Most basic monitoring tools only run HTTP checks. That means they miss four distinct categories of failure that can take your site effectively offline even when your web server is healthy. A complete monitoring setup covers all five failure modes.
Check 1: HTTP — is your application responding?
HTTP checks send a request to your URL and verify the response status code and optionally the response body. This catches: web server crashes, application errors (500s), missing content, broken reverse proxy configurations, and CDN issues that return unexpected responses. HTTP is the baseline check every service needs.
Check 2: TCP — is the socket open?
TCP checks verify a specific port is accepting connections. This is essential for non-HTTP services: email servers, databases, FTP, SSH, and custom APIs. A TCP check can also complement HTTP checks — if TCP passes but HTTP fails, your web server is running but your application is broken. If TCP fails, nothing is getting through at all.
Check 3: DNS — does your domain resolve?
DNS checks verify your domain name resolves to the expected IP address. DNS failures make your site completely unreachable regardless of how healthy your servers are. They happen after domain renewals lapse, after CDN provider changes, after manual DNS edits, and during DNS provider outages. Standard HTTP and TCP monitors never catch DNS failures because they typically resolve the domain once and cache the result.
Check 4: SSL — is your certificate valid?
SSL checks verify your certificate is valid, not expired, and issued for the correct domain. An expired SSL certificate produces a browser security warning that most users will not click through — effectively taking your site offline without any server error. SSL checks with expiry countdown give you days of warning before the certificate expires.
Check 5: SEO — is Google seeing the right page?
SEO checks measure your PageSpeed score and verify your meta tags are intact. A deployment that accidentally adds a noindex directive, removes your title tag, or triples your JavaScript bundle will not trigger any of the first four checks — but it will cost you search rankings within days. SEO checks catch this within the hour.