# Guided Exercise: Order These Instructions for Maximum Cache Efficiency

This is a paper exercise — commit to an order before checking it in the Lab.

## Your task

You need a Dockerfile with these five things, in SOME order. Application code changes very
often; dependencies change rarely. Write the most cache-efficient order:

- `FROM python:3.11-slim`
- `COPY app.py .` (your actual application code — changes often)
- `COPY requirements.txt .` (dependency list — changes rarely)
- `RUN pip install -r requirements.txt` (installs dependencies — slow)
- `WORKDIR /app`

## A worked shape to check yourself against

```
FROM python:3.11-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install -r requirements.txt
COPY app.py .
```

Copying `requirements.txt` and installing dependencies BEFORE copying the actual application
code means a code-only change never invalidates the (slow) dependency-install layer — only the
final `COPY app.py .` step needs to rerun.

## Instructor Notes

**Facilitation:** This is one of the most practically useful habits in the entire module — worth
connecting explicitly to real build-time savings on larger, slower dependency installs.

**Approximate timing:** 8-10 minutes live discussion; 5 minutes self-paced.