Tech, Rust, Cybersecurity, and Notes Zola 2025-07-18T00:00:00+00:00 https://blogs.alokranjan.me/tags/fastapi/atom.xml 2025-07-18T00:00:00+00:00 2025-07-18T00:00:00+00:00 ryu https://blogs.alokranjan.me/blog/inv/ <hr /> <h2 id="how-it-started">How it started?</h2> <p>Yesterday, I received a backend assignment from Invsto. the goal was to build a containerized API service that could store, serve, and analyze Stock price data.</p> <hr /> <h2 id="what-s-inside">What’s inside?</h2> <p>I built a complete FastAPI backend with the following key components:</p> <ul> <li>a <strong>PostgreSQL</strong> database to store time-series price data</li> <li>a clean rest <strong>API</strong> to ingest and retrieve it</li> <li>a <em>moving average crossover strategy</em> to analyze market trends</li> <li><strong>unit tests</strong> with over <strong><em>90% coverage</em></strong></li> <li>a fully <strong>dockerized</strong> environment, orchestrated via <strong>make</strong> commands</li> </ul> <p>but beyond the checklist, what made this project worthwhile was how it <em>felt</em> like building something production-ready—debugging tests, handling edge cases, and documenting everything like it mattered; because I had planned to log it in this blogging website (finally)</p> <hr /> <h2 id="the-trading-logic">The Trading Logic!</h2> <p>at the core is a simple but <strong>Effective Trading Strategy</strong>:</p> <ul> <li>take two moving averages—one short (20-day), one long (50-day)</li> <li>when the short average crosses <em>above</em> the long, that’s a <strong>Buy</strong> signal</li> <li>when it crosses <em>below</em>, that’s a <strong>Sell</strong></li> </ul> <p>the system scans the data for crossover points and calculates basic profit/loss from those trades. simple logic, but implementing it cleanly, testably, and in a way that integrates into an API—that’s where the challenge lies.</p> <hr /> <h2 id="uff-bugs">Uff! Bugs</h2> <h3 id="beetle-test-db-nightmare">🐞 Test DB nightmare</h3> <p>Early on, tests kept failing even though they used a separate sqlite database. turned out they were accidentally connecting to the dev postgres container—resulting in state bleed and weird errors.</p> <p><strong>fix</strong>: I updated the test setup to fully isolate the sqlite test DB, and cleaned stale containers using <code>make down</code>. <strong>lesson</strong>: To double-check your test environment assumptions.</p> <hr /> <h3 id="beetle-strategy-returns-0-trades">🐞 Strategy returns “0 trades”</h3> <p>My tests expected a buy signal—but the logic returned nothing.</p> <p><strong>fix</strong>: Turns out my strategy only counts <em>full trades</em> (buy + sell). the test data had a buy signal, but no corresponding sell. I had to redesign the test to simulate a full market cycle. <strong>lesson</strong>: Test data should reflect the exact logic you’re testing—not just part of it.</p> <hr /> <h2 id="reflections">Reflections ~</h2> <p>This project taught me a lot—not just about FastAPI, Docker, or (<em>specially</em>)<strong>pandas</strong>—but about treating small assignments like real systems. the biggest win wasn’t just getting it to work, but making it clean, testable, and explainable.</p> <hr /> <h2 id="visuals">Visuals</h2> <blockquote> <p>There is a live recorded demo in the GitHub Repo as well!</p> </blockquote> <h3 id="raw-data">raw data</h3> <figure> <p><img src="https://blogs.alokranjan.me/blog/inv/data.webp" alt="data" /></p> <figcaption>the raw data in the endpoint `/data`</figcaption> </figure> <br /> <h3 id="strategy-performance">strategy performance</h3> <figure> <p><img src="https://blogs.alokranjan.me/blog/inv/perf.webp" alt="strategy" /></p> <figcaption>preview of the `/strategy/performance` endpoint</figcaption> </figure> <br /> <h3 id="test-coverage">test coverage</h3> <figure> <p><img src="https://blogs.alokranjan.me/blog/inv/test.webp" alt="test" /></p> <figcaption>tests report</figcaption> </figure> <br /> <h2 id="source-code">source code</h2> <p>Check out the full code, project structure, and setup instructions here: <strong><a href="https://github.com/ryu-ryuk/inv">github.com/ryu-ryuk/inv</a></strong></p>