This is a slight departure from our regular programming. Instead of just concentrating on the sys admin side of things I want to show how to add a Nagios check to an existing application. In this case we have a Java application for which we want to monitor whether it is running or not. Later on we can make this more detailed by monitoring error codes in the application but for the moment let’s keep it simple.
On the Nagios end of things we need to define a command to perform a check on a specific port of the server where the application is running. Add a line like this to the objects/commands.cfg file of your Nagios installation.
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The -e parameter checks for a specific text that is to be returned by the application. This we can use later on to check for more detailed information. Next we need to add a service to Nagios for using this command. We do this by adding the following lines to the objects/localhost.cfg file. To keep this short I left out some lines which configure the frequency of the checks and the types of alerts.
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Creating a listener port in Java
In the second part I will show you the actual code to add to your application. Because this is a blog post I left out the package definition and the includes, but other than that the class itself is usable. To add the check to the Java app we need to add a listener thread to application. We do this by creating a class that is derived from Thread. This listener will open a port which is specified by the main application and a respond to any incoming data with a preset text. We really don’t care about the input on this end so any input will be ignored:
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In case you’re still reading this you’re probably interested in how to call this class. The following code should be executed in the initialization of the application. It creates the actual socket for port 2222 and starts the listener class. After this the listener class will run indefinitely until the application terminates.
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