The answer is UserID. This is the correct partition key because the most common query pattern filters on UserID, and Azure Cosmos DB can route the query directly to the single physical partition containing that user’s data, avoiding costly cross-partition fan-out and minimizing Request Unit consumption. On the Microsoft Azure Data Fundamentals DP-900 exam, this scenario tests your understanding that partition key selection must align with the primary query filter to achieve efficient, single-partition queries. A common trap is choosing BookingID because it is unique, but that would scatter each booking across partitions, forcing a fan-out for every UserID-based query. Remember the memory tip: “Filter first, partition key follows”—always pick the property you filter by most often, not the one that is simply unique.
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 travel booking application stores booking data in Azure Cosmos DB using the NoSQL API. Each booking document contains: BookingID (unique), UserID, Destination, TravelDate, Price. The most common query is: 'Retrieve all bookings for a specific UserID, sorted by TravelDate descending.' To minimize Request Unit (RU) consumption, which property should be chosen as the partition key?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "minimum / minimize"
Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
Why wrong: BookingID is unique, so queries by UserID would become cross-partition scans, increasing RU consumption as the service must fan out to all partitions.
B
UserID
UserID is the filter in the common query. With UserID as partition key, all bookings for a user reside in one partition, making queries efficient and reducing RU consumption.
C
Destination
Why wrong: Destination may group bookings from multiple users, but the query filters by UserID, not Destination. Using Destination would lead to cross-partition queries for UserID filters.
D
TravelDate
Why wrong: TravelDate has high cardinality but is not the filter field. Queries filtering by UserID would still need to access multiple partitions, increasing RU consumption.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
UserID
UserID is the correct partition key because the most common query filters on UserID, and Cosmos DB routes queries to the exact physical partition(s) containing that UserID's data. This minimizes cross-partition fan-out, reducing RU consumption. A partition key should align with the primary query filter to enable efficient point-read or single-partition query execution.
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.
✗
BookingID
Why it's wrong here
BookingID is unique, so queries by UserID would become cross-partition scans, increasing RU consumption as the service must fan out to all partitions.
✓
UserID
Why this is correct
UserID is the filter in the common query. With UserID as partition key, all bookings for a user reside in one partition, making queries efficient and reducing RU consumption.
Clue confirmation
The clue word "minimum / minimize" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Destination
Why it's wrong here
Destination may group bookings from multiple users, but the query filters by UserID, not Destination. Using Destination would lead to cross-partition queries for UserID filters.
✗
TravelDate
Why it's wrong here
TravelDate has high cardinality but is not the filter field. Queries filtering by UserID would still need to access multiple partitions, increasing RU consumption.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often pick a high-cardinality key like BookingID or a date-based key like TravelDate, thinking uniqueness or time-ordering helps, but they ignore that the partition key must match the most frequent query filter to avoid cross-partition queries and high RU costs.
Detailed technical explanation
How to think about this question
Cosmos DB uses the partition key hash to distribute data across logical partitions, each stored on a physical partition. When a query includes the partition key in the filter, the query engine can route directly to the partition(s) containing that key value, avoiding a full scan. For the query 'WHERE UserID = X ORDER BY TravelDate DESC', using UserID as the partition key ensures all documents for that user reside in the same logical partition, and the ORDER BY can be served from an index within that single partition without cross-partition sorting overhead.
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: UserID — UserID is the correct partition key because the most common query filters on UserID, and Cosmos DB routes queries to the exact physical partition(s) containing that UserID's data. This minimizes cross-partition fan-out, reducing RU consumption. A partition key should align with the primary query filter to enable efficient point-read or single-partition query execution.
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.
Are there clue words in this question I should notice?
Yes — watch for: "minimum / minimize". Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A travel booking application stores user itineraries in Azure Cosmos DB using the NoSQL API. Each itinerary document contains: UserID (unique to user), ItineraryID, Destination, BookingDate, and a nested array of Activities. The most common query is: 'Retrieve all itineraries for a specific UserID sorted by BookingDate descending.' To minimize Request Unit (RU) consumption, which partition key should be chosen?
medium
A.ItineraryID
✓ B.UserID
C.Destination
D.BookingDate
Why B: UserID is the correct partition key because the most common query filters on UserID, ensuring that all itineraries for a specific user are stored in the same physical partition. This allows the query to target a single partition, minimizing cross-partition queries and reducing Request Unit (RU) consumption. A partition key that aligns with the primary query filter is essential for optimal performance and cost efficiency in Azure Cosmos DB.
Variation 2. A ride-sharing application uses Azure Cosmos DB for trip data. Each trip record contains TripID (unique), DriverID, RiderID, TripDate, and other details. The most common query retrieves all trips for a specific driver within a given date range. Which partition key should be chosen to minimize Request Unit (RU) consumption and ensure even data distribution?
medium
A.TripID
✓ B.DriverID
C.TripDate
D.RiderID
Why B: DriverID is the optimal partition key because the most common query filters on DriverID and a date range. Partitioning by DriverID ensures that all trips for a specific driver are stored in the same physical partition, making the query a single-partition operation that consumes minimal Request Units (RUs). It also provides even data distribution across partitions because each driver generates a roughly similar number of trips, avoiding hot spots.
Variation 3. A social media application stores user posts in Azure Cosmos DB using the NoSQL API. Each document includes: PostID (unique), UserID, Timestamp, Content. The most common query is: 'Get all posts for a specific UserID, sorted by Timestamp descending.' Which partition key should be chosen to distribute load evenly across physical partitions while also supporting this query efficiently?
medium
A.PostID
✓ B.UserID
C.Timestamp
D.Content
Why B: UserID is the correct partition key because it evenly distributes write operations across physical partitions (each user has a unique ID) and directly supports the most common query: filtering by UserID. With UserID as the partition key, the query 'Get all posts for a specific UserID, sorted by Timestamp descending' becomes a single-partition query (using the partition key in the WHERE clause), which is efficient and avoids cross-partition fan-out. This design also allows Cosmos DB to use the Timestamp field as a sort key within each logical partition, enabling efficient sorting without additional indexing overhead.
Variation 4. A social media application stores user profiles in Azure Cosmos DB using the NoSQL API. Each profile includes UserID, Name, Email, and an array of Posts. The most common query retrieves a user's profile by UserID. The application requires strong consistency for writes so that once a profile is updated, all subsequent reads see the latest data. To minimize Request Unit (RU) consumption, which partition key should be chosen?
medium
✓ A.UserID
B.Email
C.Name
D.A synthetic partition key combining UserID and Region
Why A: UserID is the correct partition key because it is the primary filter in the most common query (retrieving a profile by UserID), ensuring each query targets a single logical partition. This minimizes cross-partition queries and RU consumption. Additionally, UserID provides high cardinality and even distribution, which prevents hot partitions and supports the required strong consistency for writes.
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