10. Quality Requirements

10.1 Quality Tree

Quality
├── Performance
│   ├── Timeline feed response < 500 ms (p95)
│   └── Post creation response < 200 ms (p95)
├── Availability
│   └── Uptime ≥ 99.9% per calendar month
├── Scalability
│   └── Sustain 10 000 concurrent users without latency degradation
├── Security
│   ├── Unauthenticated access to protected endpoints returns 401
│   └── Users can only modify their own resources (403 otherwise)
├── Maintainability
│   └── A feature confined to one bounded context requires no changes in others
└── Usability
    └── API errors include actionable, human-readable messages

10.2 Quality Scenarios

ID	Quality Attribute	Stimulus	Response	Measure
QS-01	Performance	User requests timeline feed (follows 500 users)	Feed returned	p95 latency < 500 ms
QS-02	Performance	User creates a post	Post persisted and returned	p95 latency < 200 ms
QS-03	Availability	Single service instance crashes	Load balancer routes to healthy replica	Recovery < 30 s, zero data loss
QS-04	Scalability	10 000 concurrent users active simultaneously	All requests served	p95 latency stays within QS-01/02 bounds
QS-05	Security	Unauthenticated request to GET /feed	Request rejected	HTTP 401, no data leaked
QS-06	Security	User A attempts to delete User B’s post	Request rejected	HTTP 403, audit log entry written
QS-07	Maintainability	New feature added to Messaging context	No changes required in Users or Posts services	Zero cross-context file changes in PR
QS-08	Usability	Client sends invalid request body	Error response returned	HTTP 422 with per-field error detail