So you are monitoring your web application, but are you really covering the 3 basic levels that will fully ensure it is available and responsive?  Monitoring of a web application can be as simple as testing a single web page; however, going deeper and covering your bases is better.

Level 1: Monitor web server availability - if the web server allows connections and returns a response, positive or negative, then it is available.  For example, if you know the ip address or hostname of a web server, you can monitor it with a ping, open port 80, or request a web page. 

At this level, a response is required and optionally, the HTTP status code is checked.  If the status code is not HTTP 200: OK, then it might indicate a problem. 

The actual amount of time it takes to respond is not important.  Almost all website monitoring software and services provide this level of testing.

Level 2: Monitor web page responsiveness - measure response time that it takes to download a specified web page on a web server.  After the condition of level 1 is met, it is important to ensure that a particular web page does not take too long to return HTML to allow the client’s web browser to render the page.

Think of web page responsiveness this way: a speedy page is a happy user.  Think of a slow responding web page from the user’s perspective: they think the website is either down or broken.

Level 3: Monitor web page content.  By far, this is one of the most important ways to monitor a web page and web application.  If the content returned from a web server for a particular web page is not what was intended, then the web application is not doing its job.  The web server can be available and serving web pages in a timely fashion, but, if a web page is returning an error to the user, then the web application has simply failed in the eyes of that user.

In conclusion, when setting up monitoring of your web application, consider covering all three levels.   At the very least, you can be assured that you will know what is going on at all times.