WHERE c.VehicleId = 'V-12345' AND c.Timestamp > '2023-01-01'.
You need to select a partition key that ensures even distribution of storage and throughput, and minimizes cross-partition queries.
Which property should you choose as the partition key?
You are designing an Azure Cosmos DB (NoSQL API) database for an IoT fleet management system.
The system receives telemetry from 100,000 vehicles. Each vehicle sends a JSON document every minute. The most common query run by the application is: SELECT * FROM c WHERE c.VehicleId = 'V-12345' AND c.Timestamp > '2023-01-01'.
You need to select a partition key that ensures even distribution of storage and throughput, and minimizes cross-partition queries.
Which property should you choose as the partition key?
Answer options:
A.
/Timestamp
B.
/VehicleId
C.
/FleetRegion
D.
/id
How to approach this question
A good partition key needs high cardinality (many unique values) for write distribution, and should be present in the WHERE clause of common queries for read efficiency.
Full Answer
B./VehicleId✓ Correct
/VehicleId
In Cosmos DB, choosing the right partition key is critical. `/VehicleId` is the best choice. It has high cardinality (100,000 vehicles), meaning writes will be evenly distributed across physical partitions, avoiding hot spots. Furthermore, the most common query filters by `VehicleId`. By partitioning on `VehicleId`, Cosmos DB can route the query to a single physical partition, making the read highly efficient. Partitioning by `/Timestamp` would cause all current writes to hit a single partition (hot spot).
Common mistakes
Choosing `/id`. While it distributes writes perfectly, it ruins read performance because querying by `VehicleId` would require scanning every single partition (cross-partition query).
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