DP-203 Design and implement data storage Practice Question
You are designing a solution to store semi-structured JSON logs from a web application in Azure Cosmos DB. The logs are written once and rarely read. The application writes up to 10,000 documents per second, and each document is about 2 KB. You need to minimize RU/s cost. Which API and indexing policy should you choose?
⚠ Common exam trap
Many candidates assume the NoSQL API (Option C) is always the best choice for JSON data, overlooking that its default indexing policy incurs significant RU overhead for write-heavy workloads, whereas the MongoDB API allows disabling indexing to minimize cost.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Azure Cosmos DB for MongoDB with a custom indexing policy that only includes a wildcard index on _id and disables automatic indexing.
Azure Cosmos DB for MongoDB with a custom indexing policy that disables automatic indexing and only includes a wildcard index on _id minimizes RU/s cost for write-heavy, rarely read workloads. Each write operation consumes RUs proportional to the number of indexed paths; by eliminating automatic indexing of all fields, you drastically reduce the per-document write RU charge. The MongoDB API supports this fine-grained indexing control, making it ideal for high-throughput ingestion of semi-structured JSON logs.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Azure Cosmos DB for Cassandra with default indexing policy.
Why it's wrong here
Cassandra API indexes all columns by default, increasing write cost.
- ✗
Azure Cosmos DB for Table with default indexing policy.
Why it's wrong here
Table API indexes all properties, leading to high RU/s for writes.
- ✗
Azure Cosmos DB for NoSQL with default indexing policy (automatic indexing of all fields).
Why it's wrong here
Default indexing indexes every field, consuming significant RU/s for writes.
- ✓
Azure Cosmos DB for MongoDB with a custom indexing policy that only includes a wildcard index on _id and disables automatic indexing.
Why this is correct
Disabling automatic indexing and indexing only _id minimizes RU consumption for writes.
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