Follow a Web Request from DNS to Database

A customer says the checkout page is down. Your monitoring dashboard says the server is up. Both can be right. A server can accept a connection while the application waits on a database, and a working application can be unreachable through a misconfigured domain. Following one request gives the incident a useful shape.

Start with the exact address

Write down the hostname, path, time and symptom. A browser error before a page appears is different from an HTTP error returned by the site. Compare the same URL from another network. A second location is a useful comparison, but a successful result there does not invalidate the customer's report.

DNS supplies an address for a name. A successful lookup establishes only that the resolver returned an answer. Compare that answer with the intended destination. If a CDN sits in front of the site, its address may be exactly what you should see.

Follow the response across boundaries

For a fresh HTTPS connection over TCP, the client connects and negotiates TLS before requesting the page. An HTTP response proves that some HTTP server answered. Inspect the status, headers and body to establish whether that answer came from an edge cache, a reverse proxy or the application itself. An old cached home page can look perfectly healthy during an origin outage.

Suppose the home page loads but a checkout preview times out. Compare an application request ID with the proxy log. If the application received it, inspect its timing for connection-pool waits, database queries and downstream calls. A long database span can include waiting for a lock; it does not automatically mean the query needs an index.

Change one thing after locating the delay

Keep a short record of the last boundary that worked, the first that did not and the observation that separates them. Verify the fix with the customer's original path. A green home-page check will not tell you whether checkout recovered.

Further reading

MDN: How the web works

Related guide: Trace a Slow Network Request One Phase at a Time.