The answer is Azure Database for PostgreSQL - Hyperscale (Citus). This service is the correct choice because it delivers a horizontally scalable relational database that maintains full SQL support, including complex joins and ACID transactions, by using a coordinator node to distribute data across multiple worker nodes for parallelized writes. On the Microsoft Azure Solutions Architect Expert AZ-305 exam, this scenario tests your ability to distinguish between services that scale vertically, like Azure SQL Database, and those that scale horizontally while preserving relational integrity—a common trap is assuming Azure SQL Database’s elastic pools provide horizontal scaling, but they only pool resources vertically. For write-intensive analytics requiring sharding and relational consistency, Citus is the only Azure managed database that combines horizontal scale with full SQL and ACID compliance. Memory tip: think “Citus = Shards + SQL + ACID” to recall it’s the relational database that truly scales out.
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.
A company runs a data analytics application that stores large volumes of structured data in a relational format. The data is write-intensive and the application needs to scale horizontally for high throughput. The solution must support SQL queries, including joins and ACID transactions. Which Azure database service should they choose?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Azure Database for PostgreSQL - Hyperscale (Citus)
Azure Database for PostgreSQL - Hyperscale (Citus) is correct because it provides horizontal scaling (sharding) across multiple nodes while preserving full SQL support, including JOINs and ACID transactions. Citus distributes data across worker nodes using a coordinator node, enabling write-intensive workloads to achieve high throughput through parallelized writes. This makes it ideal for large-volume, relational, write-heavy analytics applications that require relational integrity.
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.
✓
Azure Database for PostgreSQL - Hyperscale (Citus)
Why this is correct
Citus is a managed PostgreSQL service that scales out horizontally across multiple nodes and supports full SQL, joins, and ACID transactions.
Related concept
Read the scenario before looking for a memorised answer.
✗
Azure SQL Database Hyperscale
Why it's wrong here
Hyperscale is a single database with read scale-out but does not provide horizontal sharding across multiple databases.
✗
Azure Cosmos DB (SQL API)
Why it's wrong here
Cosmos DB is a NoSQL database; it does not support SQL joins or ACID transactions across partitions.
✗
Azure Synapse Analytics
Why it's wrong here
Synapse is a data warehouse optimized for analytical queries, not for high-transaction OLTP workloads.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse Azure SQL Database Hyperscale's 'scale-out' read replicas with true horizontal write scaling, or they assume Cosmos DB's SQL API supports relational queries and transactions, when in fact it is a NoSQL store with limited consistency and no JOIN support.
Detailed technical explanation
How to think about this question
Citus achieves horizontal scaling by distributing table rows across worker nodes using a hash-based sharding strategy, while the coordinator node routes queries and manages distributed transactions via two-phase commit (2PC) for ACID compliance. Under the hood, it uses PostgreSQL's native MVCC (Multi-Version Concurrency Control) to handle concurrent writes without locking, and the shard rebalancer can redistribute data online to maintain performance. In a real-world scenario, a time-series analytics application ingesting millions of events per second can use Citus to partition data by time and tenant, ensuring both high write throughput and the ability to run complex JOINs across shards.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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: Azure Database for PostgreSQL - Hyperscale (Citus) — Azure Database for PostgreSQL - Hyperscale (Citus) is correct because it provides horizontal scaling (sharding) across multiple nodes while preserving full SQL support, including JOINs and ACID transactions. Citus distributes data across worker nodes using a coordinator node, enabling write-intensive workloads to achieve high throughput through parallelized writes. This makes it ideal for large-volume, relational, write-heavy analytics applications that require relational integrity.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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