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Monitoring web applications with the Apache RA One of typical uses of apache is as an interface to the one or the other kind of web application. It could be expressed thus in terms of a resource group: IP address apache web_app where web_app is a JSP application (tomcat,jeronimo) or similar. Rumour has it that the web applications suffer from occasional instability which may make them an administration nightmare. But, typical remedy is simply an application restart. How do we increase availability in this situation? The web applications are most commonly represented as one or more processes in a UNIX environment. The afore mentioned instability is most commonly not reflected in the process state. Hence, checking the process status makes us no wiser. What could help, though, is probing the application just as our unhappy user does---through the web interface. We can ask the application developers to provide a URL which should exercise the application and then provide predictable output. Now, given our generic resource group and the failed web application, which we established using a http client, we have the following situation: IP address apache FAILED web_app Some might argue that it's not apache that is the culprit or has failed, but this nevertheless should serve our purpose well. The cluster will stop web_app and apache and then start them, either on the some node or elsewhere. There's an extra apache restart which was not needed, but then again it cannot really hurt. What to monitor? Choose carefully the URL to monitor. It should probe exactly what is further up in the resource group, no more and no less. In other words, if you have a database backend running elsewhere, it would be of no use to specify a URL which depends on the database. You should monitor only what is within reach. Configuration and usage It is possible to configure the monitoring either through CIB or using an extra configuration file. If your monitoring spec consists only of a URL and a regular expression to be matched in the output, then something like this should suffice: primitive apache_a1 ocf
💓apache \ params configfile="/apps/a1.conf" \ op monitor interval=120s timeout=60s start-delay=120s \ OCF_CHECK_LEVEL=10 testurl="/webapp1_mon" testregex="This application is alive" The testurl parameter is where we connect and the testregex is what we should look for. The OCF_CHECK_LEVEL must be set to "10". Note that testurl specifies a URL which is relative to where the apache listens for connections. Obviously, this should be preferred to specifying the full URL. It is important to set start-delay to a value larger than the time needed to start the web application (the next resource). If we don't, then the first monitor operation is likely to fail. In case you need more complex configuration, it can be set in an extra configuration file: primitive apache_a1 ocf 💓apache \ params configfile="/apps/a1.conf" testconffile="/apps/webmon.cf" \ op monitor ... OCF_CHECK_LEVEL=10 /etc/apache2/webmon.cf: test webapp1 url /webapp1_mon match This application is alive client curl end This test configuration is equivalent to the first one, it's just that in the latter we want to use curl(1) as an http client instead of wget(1). Another example: test webapp1 url /webapp1_mon match This application is alive client curl client_opts --header 'Host: www.webapp1.megacorp.com' end Here we use the curl's --header option to specify the virtual host we want to talk to. It is also possible to set the credentials using the "user" and "password" keywords. The configuration file may contain more than one test definition which is handy in case one should monitor more than one web application. In that case you should specify the test name in the CIB: primitive apache_common ocf 💓apache \ params configfile="/apps/httpd.conf" testconffile="/apps/webmon.cf" \ op monitor ... OCF_CHECK_LEVEL=10 testname="a1" \ op monitor ... OCF_CHECK_LEVEL=10 testname="b1" The apache OCF RA supports wget(1) (the default) and curl(1) http clients. If neither will do, then you can specify your own using the client and client_opts keywords. Your client must allow URL as the last parameter and it must dump output from the web server to stdout. All configuration file keywords: test The name of the text. url The url to test. If it doesn't start with http, it's considered to be relative to the apache Listen directive. match The regular expression to match. user Username to authenticate with. password Password to authenticate with. client The http client. client_opts Options for the http client. end Marks the end of the test definition. # Comment. May be used only at the start of line. Notes We could support more depth levels, but it is not clear if anybody really needs that. Different check levels could be defined as different monitor operations. In case you are using the external configuration file, don't forget to replicate it to all cluster members and to keep it synchronized.