AZ-305 Design data storage solutions Practice Question
An application requires a highly available key-value store with sub-millisecond read and write latencies across multiple Azure regions. The data model is simple and does not require complex queries. Which Azure data store should they choose?
⚠ Common exam trap
It's easy for candidates to confuse Azure Cache for Redis as a primary data store due to its sub-millisecond performance, but it lacks global distribution and durability guarantees, making Cosmos DB the correct choice for a highly available, multi-region key-value store.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Azure Cosmos DB
Azure Cosmos DB is the correct choice because it provides a globally distributed, multi-region key-value store with guaranteed sub-10-millisecond read and write latencies at the 99th percentile, and supports multiple consistency models including eventual consistency for even lower latency. Its turnkey global distribution enables active-active replication across Azure regions, meeting the high availability and sub-millisecond performance requirements without complex query support.
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 SQL Database
Why it's wrong here
Azure SQL Database is a relational database service built around tables, schemas, and ACID transactions, not a purpose-built key-value store. While it can technically store key/value pairs, its query engine and indexing structures are optimized for relational operations (joins, aggregations, complex predicates), not for simple PUT/GET access patterns at massive scale. Running a highly available key-value workload on SQL Database would also require a more expensive compute and storage configuration, and it still lacks the global multi-master replication implied by the requirement.
- ✗
Azure Table Storage
Why it's wrong here
Azure Table Storage is a NoSQL key-value service, but it is a single-region data store with only read-access geo-redundant replication, not active multi-region writes. This means a regional outage can force manual failover or cause longer downtime windows, and throughput is constrained by per-account partition scale limits. Its read and write latencies are also higher than Cosmos DB's single-digit millisecond SLAs, making it a weaker fit for a highly available, low-latency key-value requirement.
- ✓
Azure Cosmos DB
Why this is correct
Azure Cosmos DB is a globally distributed, multi-model database purpose-built for key-value workloads, offering turnkey multi-region writes and a 99.999% availability SLA for multiple-region configurations. Requests are served with single-digit millisecond latencies, and you can select from multiple consistency levels (eventual, session, bounded staleness, strong) to trade consistency for performance. Cosmos DB supports key-value schemas through its core (SQL), Table API, Cassandra API, and MongoDB API, making it the only option that fully satisfies the combination of global scale, high availability, and low latency.
- ✗
Azure Cache for Redis
Why it's wrong here
Azure Cache for Redis is an in-memory, distributed cache, not a durable persistent data store. Even with Redis persistence enabled, data lives primarily in memory and can be lost during failovers, evictions, or resharding, so it cannot serve as the authoritative source of truth for a highly available key-value application. It is best used to accelerate reads in front of a durable store, not to replace one, and it also lacks native multi-region active-active replication.
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