AZ-305 Design data storage solutions Practice Question
You need to store billions of small JSON files (average 50 KB) that are accessed infrequently but must be available within seconds when requested. Which Azure storage solution is most cost-effective?
⚠ Common exam trap
A common mix-up: candidates choose Azure Cosmos DB (Option D) because of its low-latency guarantees, but they overlook the massive cost difference for storing billions of small files, where Blob Storage's object storage model is far more economical for infrequently accessed data.
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 Blob Storage Cool tier
Azure Blob Storage Cool tier is the most cost-effective solution for storing billions of small JSON files (average 50 KB) that are infrequently accessed but require low-latency retrieval within seconds. The Cool tier offers low storage costs for data accessed less than once per month, while still providing sub-second access latency for individual blobs via HTTPS REST API, matching the 'available within seconds' requirement without the higher costs of Hot or premium tiers.
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 with a table storing JSON
Why it's wrong here
Azure SQL Database is a transactionally consistent relational engine optimized for structured, queryable data, not for zero-index blob payloads. Storing billions of 50 KB JSON documents would consume database storage that is priced at a premium per GB, and every file insert/update generates transaction log, checkpointing, and indexing overhead that does not scale economically or operationally at this volume. Additionally, concurrent read/write operations would compete for the database's limited IOPS and DTUs, making it impractical as a file repository.
- ✗
Azure Files with SMB shares
Why it's wrong here
Azure Files is a fully managed SMB-based file service designed for shared network drives, not for massive object-style workloads. The SMB protocol imposes per-operation handshake, metadata, and directory enumeration overhead, while each share has documented throughput and IOPS limits that a 50 KB * billions workload would saturate almost immediately. Cost also climbs steeply with capacity and transactions, and there is no native content-addressed or flat-namespace design to efficiently locate individual files out of billions.
- ✓
Azure Blob Storage Cool tier
Why this is correct
Azure Blob Storage Cool tier is purpose-built for massive, flat namespace object storage where a single account can hold trillions of blobs. Its storage cost per GB is significantly lower than hot tier, SQL Database, or Cosmos DB, and the access latency remains low enough to serve 50 KB JSON objects on demand. While Cool tier carries a minimum retention period and early-delete fee, for infrequently accessed small files it is the most cost-effective and scalable choice among the options.
- ✗
Azure Cosmos DB with a container for each file
Why it's wrong here
Azure Cosmos DB is a multi-model NoSQL database that charges for request units (RUs) and provisioned throughput, making it prohibitively expensive for storing billions of tiny JSON files as individual documents or containers. Each container adds metadata and indexing overhead, and the per-byte storage cost is far higher than object storage. Reading each file would consume RU-per-request charges, and 'a container for each file' would create an unmanageable number of containers, pushing capacity and cost into unacceptable territory.
Quick reference
Azure Blob Storage Tier Comparison
| Tier | Storage Cost | Retrieval Cost | Latency | Use Case |
|---|---|---|---|---|
| Hot | Highest | Lowest | Immediate | Active data, frequent reads |
| Cool | Lower | Higher | Immediate | Data accessed < once / month |
| Cold | Lower still | Higher | Immediate | Data accessed < once / quarter |
| Archive | Lowest | Highest + rehydration delay | Hours | Long-term compliance retention |
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