The latest articles on DEV Community by JEONSEWON (@jeonsewon). https://dev.to/jeonsewon https://media2.dev.to/dynamic/image/width=90,height=90,fit=cover,gravity=auto,format=auto/https:%2F%2Fdev-to-uploads.s3.amazonaws.com%2Fuploads%2Fuser%2Fprofile_image%2F3781888%2Fe7d6377e-9f5c-4964-8938-bf0a1317f26e.jpg https://dev.to/jeonsewon en JEONSEWON Thu, 02 Apr 2026 17:20:23 +0000 https://dev.to/jeonsewon/i-built-an-open-source-api-monitor-that-catches-silent-failures-http-200-doesnt-mean-your-api-is-33n1 https://dev.to/jeonsewon/i-built-an-open-source-api-monitor-that-catches-silent-failures-http-200-doesnt-mean-your-api-is-33n1 <h2> The problem with standard API monitoring </h2> <p>Most monitoring tools work like this:</p> <ol> <li>Send a request to your endpoint</li> <li>Check the HTTP status code</li> <li>If 200 → ✅ healthy. If 5xx → ❌ alert.</li> </ol> <p>This misses an entire category of failures.</p> <h2> What is a silent failure? </h2> <p>Your API returns <code>200 OK</code> — but something is actually wrong:<br> </p> <div class="highlight js-code-highlight"> <pre class="highlight json"><code><span class="err">//</span><span class="w"> </span><span class="err">HTTP</span><span class="w"> </span><span class="mi">200</span><span class="err">,</span><span class="w"> </span><span class="err">but...</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nl">"error"</span><span class="p">:</span><span class="w"> </span><span class="s2">"database_connection_failed"</span><span class="p">,</span><span class="w"> </span><span class="nl">"data"</span><span class="p">:</span><span class="w"> </span><span class="kc">null</span><span class="w"> </span><span class="p">}</span><span class="w"> </span></code></pre> </div> <p>Or worse — an empty response body. Or a completely different payload than expected.</p> <p>Standard monitors report this as healthy. Your users experience a broken product.</p> <h2> How CheckAPI handles this </h2> <p>Three levels of response body validation:</p> <p><strong>1. Keyword validation</strong><br> Specify a keyword that must be present or absent in the response body.</p> <ul> <li>Present: <code>"status":"ok"</code> must exist → fails if missing</li> <li>Absent: <code>"error"</code> must not exist → fails if found</li> </ul> <p><strong>2. Regex support</strong><br> Match patterns for structured validation:<br> /"user_id":\d+/</p> <p><strong>3. JSON Path (coming soon)</strong><br> Target nested fields directly without parsing the full response.</p> <h2> The rest of the feature set </h2> <ul> <li>HTTP methods: GET, POST, PUT, DELETE, PATCH, HEAD, OPTIONS</li> <li>Custom headers + request body</li> <li>5 alert channels: Email, Slack, Telegram, Discord, Custom Webhook</li> <li>SSL certificate expiry alerts (14 days before)</li> <li>Public status pages</li> <li>Response time tracking + uptime % (24h / 7d / 30d)</li> <li>Check intervals: 5min (free) → 1min → 30sec → 10sec</li> </ul> <h2> Tech stack </h2> <p><strong>Backend:</strong> FastAPI + Celery + Redis + PostgreSQL → Railway<br><br> <strong>Frontend:</strong> Next.js 14 + TypeScript + Tailwind CSS → Vercel<br><br> <strong>Payments:</strong> LemonSqueezy<br><br> <strong>Email:</strong> Resend </p> <h2> Pricing </h2> <div class="table-wrapper-paragraph"><table> <thead> <tr> <th>Plan</th> <th>Price</th> <th>Monitors</th> <th>Interval</th> </tr> </thead> <tbody> <tr> <td>Free</td> <td>$0</td> <td>10</td> <td>5 min</td> </tr> <tr> <td>Starter</td> <td>$5/mo</td> <td>20</td> <td>1 min</td> </tr> <tr> <td>Pro</td> <td>$15/mo</td> <td>100</td> <td>30 sec</td> </tr> <tr> <td>Business</td> <td>$49/mo</td> <td>Unlimited</td> <td>10 sec</td> </tr> </tbody> </table></div> <p>Free plan has <strong>no commercial restrictions</strong> — unlike UptimeRobot which restricted <br> commercial use on free plans in December 2024.</p> <h2> Links </h2> <ul> <li>🌐 Live: <a href="https://checkapi.io" rel="noopener noreferrer">checkapi.io</a> </li> <li>💻 GitHub (MIT): <a href="https://github.com/JEONSEWON/CheckAPI" rel="noopener noreferrer">JEONSEWON/CheckAPI</a> </li> </ul> <p>Built by a solo dev from Seoul 🇰🇷. Happy to answer questions about the architecture <br> or the silent failure detection approach.</p> opensource webdev devops showdev JEONSEWON Sat, 07 Mar 2026 05:37:44 +0000 https://dev.to/jeonsewon/i-built-an-api-monitoring-tool-because-every-existing-one-either-costs-too-much-or-restricts-406a https://dev.to/jeonsewon/i-built-an-api-monitoring-tool-because-every-existing-one-either-costs-too-much-or-restricts-406a <h2> The problem </h2> <p>I was building a side project and needed to monitor a few API endpoints.</p> <p>I tried the usual suspects:</p> <ul> <li> <strong>UptimeRobot free</strong>: 50 monitors, sounds great — until you read "non-commercial use only"</li> <li> <strong>Better Uptime</strong>: $20/mo minimum for anything useful</li> <li> <strong>Pingdom</strong>: No free tier at all</li> <li> <strong>Datadog</strong>: Powerful, but $100+/mo overkill</li> </ul> <p>All of them also share another problem: <strong>they only check status codes</strong>.</p> <p>Your API can return <code>200 OK</code> with a body of <code>{"error": "database connection failed"}</code> and most uptime tools will happily mark it as "up."</p> <h2> What I built </h2> <p><strong>CheckAPI</strong> (<a href="https://checkapi.io" rel="noopener noreferrer">https://checkapi.io</a>) — API health monitoring with a focus on what actually matters to developers.</p> <h3> Key features </h3> <p><strong>1. Response body keyword validation</strong><br> Set a keyword that must be present (or absent) in the response body. <br> Example: check that <code>"status":"ok"</code> exists in the response — not just that the server responded.</p> <p><strong>2. SSL certificate expiry alerts</strong><br> Get notified 14 days before your cert expires. Especially useful if you're using Let's Encrypt's 90-day certificates.</p> <p><strong>3. Free tier with no commercial restrictions</strong></p> <ul> <li>10 monitors</li> <li>5-minute check intervals</li> <li>All 5 alert channels (Email, Slack, Telegram, Discord, Webhook)</li> <li>No "non-commercial use only" restriction</li> </ul> <p><strong>4. Paid plans from $5/mo</strong><br> Starter ($5): 20 monitors, 1-minute checks, 30-day history<br><br> Pro ($15): 100 monitors, 30-second checks, team sharing<br><br> Business ($49): Unlimited monitors, 10-second checks, 1-year history</p> <h2> Tech stack </h2> <ul> <li> <strong>Backend</strong>: FastAPI + Celery + Redis + PostgreSQL (Railway)</li> <li> <strong>Frontend</strong>: Next.js 14 + TypeScript + Tailwind CSS (Vercel)</li> <li> <strong>Alerts</strong>: Resend (email), direct API calls for Slack/Telegram/Discord/Webhook</li> </ul> <h2> Current status </h2> <p>Just launched. $0 MRR. Building in public at <a href="https://x.com/imwon_dev" rel="noopener noreferrer">@imwon_dev</a>.</p> <p>Would love feedback — especially if you've hit the "non-commercial use" wall with other tools, or if keyword validation would have caught a bug for you.</p> <p>👉 <a href="https://checkapi.io" rel="noopener noreferrer">https://checkapi.io</a> — free forever tier, no credit card required</p> opensource saas webdev ai JEONSEWON Sat, 21 Feb 2026 11:50:52 +0000 https://dev.to/jeonsewon/building-checkapi-from-idea-to-production-in-6-weeks-with-fastapi-and-nextjs-1b8h https://dev.to/jeonsewon/building-checkapi-from-idea-to-production-in-6-weeks-with-fastapi-and-nextjs-1b8h <p>I just launched CheckAPI - an open source API monitoring tool built in 6 weeks. Here's how I did it.</p> <h2> The Problem </h2> <p>Existing monitoring tools are either too expensive ($75-100/mo) or have outdated UIs. I wanted something affordable for side projects with modern tech.</p> <h2> Tech Stack </h2> <p><strong>Backend:</strong></p> <ul> <li>FastAPI (Python)</li> <li>Celery + Beat for background workers</li> <li>Redis as message broker</li> <li>SQLAlchemy ORM</li> <li>PostgreSQL</li> </ul> <p><strong>Frontend:</strong></p> <ul> <li>Next.js 14 (App Router)</li> <li>TypeScript</li> <li>Tailwind CSS</li> <li>Zustand for state</li> </ul> <p><strong>Deployment:</strong></p> <ul> <li>Railway (backend + worker)</li> <li>Vercel (frontend)</li> </ul> <h2> Key Features </h2> <ul> <li>Real-time health checks (30s - 5min intervals)</li> <li>Multi-channel alerts (Email, Slack, Telegram, Discord, Webhooks)</li> <li>Response time tracking</li> <li>Public status pages</li> <li>Payment integration (LemonSqueezy)</li> </ul> <h2> Technical Challenges </h2> <h3> 1. Celery + Beat in One Process </h3> <p>Railway needed combined worker+scheduler:</p> <div class="highlight js-code-highlight"> <pre class="highlight plaintext"><code> python celery -A app.celery_app worker --beat --concurrency=2 2. Multi-Channel Alerts Created abstraction in alerts.py for 5 different APIs. 3. Token Refresh Axios interceptor handles JWT expiry smoothly. Results • 34 API endpoints • 5 alert channels • Full payment system • Open source (MIT) Live Demo  • Website: [https://checkapi.io](https://checkapi.io ) • GitHub: [https://github.com/JEONSEWON/api-health-monitor] Built with AI assistance (OpenClaw) which sped up development significantly. Questions? Ask in comments! #buildinpublic #indiehacker </code></pre> </div> webdev python nextjs opensource