About

This Client-Side Uptime Monitor enables real-time tracking of web infrastructure availability directly from your browser. Unlike server-side monitors (e.g., Pingdom), this tool measures connectivity from the user's perspective, detecting local ISP issues, DNS propagation delays, and edge-case routing failures that centralized servers might miss.

The tool utilizes fetch with mode: "no-cors" to bypass Cross-Origin Resource Sharing (CORS) restrictions. This allows the dashboard to detect server reachability (TCP handshake completion) even for servers that do not explicitly allow cross-origin requests. While this method returns an opaque response (hiding specific HTTP status codes like 200 or 404), it provides a binary status of Reachable versus Network Failure.

Key Capabilities:

1. Latency Analysis: Calculates the Round Trip Time (RTT) using high-precision performance.now() timestamps.
2. Heuristic Failure Detection: Identifies downtime via DNS resolution failures, connection timeouts, or SSL handshake aborts.
3. State Retention: All configuration and historical metrics are persisted locally, allowing long-term uptime percentage calculation without external databases.

Formulas

The core metric for availability is the Uptime Percentage (U), calculated over a sliding window of check events (E).

U = N∑i=1 is_up(e_i)N × 100%

Where the indicator function is_up is defined by the network promise resolution:

{

1 if Promise → RESOLVED0 if Promise → REJECTED ∨ t_lat > t_max

Latency (t_lat) is derived from the High Resolution Time API:

t_lat = t_end − t_start

Reference Data

Protocol	Method	Capability	Limitation
Standard Fetch	GET / HEAD	Full HTTP Status (200, 404, 500)	Blocked by CORS on 99% of external sites.
Opaque Fetch	mode: "no-cors"	Verifies TCP/IP Connectivity	Cannot distinguish 200 OK from 404 Not Found.
Img Beacon	new Image()	Legacy Support	Restricted to image MIME types; slower execution.
Resource Timing	performance.getEntries()	Detailed DNS/TCP Metrics	Requires Timing-Allow-Origin header.

Frequently Asked Questions

This tool uses "Opaque Mode" to bypass security restrictions (CORS) that browsers enforce. This mode can confirm that the server exists, accepted the connection, and replied, but the browser security layer hides the specific content and status code (like 404 or 500) from the JavaScript code. It confirms "Reachability", not "Application Health".

No. This is a client-side tool running entirely in your browser's memory. If you close the tab or the browser, the monitoring loop stops. However, it can run in a background tab, and we utilize the Notifications API to alert you even if you are looking at a different window.

The latency represents the Time to First Byte (TTFB) plus the overhead of the browser's internal processing. Sudden spikes indicate network congestion or server load. A flat line is ideal. Red gaps indicate packet loss or timeouts.

Browsers enforce "Mixed Content" security. An HTTPS page (like this tool) is forbidden from making insecure HTTP requests. To monitor HTTP-only legacy sites, you must run this tool from an HTTP environment or upgrade your targets to HTTPS.