Pros: (one service I reviewed)

3 Monitors
5/15 Minute Checks (with some plans they do 2 minute monitoring)
DNS monitoring
Statistics Download
XML Statistics
Multiple Alert Contacts

Ok, and now for the Cons:

The drawbacks to using a website monitoring service (and they usually don't mention them) are that they usually ping (or use other methods) the server your site is on, and if they get a response then they say your site is up, if they don't, they say your site is down. 

The flaw in this method, is that the data has to move from their servers, through 15-30 hops (on average) to get to your server, moving through other routers to do this.  If even 1 of them routers has a problem, they wont get a response, and therefor report your site as down, while it may be up with no issues.

Do they provide you with information on if all of their servers said the site was down at a specific time, or do they just say the site was down?   If they use multiple servers to test, but report site as down if only one of them reports an error, it may just be standard net congestion or a router problem along the route.

A good monitoring service will provide traceroutes from thier server to your site for the period that the site was "down".

Also, if they use multiple servers, they can actually be causing your site to go down (especially in the case of tests every 2 minutes as I seen on one plan)as they can be causing server loads to skyrocket if they are constantly pinging the server.

In some cases, the method they are using may cause extreme usage, or be detected as attacks on our systems, in which case thier ip will be blocked at any point in the route.  Well, if thier ip is blocked, of course the data isnt going from thier systems to your website.  In this case, they report the server as "down" when its actually running perfectly fine.

These are just a few of the flaws/cons in the way these services work.



WHAT TO DO IF THEY SAY YOUR SITE IS "DOWN"

If you get constant reports from a monitoring service saying that your site is down, please ask them to provide a traceroute for the time they say your site was "down".  These traceroutes provide a LOT of information that can be helpful, and will show if there are any outages between thier servers and the server your site may be on.

When reporting to support that your monitoring service says your site is down, its best if you can provide the traceroute your service provides to you as well as the IP address of all the servers they use to monitor with.

If you can also provide the name of the service you are using, and the package you are using, we can see if perhaps they are causing any issues.

EXAMPLE:

(ADDITION) - As a check, I had a monitoring service (sorry cant say which) setup to monitor my server (on an LP dedicated 2 plan) and got some interesting results.   

Had a day I couldn't access my domain using my cable connection, site monitoring service said server was down, however on my DSL connection, could access no issue.

Odd?

It appears that there was an issue with a level 3 router that day, and a traceroute from my cable connection and one provided by the site monitoring service were both hitting this router and getting no further.   Traceroute from my DSL connection (remember, this one could connect) showed no issue, and it appears the dsl was taking a different route.

Lesson on this --  My cable connection and website monitoring service were both unable to connect at the time, however server was not down.  The issue was my connection and the monitoring service were both hitting the same faulty router.

IN CONCLUSION:

Using an outside monitoring service to "watch" your site isn't always the best option, as with any service there can be extra circumstances involved.



Soon I hope to provide a tool that will allow you to check your own uptime, and keep track of it, without the need for someone who is miles (in some cases other countries and that is another kettle of worms) away from the server to monitor your sites.

 

EDIT ----   Just realized on con I forgot about, these services send data to your server, then the server sends data back.   Notice an large increase in your bandwith usage after signing up for one of these plans?

UPDATE --- my monitoring script is still being developed, hopefully have it done soon.

While I generally agree with your sentiments in terms of using one of these services as your only source of monitoring or uptime reports, I think they are perfectly valid in a multi-pronged monitoring approach.

For the last year or so I've been using a web based service such as the one you describe (ping and http request every 15 minutes) along with a more robust monitoring program (watches dns, smtp, pop, imap, ftp, ssl, ssh, and so on) running on a machine in my home.  That way I get results from different locations.

Bottom line is that the more information you can get about an outage the better.

Thanks for the tips, and I look forward to checking out your new toy when you introduce it.

One uptime service that I discovered is http://uptime.pingdom.com/

It uses the concept of Distributed Computing so the IP addresses checking your website will be different each time or from a large number of users from around the world. In order to use the free service you will need to download and run the client to run tasks on your machine.

I've been running it on my dedicated server in 1 minute intervals just to see what was going on. According to the latest stats, Apache was down for about 45 seconds today (which was prolly the restart I did to fix something).

Either way, seems very good. For shared hosting I think about 5 or 10 minute intervals will be fine at max. 3 and 1 minute I will not recommend unless you have a few dedicated servers clustered together.

A little more information about this can be found at http://uptime.pingdom.com/general/methodology

I agree, if you don´t put all your eggs in one basket these monitoring services should give most users a good overview of total site uptime.

My biggest problem with these sites, is they should only be used as guidelines, yet most poeple will sign up with one, and then act like its the God of monitoring.

There are to many veriables involved that cause cause a site to appear "down" to one of these site while it is actually up and running without issue.


My recommendation if your going to use one of these site, use two of them by different companies, then when one says that your site was "down" see if the other company says it was down during the same period.

If they both match up saying your site was "down" during a period of time, then contact support.   

If one says it was down, and the other says it was up, then it was probably an issue with the connection between the monitoring service and the server, and no need to contact support.     

hmm downloading a different file?  Doesnt seem like the way these services operate, plus, if you had signed up for a service, they would have mentioned thats how they monitor.

By the way, wasnt this forum a few pages long last time i looked?