WatchTower — Observability for early-stage teams

01One place for three signals

PostHog and Sentry are great when you've scaled. WatchTower is tightened down to what you actually need when your software first ships — and nothing more.

Errors

Uncaught exceptions and unhandled rejections with full stack traces, captured by a tiny SDK that hooks window.onerror.

∿

Performance

Core Web Vitals (LCP, INP, CLS) and page-load timing, sampled via PerformanceObserver. No extra config.

★

Feedback

Drop-in rating + comment widget. Real signal from real users, alongside the technical data.

02Who it's for

WatchTower is built for the moment when "my software is in production now and I have no idea what it's doing."

The on-call dev at 2am

Open the dashboard. See errors spiking after a 1:47am deploy. Click into the trace, find the line, ship a fix. Go back to bed.

The solo founder

Ship from your laptop. WatchTower lives on Cloudflare Workers' free tier; one SDK include on your site is the whole integration.

The student team

Built by 13 CSE 110 students in five sprints. Vanilla JS, zero framework lock-in, every decision captured as an ADR you can read on your way in.

The future maintainer

22 ADRs, an architecture doc, a changelog, sprint retros, and a wiki — written so the next team can pick this up without a handoff meeting.

03Quickstart

Three steps. About five minutes total.

Register your project

Create an account on the dashboard. You'll get a projectId to identify your app.

Drop in the SDK

One <script> tag. No build step, no bundler, no npm install. Served from jsDelivr.

<script
  src="https://cdn.jsdelivr.net/gh/cse110-sp26-group7/Watchtower@main/client/watchtower.min.js"
  defer></script>

Initialize

Configure with your projectId and tell WatchTower which environment this is.

const wt = new Watchtower({
  projectId: "your_project_id",
  endpoint: "https://watchtower-ingest.cse110piedpiper7.workers.dev/ingest",
  environment: "prod",
});
wt.init();

Trigger a test error

Drop this button into your page to confirm the round-trip ends up in the dashboard.

<button id="trigger-error">Trigger Test Error</button>
<script>
  document.getElementById("trigger-error").addEventListener("click", () => {
    throw new Error("Manual test error triggered");
  });
</script>

Full setup instructions are in the repo README.

04How it's built

WatchTower is a five-layer pipeline: SDK → Ingest Worker → D1 → API Worker → Dashboard SPA. Every layer is one file or one folder you can read in an afternoon.

What's inside

Single-file vanilla-JS SDK, served from a CDN.
Two Cloudflare Workers (ingest, API) with isolated blast radius.
Cloudflare D1 (SQLite) with one polymorphic events table.
Vanilla-JS dashboard on Cloudflare Pages — no framework, no bundler.
Tag-triggered CI/CD via GitHub Actions; the same tag fires a deploy event back into WatchTower.

What's deliberately not

No queueing layer in front of D1. Bursts above the rate limit drop and we say so.
No staging environment — tag-gated production deploys instead.
No third-party auth (OAuth). Signed-cookie sessions.
No long-term retention beyond 30 days. The data window matches the use case.

See the architecture doc and the 22 architectural decision records for the reasoning.