AZ-305 Design data storage solutions Practice Question
You need to design a data storage solution for a mobile app that requires low-latency reads and writes globally. The data is JSON documents with varying schemas. Which Azure service should you choose?
⚠ Common exam trap
Many candidates choose Azure Cache for Redis because they associate 'low-latency' with caching, but they overlook that the requirement is for a durable, globally distributed primary data store with varying JSON schemas, which Redis as a cache cannot fulfill as a persistent, globally writable database.
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 native global distribution with multi-region writes and single-digit-millisecond latency at the 99th percentile, making it ideal for a mobile app requiring low-latency reads and writes worldwide. It natively supports JSON documents with varying schemas through its document model and offers multiple consistency levels to balance performance and data integrity.
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 Cache for Redis
Why it's wrong here
Azure Cache for Redis is an in-memory key-value store designed for low-latency data access, but it is not a durable primary data store. Without persistence, data is lost on cache node restarts, and even with Redis persistence (RDB/AOF), it does not offer the same durability, availability SLAs, or disaster recovery guarantees as a managed database. Its native JSON support (RedisJSON) still requires external indexing and lacks the rich query and transactional capabilities of a document database, making it inappropriate as the mobile app's authoritative storage layer.
- ✓
Azure Cosmos DB
Why this is correct
Azure Cosmos DB is the correct choice because it is a globally distributed, multi-model NoSQL database built for mission-critical applications that need low-latency reads and writes anywhere in the world. Its turnkey global distribution with active-active multi-region writes, automatic indexing, and support for native JSON documents directly matches the mobile app's need for frequently varying schemas and fast, scalable access. Cosmos DB offers multiple consistency levels (strong, bounded staleness, session, consistent prefix, eventual) and a 99.999% availability SLA, which is essential for a mobile back end that must serve users across regions without sacrificing availability.
- ✗
Azure SQL Database
Why it's wrong here
Azure SQL Database is a relational service not designed for globally distributed, low-latency *writes* across active-active regions, nor is it optimised for the performance demands of frequently varying JSON document schemas. While it supports JSON data types, its core strength lies in providing strong transactional consistency and complex querying for structured data with predefined schemas. It would be the correct choice for applications requiring robust ACID compliance, complex SQL joins, and predictable data structures, even with some JSON storage.
- ✗
Azure Table Storage
Why it's wrong here
Azure Table Storage is a NoSQL key-value service that excels at storing large volumes of structured, entity-based data, but it does not support JSON document queries or flexible, nested schemas. Its query capabilities are limited to primary key and property-value lookups with basic filters; it cannot perform complex queries, cross-document joins, or use a JSON document model like Cosmos DB. While Table Storage is durable and highly scalable, it is not designed for globally distributed, multi-region writes with tunable consistency, nor does it natively handle the schema evolution required by a mobile app's frequently changing JSON payloads.
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