Build guide: one real slice

The reference service app.py lives next to this file. Read it, run it, then build the equivalent slice of your own capstone.

Run the reference

cd labs/day-56-capstone-build
python3 app.py            # listens on http://127.0.0.1:8080

In another terminal:

curl -s -X POST http://127.0.0.1:8080/shorten -d '{"url":"https://example.com"}'
curl -s -i http://127.0.0.1:8080/u/1
curl -s http://127.0.0.1:8080/metrics
curl -s http://127.0.0.1:8080/healthz

You will get a short code back, a 302 redirect when you resolve it, and live counters from /metrics. Stop it with Ctrl-C. It writes a shortener.db file (gitignored), which you can delete to reset.

What to copy (the shape, not the domain)

a persistent store (SQLite via sqlite3, or an append-only file), not a dict
a read path and a write path from your Day 55 design
an in-memory cache if your read path benefits from one
/healthz so the load generator knows you are up
/metrics exposing the three or four numbers that matter for your system
a threaded HTTP server so many clients can connect at once

The build checklist

Walking skeleton first: the thinnest end-to-end path that genuinely works.
Then add the cache / queue / extra endpoints your design calls for.
Run it, curl it, watch /metrics move as you use it.
Write down, before tomorrow, where you think it will break under load.

The deliberate bottleneck

In app.py, every write and every uncached read takes one database lock, so the database serves one request at a time. The cache hides this for repeated reads; the write path cannot escape it. That is on purpose. Day 57’s load generator will make it visible as a flat write-throughput line and a climbing p99. Your own slice has a bottleneck too. Guess where it is now, prove it tomorrow.

It is fine to start from app.py

If your capstone is close enough to a shortener, or you are short on time, copy app.py and modify it into your slice. The goal is a running, measurable service by end of day, not heroics from scratch.