Practice English
with real AI
conversations

Rather than proxying audio through our servers, the app gets a short-lived token and opens a WebRTC session directly with OpenAI. This eliminates infrastructure audio latency, avoids storing sensitive audio data, and drastically reduces bandwidth costs. The backend's only role in a voice session is minting the token and evaluating the transcript afterward.

Each microservice owns its own PostgreSQL database. The conversation service never reads the subscription DB directly — it calls the subscription service via Feign when it needs plan or level data. This enforces proper domain boundaries and makes each service independently deployable and testable.

Usage limits (voice minutes, text conversations, AI evaluations per plan) are tracked in Redis with TTL-based windows that reset on schedule. This avoids expensive DB writes on every user action and enables sub-millisecond limit checks at the API layer without adding a caching proxy.

AI evaluation prompts are stored as Flyway repeatable migrations (R__prompts.sql). This allows prompt iteration — adjusting tone, evaluation criteria, output format — without code changes or redeployment. A DB migration is all that's needed, keeping prompt engineering fully decoupled from application releases.

Both services spin up real PostgreSQL and Redis instances in CI via Testcontainers — no mocks, no in-memory substitutes. JaCoCo merges unit and integration coverage into a single report, so coverage numbers actually reflect what runs in production. The test suite runs with mvn verify, not just mvn test.

Firebase remains the identity provider, while customer-subscription owns email normalization, eligibility, anti-abuse controls, anti-enumeration responses, branding, and delivery orchestration. Every request creates a temporary, one-time link that is sent by notifications-app and never returned by the public endpoint.

Crashlytics captures native and fatal JavaScript failures plus selected handled technical errors. Reports contain only controlled operation, reason, and optional HTTP status — never email, Firebase UID or ID Token, Authorization headers, magic links, oobCode, payloads, raw responses, SDP, ephemeral keys, audio, messages, or transcripts.