Memory hook: Good keys spread writes and target reads.
Must remember
- An account contains databases and containers; items live within logical partitions determined by their partition-key values. Choose keys from access patterns, cardinality and write distribution, not only a field that looks unique.
- Hierarchical partition keys use multiple key levels to support suitable tenant and scale patterns. Synthetic keys combine or distribute values deliberately; random sharding can reduce hotspots but complicate reads.
- Embed related data when it is read/updated together and bounded; reference when independent growth or update patterns make duplication expensive. Model for the actual queries rather than copying a relational schema unchanged.
- The five consistency choices are strong, bounded staleness, session, consistent prefix and eventual. Session consistency preserves supported session guarantees through tokens; consistency is not the same as transactional scope.
- Request Units normalize operation cost. Provisioned manual/autoscale and serverless models suit different traffic patterns; evaluate account, database and container throughput settings and service constraints.
- Use a long-lived SDK client where recommended, appropriate connection mode and region preferences. Capacity planning includes hot partitions, item size, indexing, query shape and concurrency.
Choose under exam pressure
| Requirement | Choice and reason |
|---|---|
| Most requests target one tenant’s records | A tenant-aware key design, with growth/hot-tenant analysis and HPKs where suitable. |
| Occasional unpredictable traffic | Evaluate serverless against provisioned/autoscale pricing and supported limits. |
Traps
- High total RU capacity cannot always rescue one hot logical partition.
- A unique random partition key can make common multi-item queries expensive.
Active recall
1. What makes a useful partition key?
It distributes load while allowing important queries to target a manageable partition set.
2. What does a session token help preserve?
The supported consistency guarantees for that client session.
3. Why distinguish logical and physical partitions?
Application keys define logical grouping while the service manages physical placement and capacity.
4. When embed documents?
When related data is bounded and commonly read or updated together.
5. What does a 429 usually signal?
Rate limiting due to available throughput/capacity constraints; inspect diagnostics and retry guidance.