The answer is a query consuming increasing RU over time, likely due to a hot partition. This is correct because a hot partition occurs when a single logical partition receives a disproportionate share of requests, exhausting its allocated RU budget while other partitions remain underutilized. As the partition’s throughput saturates, the database throttles or queues requests, causing query latency to climb steadily. On the AZ-305 exam, this scenario tests your ability to correlate diagnostic log metrics—specifically rising RU consumption with latency—to identify partition-level bottlenecks. A common trap is to blame indexing or network issues, but the key clue is that RU per execution is also increasing, not just overall load. Remember the memory tip: “Hot partition, high RU fraction” — if one partition’s RU usage spikes while others stay flat, that’s your smoking gun for latency growth.
AZ-305 Design data storage solutions Practice Question
This AZ-305 practice question tests your understanding of design data storage solutions. 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.
Exhibit
{
"query": "AzureDiagnostics | where ResourceProvider == 'MICROSOFT.DOCUMENTDB' | where Category == 'DataPlaneRequests' | summarize avg(DurationMs) by bin(TimeGenerated, 1h), OperationName | order by avg_DurationMs desc",
"resultSample": "TimeGenerated, OperationName, avg_DurationMs\n2024-01-01 10:00:00, Query, 150\n2024-01-01 11:00:00, Query, 300\n2024-01-01 12:00:00, Query, 450"
}
Refer to the exhibit. You run a KQL query against Azure Cosmos DB diagnostics logs. The query shows increasing latency for Query operations over time. Which is the most likely root cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A query is consuming increasing RU over time, possibly due to a hot partition
The query latency increase over time, combined with the fact that the query is consuming more Request Units (RU) per execution, strongly indicates a hot partition. In Azure Cosmos DB, a hot partition occurs when a disproportionate amount of traffic hits a single physical partition, causing that partition's RU budget to be exhausted while others remain underutilized. This leads to increased latency for queries targeting that partition, as the partition's resources become saturated.
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 query is consuming increasing RU over time, possibly due to a hot partition
Why this is correct
Increasing latency indicates inefficient queries or hot partitions consuming more RUs.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The Cosmos DB account has reached its storage limit
Why it's wrong here
Storage limit would cause write errors, not necessarily increasing query latency.
✗
The Cosmos DB account is being throttled due to insufficient RUs
Why it's wrong here
Throttling would show 429 errors, not just increasing latency.
✗
There is network latency between the client and the Cosmos DB endpoint
Why it's wrong here
Network latency would be constant, not increasing over time.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse throttling (Option C) with latency degradation, but throttling is an immediate rejection (HTTP 429), not a gradual latency increase; the key clue is the 'increasing latency over time' combined with 'increasing RU consumption,' which points to a hot partition, not a capacity issue.
Trap categories for this question
Command / output trap
Throttling would show 429 errors, not just increasing latency.
Detailed technical explanation
How to think about this question
Under the hood, Cosmos DB partitions data based on the partition key; a hot partition means that a single logical partition key value (e.g., a popular user ID) receives a disproportionate share of requests, causing that physical partition's RU/s to be consumed faster than others. The query's RU charge increases because the engine must scan more data within that partition to satisfy the query, often due to a lack of a covering index or a filter that doesn't align with the partition key. In real-world scenarios, this is commonly seen in IoT or logging workloads where a single device or timestamp range becomes the bottleneck.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this AZ-305 question in full detail.
Design data storage solutions — This question tests Design data storage solutions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: A query is consuming increasing RU over time, possibly due to a hot partition — The query latency increase over time, combined with the fact that the query is consuming more Request Units (RU) per execution, strongly indicates a hot partition. In Azure Cosmos DB, a hot partition occurs when a disproportionate amount of traffic hits a single physical partition, causing that partition's RU budget to be exhausted while others remain underutilized. This leads to increased latency for queries targeting that partition, as the partition's resources become saturated.
What should I do if I get this AZ-305 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: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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