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Because it does generate revenue, I am thinking about using an uptime monitoring service of some kind. However, while I see this as being useful to alert me if the site is not accessible at all, what happened yesterday is a little different. For some reason the Joomla session table crashed and when you went to the site a message displayed indicating there was this problem.
My questions is would a monitoring service really view this as the site being down since, in effect, a page did load, granted with an error message? If monitoring services are capable of doing this, would most all monitoring services do this or should I just look for certain types?
Some monitoring services do something called string based monitoring. In string based monitoring, the monitor pulls the monitor page and checks it for a string. If the string is present, it either alarms or passes depending on the test.
For example the search string is copyright and you pass if it is found. If the page loads the footer that contains copyright, it passes. If there is a server error and the error message doesn't contain copyright or it white screens, it fails and alarms.
A different example is the search string failure and you want to alarm it is present. If you have a test page, you have it run a couple of test( can i connect to mysql, write a file, is there enough diskspace, etc) then print success. If not print failure and it will alarm. This is good for running multiple tests and having it alarm if only one fails but things like static pages works. I've seen people setup scripts to check server load and fail if is to high or disk space too low to alert someone of a possible future problem before it becomes a issue.
You probably want to try a monitoring service like this.