- A
Partition key on email
Why wrong: Email is not the primary access pattern, and point reads by userId would become cross-partition queries, increasing latency and RU consumption.
- B
Partition key on userId
Correct. Partitioning by userId ensures even distribution and efficient point reads. The subscription query can still run as a cross-partition query, which is acceptable for this workload.
- C
Partition key on serviceName (extracted from subscriptions array)
Why wrong: A partition key on serviceName would cause hot partitions if few services are popular, and point reads by userId would become cross-partition, slowing the primary access pattern.
- D
Partition key on a composite key combining userId and serviceName
Why wrong: Azure Cosmos DB does not natively support composite partition keys in the same sense; you would have to concatenate values, which could be complex and still not ideal for the primary point-read pattern.
DP-900 Practice Question: Describe considerations for working with non-relational data on Azure
This DP-900 practice question tests your understanding of describe considerations for working with non-relational data on azure. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A media company stores user profiles in Azure Cosmos DB using the Core (SQL) API. Each profile document contains a userId (unique), name, email, and a subscriptions array containing objects with a serviceName and startDate. The application needs to efficiently retrieve a single user by userId and also run a query to find all users who have a subscription to the service 'PremiumVideo'. Which partition key design is most appropriate for this workload?
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
Partition key on userId
Option B is correct because partitioning on userId ensures each document is evenly distributed across physical partitions, as userId is unique and used for point reads (the most efficient operation in Cosmos DB). The query for users with a 'PremiumVideo' subscription will be a cross-partition query regardless of partition key choice, but the primary workload—retrieving a single user by userId—is optimized with this design. Partitioning on userId also avoids hot partitions and adheres to the best practice of using a high-cardinality, frequently queried field as the partition key.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Partition key on email
Why it's wrong here
Email is not the primary access pattern, and point reads by userId would become cross-partition queries, increasing latency and RU consumption.
- ✓
Partition key on userId
Why this is correct
Correct. Partitioning by userId ensures even distribution and efficient point reads. The subscription query can still run as a cross-partition query, which is acceptable for this workload.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Partition key on serviceName (extracted from subscriptions array)
Why it's wrong here
A partition key on serviceName would cause hot partitions if few services are popular, and point reads by userId would become cross-partition, slowing the primary access pattern.
- ✗
Partition key on a composite key combining userId and serviceName
Why it's wrong here
Azure Cosmos DB does not natively support composite partition keys in the same sense; you would have to concatenate values, which could be complex and still not ideal for the primary point-read pattern.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume partitioning on a frequently queried field like serviceName will optimize the subscription query, but they overlook that Cosmos DB requires the partition key to be a top-level property with high cardinality, and that point reads (by userId) are the most common and cost-sensitive operation in this workload.
Detailed technical explanation
How to think about this question
Under the hood, Cosmos DB uses the partition key to hash documents into physical partitions; a point read (e.g., ReadDocumentAsync with userId and partition key) costs 1 RU if the document is under 1 KB, while cross-partition queries consume at least 2.5 RU plus additional RU per partition touched. The query for users with 'PremiumVideo' subscriptions will always be a cross-partition query because the filter is on a nested array field, not the partition key, so the goal is to minimize RU cost for the more frequent point reads. In real-world scenarios, media companies often have millions of users, and a low-cardinality partition key like serviceName would cause all documents with 'PremiumVideo' to land on the same physical partition, creating a hot partition that throttles requests under high concurrency.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
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FAQ
Questions learners often ask
What does this DP-900 question test?
Describe considerations for working with non-relational data on Azure — This question tests Describe considerations for working with non-relational data on Azure — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Partition key on userId — Option B is correct because partitioning on userId ensures each document is evenly distributed across physical partitions, as userId is unique and used for point reads (the most efficient operation in Cosmos DB). The query for users with a 'PremiumVideo' subscription will be a cross-partition query regardless of partition key choice, but the primary workload—retrieving a single user by userId—is optimized with this design. Partitioning on userId also avoids hot partitions and adheres to the best practice of using a high-cardinality, frequently queried field as the partition key.
What should I do if I get this DP-900 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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