The correct action is to change the partition key to a synthetic key combining userId and postId. This resolves the Azure Cosmos DB logical partition size limit exceeded issue because a single logical partition cannot exceed 20 GB of storage, and using only userId as the partition key forces all data for a popular user into one partition, creating a hot partition that throttles performance. By introducing a synthetic key like “userId_postId,” you distribute that user’s posts and comments across multiple logical partitions, keeping each under the 20 GB limit and restoring balanced throughput. On the DP-900 exam, this scenario tests your understanding of partition key design and the 20 GB logical partition cap—a common trap is assuming you can simply increase the limit or delete old data, but the correct fix is always to choose a more granular key. Remember the memory tip: “One user, many posts—split the key to split the load.”
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 social media startup stores user profile data, posts, and comments in Azure Cosmos DB. They notice that the logical partition size for a popular user's profile is growing beyond 20 GB, causing performance issues. The current partition key is 'userId'. Which action should they take to solve this?
Change the partition key to a synthetic key combining userId and postId
A synthetic key like 'userId_postId' ensures that each post gets its own logical partition, preventing any single partition from exceeding 20 GB.
B
Increase the RU/s
Why wrong: Increasing RU/s improves throughput but does not resolve the logical partition size limit.
C
Split the container into multiple containers by userId range
Why wrong: Azure Cosmos DB handles physical partition splitting automatically. Manually splitting containers is not supported and would complicate the application.
D
Use a different API like MongoDB
Why wrong: Changing the API does not affect the logical partition size limit, which is a core constraint of Cosmos DB.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Change the partition key to a synthetic key combining userId and postId
A is correct because the logical partition size limit in Azure Cosmos DB is 20 GB. By using a synthetic partition key that combines 'userId' and 'postId', you distribute the data for the popular user across multiple logical partitions, preventing any single partition from exceeding the 20 GB limit and resolving the performance bottleneck.
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.
✓
Change the partition key to a synthetic key combining userId and postId
Why this is correct
A synthetic key like 'userId_postId' ensures that each post gets its own logical partition, preventing any single partition from exceeding 20 GB.
Related concept
Read the scenario before looking for a memorised answer.
✗
Increase the RU/s
Why it's wrong here
Increasing RU/s improves throughput but does not resolve the logical partition size limit.
✗
Split the container into multiple containers by userId range
Why it's wrong here
Azure Cosmos DB handles physical partition splitting automatically. Manually splitting containers is not supported and would complicate the application.
✗
Use a different API like MongoDB
Why it's wrong here
Changing the API does not affect the logical partition size limit, which is a core constraint of Cosmos DB.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse throughput (RU/s) scaling with storage limits, thinking that increasing RU/s will fix a partition size issue, when in fact the 20 GB logical partition limit is a hard storage constraint that requires partition key redesign.
Detailed technical explanation
How to think about this question
Under the hood, Azure Cosmos DB uses hash-based partitioning: the partition key value is hashed to determine the physical partition. A synthetic key like 'userId_postId' creates a composite hash that distributes each post of the popular user across different physical partitions, staying within the 20 GB logical partition limit. In real-world scenarios, this pattern is common for social media apps where a single user generates massive amounts of data (e.g., tweets, comments), and the synthetic key ensures even distribution without redesigning the data model.
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 cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this DP-900 question in full detail.
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: Change the partition key to a synthetic key combining userId and postId — A is correct because the logical partition size limit in Azure Cosmos DB is 20 GB. By using a synthetic partition key that combines 'userId' and 'postId', you distribute the data for the popular user across multiple logical partitions, preventing any single partition from exceeding the 20 GB limit and resolving the performance bottleneck.
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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