12. Glossary

Term	Definition
Bounded Context	A explicit boundary within which a domain model applies. The system has three: Users, Posts/Timeline, Messaging.
JWT (JSON Web Token)	A compact, self-contained token used to transmit authentication claims between parties. Signed with RS256 in this system.
RS256	RSA Signature with SHA-256. An asymmetric signing algorithm where the private key signs and the public key verifies.
Fan-out on Read	A feed strategy where the timeline is assembled at query time by fetching posts from all followees.
Fan-out on Write	A feed strategy where a post is pushed into each follower’s feed at write time. Not currently used (see ADR-004).
FastAPI	An async Python web framework that generates OpenAPI documentation automatically from Pydantic models.
asyncpg	A high-performance async PostgreSQL driver for Python, used via SQLAlchemy 2’s async engine.
Alembic	A database migration tool for SQLAlchemy. Each bounded context has its own Alembic environment.
Pydantic	A Python data validation library used by FastAPI for request/response schema definition and validation.
OpenAPI	A specification standard for describing REST APIs. FastAPI generates an OpenAPI 3.x document automatically.
Trace ID	A unique identifier attached to a request and propagated across service calls via X-Trace-Id header for distributed tracing.
Schema (PostgreSQL)	A namespace within a PostgreSQL database. Used here to isolate each bounded context (users, posts, messaging).
SPA (Single-Page Application)	A web application that loads once and updates dynamically. The frontend is built with React.
ADR (Architecture Decision Record)	A document capturing a significant architectural decision, its context, rationale, and consequences.
JWKS (JSON Web Key Set)	A JSON document exposing public keys used to verify JWTs. Referenced as a future improvement for key rotation (R-03).
Circuit Breaker	A resilience pattern that stops calling a failing service after a threshold of errors, preventing cascading failures.
Mention	A reference to another user within a post or message, denoted by @username.
Follow	A directed relationship where one user subscribes to another user’s posts.
Timeline / Feed	An aggregated, chronologically sorted list of posts from users that the current user follows.