Build 2 — URL Shortener Service

# Build 2 — URL Shortener ## The brief Build a URL shortener. POST /shorten returns a short code; GET /:code redirects to the long URL. Click tracking; rate limiting; custom slugs. ## Step 1: Setup Similar to Build 1; add Redis for rate limiting: ```bash uv add fastapi 'uvicorn[standard]' sqlalchemy redis pydantic-settings ``` ## Step 2: Data model ```python from datetime import datetime from sqlalchemy import String from sqlalchemy.orm import Mapped, mapped_column class URL(Base): __tablename__ = "urls" short_code: Mapped[str] = mapped_column(String(20), primary_key=True) long_url: Mapped[str] = mapped_column(String(2048)) created_at: Mapped[datetime] = mapped_column(default=datetime.utcnow) click_count: Mapped[int] = mapped_column(default=0) creator_id: Mapped[int | None] = mapped_column(nullable=True) expires_at: Mapped[datetime | None] = mapped_column(nullable=True) ``` ## Step 3: Short-code generation ```python import secrets import string def generate_short_code(length: int = 7) -> str: chars = string.ascii_letters + string.digits return "".join(secrets.choice(chars) for _ in range(length)) ``` Each code: 62^7 ≈ 3.5T possibilities. Collision rate is low for moderate volumes; check before insert just in case. ## Step 4: Endpoints ```python from pydantic import BaseModel, HttpUrl, Field from fastapi.responses import RedirectResponse from sqlalchemy import select class ShortenRequest(BaseModel): url: HttpUrl custom_slug: str | None = Field(default=None, min_length=3, max_length=20) expires_in_days: int | None = None class ShortenResponse(BaseModel): short_code: str short_url: str long_url: str @app.post("/shorten", response_model=ShortenResponse) def shorten(payload: ShortenRequest, db: Session = Depends(get_db)): if payload.custom_slug: existing = db.get(URL, payload.custom_slug) if existing: raise HTTPException(409, "slug taken") code = payload.custom_slug else: for _ in range(5): code = generate_short_code() if not db.get(URL, code): break else: raise HTTPException(500, "could not generate unique code") expires_at = None if payload.expires_in_days: expires_at = datetime.utcnow() + timedelta(days=payload.expires_in_days) url = URL(short_code=code, long_url=str(payload.url), expires_at=expires_at) db.add(url); db.commit() return ShortenResponse(short_code=code, short_url=f"{settings.base_url}/{code}", long_url=str(payload.url)) @app.get("/{short_code}") def redirect(short_code: str, background_tasks: BackgroundTasks, db: Session = Depends(get_db)): url = db.get(URL, short_code) if not url: raise HTTPException(404) if url.expires_at and url.expires_at < datetime.utcnow(): raise HTTPException(410, "link expired") background_tasks.add_task(increment_click, db, short_code) return RedirectResponse(url.long_url, status_code=301) def increment_click(db: Session, short_code: str): url = db.get(URL, short_code) if url: url.click_count += 1 db.commit() ``` ## Step 5: Rate limiting with Redis ```python import redis r = redis.Redis(host="localhost", port=6379) def rate_limit(request: Request): ip = request.client.host key = f"ratelimit:{ip}" count = r.incr(key) if count == 1: r.expire(key, 60) # 60 sec window if count > 30: raise HTTPException(429, "rate limit exceeded") ``` Apply via dependency: `dependencies=[Depends(rate_limit)]` on the shorten endpoint. ## Step 6: Stats endpoint ```python class UrlStats(BaseModel): short_code: str long_url: str click_count: int created_at: datetime expires_at: datetime | None @app.get("/api/stats/{short_code}", response_model=UrlStats) def stats(short_code: str, db: Session = Depends(get_db)): url = db.get(URL, short_code) if not url: raise HTTPException(404) return url ``` ## Deliverables - Working shortener with redirect + stats. - Custom slug support. - Expiration support. - Rate limiting per IP. - README + simple frontend (optional). ## What you'll learn - Designing a small but real service. - RedirectResponse and HTTP status codes (301 vs 302 vs 308). - Background tasks for async-incrementing counters. - Redis as a rate-limiter store. - Pairs with the System Design course (walkthrough #1). ## Stretch - Add user accounts; track who created each URL. - Analytics: clicks by day, top referrers. - QR code endpoint. - Custom domain support.