The answer is deviceID. This is the correct partition key because it directly aligns with the most common query pattern—retrieving all sensor readings for a specific device within a time range—ensuring that all data for a single device resides in the same logical partition, which minimizes cross-partition queries and reduces Request Units (RU) consumption. Additionally, using deviceID for Azure Cosmos DB partition key selection in IoT scenarios provides even data distribution across physical partitions, as thousands of devices generate roughly equal amounts of data. On the Microsoft Azure Data Fundamentals DP-900 exam, this question tests your understanding of how partition key choice directly impacts query efficiency and cost; a common trap is choosing timestamp, which would scatter data across partitions and force expensive fan-out queries. Remember the memory tip: “Partition by the filter, not the time”—if your query always starts with deviceID, your partition key should too.
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 smart home company stores sensor readings from thousands of devices in Azure Cosmos DB. Each reading includes a deviceID, timestamp (ISO format), sensor type, and value. The most common query retrieves all readings for a specific device within a time range. To minimize Request Units (RU) consumption and ensure even data distribution, 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.
deviceID has high cardinality (many unique values) and is always used in the query filter, leading to efficient partition routing and even distribution.
B
B) timestamp
Why wrong: Using timestamp as partition key would create a new partition per second/minute, but queries for a time range would hit all partitions, and it could cause hot partitions on current time.
C
C) sensor type
Why wrong: Sensor type has low cardinality (few distinct values) leading to large partitions and uneven distribution, not meeting the 'even distribution' requirement.
D
D) value
Why wrong: Value is continuous and not used in where clauses; it would not route queries efficiently and could cause skewed partitions.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A) deviceID
DeviceID is the correct partition key because it is the primary filter in the most common query (all readings for a specific device within a time range). Partitioning by deviceID ensures that all readings for a single device are stored in the same logical partition, making queries highly efficient by targeting a single partition. It also provides even data distribution across physical partitions, as thousands of devices will have roughly equal numbers of readings, minimizing RU consumption.
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.
✓
A) deviceID
Why this is correct
deviceID has high cardinality (many unique values) and is always used in the query filter, leading to efficient partition routing and even distribution.
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.
✗
B) timestamp
Why it's wrong here
Using timestamp as partition key would create a new partition per second/minute, but queries for a time range would hit all partitions, and it could cause hot partitions on current time.
✗
C) sensor type
Why it's wrong here
Sensor type has low cardinality (few distinct values) leading to large partitions and uneven distribution, not meeting the 'even distribution' requirement.
✗
D) value
Why it's wrong here
Value is continuous and not used in where clauses; it would not route queries efficiently and could cause skewed partitions.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often choose timestamp because they think it naturally orders data by time, but they overlook that the most common query filters by deviceID first, and using timestamp as the partition key would cause cross-partition queries for every device-specific time range, dramatically increasing RU costs.
Detailed technical explanation
How to think about this question
In Azure Cosmos DB, the partition key determines the logical partition where data is stored; all items with the same partition key value reside in the same logical partition, which is then mapped to a physical partition. For time-range queries on a specific device, combining deviceID as the partition key with a timestamp as the sort key (using the 'ORDER BY' or range filter) allows Cosmos DB to efficiently serve the query from a single physical partition, leveraging the index on timestamp within that partition. A real-world scenario: a smart home company with 10,000 devices each sending 100 readings per day would see each device's data stored together, enabling fast retrieval without cross-partition queries.
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: A) deviceID — DeviceID is the correct partition key because it is the primary filter in the most common query (all readings for a specific device within a time range). Partitioning by deviceID ensures that all readings for a single device are stored in the same logical partition, making queries highly efficient by targeting a single partition. It also provides even data distribution across physical partitions, as thousands of devices will have roughly equal numbers of readings, minimizing RU consumption.
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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