Introduction

Let’s say I have a web application and I want to know how many users are online. My application has a rest API from which I can get this number in json. So I need to somehow get it and pass to machine agent. The response I got is like this:

{
  "users": {
    "online": 20
  }
}

Type of extension

AppDynamics supports three types of extensions:

• Using a script
• Using java application
• Using HTTP listener

Basically the idea of first two is when they run machine agent reads the output of them (which is in a special format) and sends it to the controller. Since writing a script is simpler in terms of support and future extension than java application I decided to use a script way. The idea is to create a ruby script which will be called by .sh script. At the end the extension will consist of three files:

• rest-monitor.sh
• rest-monitor.rb
• monitor.xml

Ruby script

The script’s task is to call HTTP service which in my case returns response in json, parse it and write to console formatted metric. Metric format should be as following (example from default extension):

name=Custom Metrics|Hardware Resources|Disks|Total Disk Usage %, value=23

And here is the method which formats the metric to such format:

def printMetric(metric, value)
    printf("name=Custom Metrics|Users|%s, value=%d\n", metric, value)
end

Now we need to call a service, in my case it is HTTPS with basic authentication. In ruby it’s quite easy to perform such call:

url  = "https://myawesomeapp.com/service/api/getStatistics.json"
uri = URI.parse(url)

http = Net::HTTP.new(uri.host, uri.port)
http.use_ssl = true
http.verify_mode = OpenSSL::SSL::VERIFY_NONE

request = Net::HTTP::Get.new(uri.request_uri)
request.basic_auth 'admin', '123'

res = http.request(request)

res.body will contain json response, but first we need to parse it, get value by key and print it:

resJson = JSON.parse(res.body)
qty = resJson['users']['online']
printMetric('online', qty)

The whole script is:

#!/usr/bin/ruby

require 'rubygems'
require 'net/http'
require 'net/https'
require 'json'

def printMetric(metric, value)
    printf("name=Custom Metrics|Users|%s, value=%d\n", metric, value)
end

url  = "https://myawesomeapp.com/service/api/getStatistics.json"
uri = URI.parse(url)

http = Net::HTTP.new(uri.host, uri.port)
http.use_ssl = true
http.verify_mode = OpenSSL::SSL::VERIFY_NONE

request = Net::HTTP::Get.new(uri.request_uri)
request.basic_auth 'admin', '123'

res = http.request(request)

resJson = JSON.parse(res.body)
qty = resJson['users']['online']
printMetric('online', qty)

Please note that I was using ruby version 1.8.7, so that’s why I needed to include explicitly net/https and rubygems. If you have newer version (ruby -v to check it) you probably won’t need this.

Shell script

Since the logic is in ruby shell script is just a wrapper for it, so it should just run ruby script:

#!/usr/bin/env bash

ruby ./rest-monitor.rb
exit 0

Monitor.xml

This file is almost the same for every monitor, it basically tells machine agent how to execute the script. You can find more details in step 7 of original documentation. For my monitor it looks like this:

<monitor>
    <name>RestMonitor</name>
    <type>managed</type>
    <description>Rest call monitoring</description>
    <monitor-configuration></monitor-configuration>
    <monitor-run-task>
        <execution-style>periodic</execution-style>
        <name>Run</name>
        <type>executable</type>
        <task-arguments></task-arguments>
        <executable-task>
            <type>file</type>
            <file os-type="linux">rest-monitor.sh</file>
        </executable-task>
    </monitor-run-task>
</monitor>

Now you just need to put these files under monitors folder and restart machine agent. Metrics will be available under Custom Metrics|Users|online