When your site goes down, you want to know about it as soon as possible. Ideally, you’d detect that something was going badly before a failure occurred, using monit or similar software running on your server. (See my article on Installing monit for Server Peace of Mind for details.) But some failures just aren’t detectable in advance. Furthermore, if the failure brings down your server entirely, or is in the connectivity to the server, no software running on your server will be able to notify you.

Fortunately, there’s a simple and inexpensive solution: third-party monitoring services that periodically retrieve a page from your server and send you an alert if it fails. There are many such services, most of which have a limited free option and a variety of paid plans.

My current favorite is site24×7. It has a rich set of features and flexible, reasonable pricing. The free plan will test two URLs every 60 minutes. With the paid plans, you can test as frequently as every 5 minutes (for $4/month per URL). The faster you poll, the more it costs, so you can decide for each site how critical it is to know right away, and how much you’re willing to pay to find out quickly. You can also get status reports via RSS.

Assuming you’re running a dynamic site, it’s not enough to know that the server responded to the page request; you want to ensure that the database and the application server are running. The simple way to do this is to have the service look for certain keywords on the retrieved page. As long as these keywords come from the database, this test gives you pretty good assurance that your application is running and is able to talk to the database. site24×7 offers this ability even on the free plan. On the paid plan, you can also test a web application using a series of URLs to step through a sequence of operations. It will even provide login credentials to the application if needed.

You can receive notifications in a variety of ways. There’s no charge for email alerts, and you can always view the status via the web. You can also view a variety of reports on the web, including uptime statistics and response time data. If you want an SMS alert sent to your phone, it costs $0.20 per message.

Hyperspin is another very capable service. Hyperspin offers a rich set of features and will test connections via a variety of protocols, not just http and https. It will also poll as frequently as once a minute, for $12/month per URL (most services have a minimum interval of 5 minutes). Monitoring every 15 minutes costs only $2/month per URL. It monitors from ten locations spread around the world, so you know that your site is available everywhere.

There’s many other such services. Here’s a partial list:

• SiteUptime (free to $10/month)
• WebMetrics (no pricing provided on their site, so it’s probably expensive)
• InternetSeer (free to $99.95/year, or $29.95/year for qualifying small businesses)
• AtWatch (advanced service from InternetSeer, $19.95 to $39.95/month)
• HostTracker (free service)
• Website Alert (free to $25/month)
• 1st Warning ($4.95 to $99.95/month)
• WebSitePulse (free trial to $42/month)
• Uptime Auditor ($7 to $14/month)
• Pingability ($9.95/10,000 checks; free for less than 1,000 checks/month)

Google for “site monitoring” or “site uptime” for even more services.

Amost all of these services go beyond just telling you if your site is down. They also report response time and provide averages and plots over time, so you can see how your site is performing and how consistent it is.

Most services provide some sort of public stats page as an option, so you can use it to demonstrate to customers the uptime that you’re providing for their site. You can also use it to document any problems your hosting company may be having, which can be especially useful if you believe they’re not meeting their service level agreement (SLA). Some services even provide uptime badges that you can put on a site to show users what the uptime percentage is.

If you’re not using any site monitoring service, set up one of the free plan to become familiar with it. Then shop the various alternatives for the specific features you’re looking for. Think about how to verify that the critical parts of your application are functioning properly, decide how quickly you want to be notified, and whether you want email, SMS, or an RSS feed. Take a look at how many locations the service checks from, what their reports look like, and whether they can handle any special protocols or authentication you need. Then compare pricing among those that offer the features you want.

For a real belt-and-suspenders approach, use one company to perform a full, frequent test on a paid plan, and use one of the free services for backup in case the paid service slips up.