HAR File Analysis

HAR stands for HTTP Archive. It is a JSON export of browser network activity and is one of the fastest ways to inspect how a page really loaded: requests, responses, redirects, timings, headers, cookies, and failed assets. The Keysight guide frames HAR as a practical application-layer debugging tool for understanding blocked requests, decrypted traffic details, and network visualization.

When to use HAR analysis

Reach for a HAR file when the browser says a page is slow, incomplete, redirected in circles, missing CSS or JavaScript, failing API calls, or repeatedly prompting for authentication. HAR is especially effective when logs alone do not explain what the browser experienced.

How to capture a strong HAR

Open browser developer tools, go to the Network tab, enable Preserve log, disable cache, reproduce the issue, and then export the network session. The Keysight guide specifically calls out Preserve log and Disable cache because without them you lose redirect history and repeated network fetches.

What a HAR contains

At a high level, a HAR log has creator, pages, and entries. Creator tells you which tool generated the capture. Pages summarizes the page load and timestamps. Entries contains the request-response records you usually care about during troubleshooting.

How to read the capture

Start from the waterfall. Look for long waits before first byte, repeated redirects, blocked requests, 4xx or 5xx responses, failed preflight calls, or static assets that never load. Then open the specific entry and compare URL, method, status, headers, cookies, timing, and payload behavior against a working path.

Patterns to practice

• Authentication loops: 302 chains, repeated 401 or 403 responses, or session cookies that never persist.
• API failures: 4xx or 5xx responses, CORS or preflight issues, wrong hostnames, or malformed request bodies.
• Cache or CDN issues: stale bundles, missing invalidation, or mixed old and new asset versions.
• Frontend dependency problems: blocked JS, CSS, fonts, or source maps that stop parts of the UI from rendering.
• Latency spikes: long queueing, DNS, connect, SSL, wait, or content-download phases that point to different fault domains.

Use HAR with layered thinking

A HAR file is application-layer evidence. It tells you what the browser saw, not CPU, memory, or packet-level conditions. That limitation is actually helpful: use HAR to isolate whether the user experience broke in the browser or request path, then move deeper only if the capture points there.