Creating a Monitor to Listen for Events

In this lesson, you will write an EPL program that monitors events coming into the correlator from a simulated event feed (the SensorTest.evt file). The correlator can understand these events because you created definitions for them in the SensorMonitorEvents.mon event definition file.

Your EPL program will instruct the correlator to monitor the Temperature events for a single sensor ID (“S001”). The correlator will check the data types of the incoming events to make sure that they match the data types provided in the event type definitions. Each time it detects a matching temperature event, the correlator will send information to the Eclipse console.

1. Right-click on the monitors folder and Create new file SensorMonitor.mon.

2. Open SensorMonitor.mon, the basic template of a monitor can be generated automatically by typing mon and selecting the auto-complete suggestion (generated by Apama Extension for Visual Studio Code). The generated content will be as following:

/**
 * This monitor is responsible for...
 */
monitor SensorMonitor {
    action onload() {
        log "Started monitor SensorMonitor.";
    }
}

As the first rule for this monitor, you will write code that references the Temperature event type in the eventdefinitions/SensorMonitorEvents.mon file (that you defined as part of the “Defining Types of Events” lesson). Note that in the SensorTest.evt file, each Temperature and Pressure event is sent in on a specific channel. The monitor must first subscribe to that channel.

1. Write the following code that uses the monitor.subscribe(“channel”) method to subscribe to the channel that the event data is sent into the correlator on:

action onload() {  
    // Subscribe to events.  
    monitor.subscribe("Factory1");
}

2. Write the following code that listens for all Temperature events where the value of the sensor ID is “S001”:

monitor SensorMonitor {  
    action onload() {  
        // Subscribe to events.  
        monitor.subscribe("Factory1");  
        // Listen for all temperature events where ID = S001.
        on all Temperature (sensorId="S001"){ }
    }
}

3. Save your changes.

1. For this listener’s action, add code that prints the text “Temperature received”.

monitor SensorMonitor {  
    action onload() {  
        // Subscribe to events.  
        monitor.subscribe("Factory1");  
        // Listen for all temperature events.  
        on all Temperature (sensorId="S001"){  
            // Print the temperature.  
            print "Temperature received.";
        }  
    }  
}  

2. Save your changes.

Testing the code.

1. Start correlator using : correlator -p 5000

2. Right click on SensorMonitorEvents.mon and select correlator: engine inject
   (This should be in eventdefinitions folder see Defining Event Types)
   You should see:

Added type AddSensor
Added type Pressure
Added type Temperature
Injected MonitorScript from file /.../EPL_Fundamentals/eventdefinitions/SensorMonitorEvents.mon

3. Right click on SensorMonitor.mon and select correlator: engine inject
   You should see:

Added monitor SensorMonitor
Injected MonitorScript from file /.../EPL_Fundamentals/monitors/SensorMonitor.mon

4. Right click on SensorTest.mon and select correlator: engine send (This should be in events folder)
   You should see this text repeated 4 times: Temperature received.

Temperature received.
Temperature received.
Temperature received.
Temperature received.

EPL supports the ability to copy an event into a variable. This process is called coassignment. Follow these steps to coassign found Temperature events to a variable:

1. Open the monitors/SensorMonitor.mon file.

2. Use the as operator to coassign each Temperature event to a variable called temperature:

on all Temperature (sensorId="S001") as temperature {...}

This variable is only in scope in the processing block of the listener and has no effect on clashing local or global variables.

Temperature temperature; // Defining an action-scope variable
  
on all Temperature (sensorId="S001"):temperature {...}

4. Update the print statement to use this variable:

print "Temperature received: "+temperature;

5. Save your changes.

Follow the above steps of testing the code:
You should now see output such as:

Temperature received: Temperature("S001",90)
Temperature received: Temperature("S001",100)
Temperature received: Temperature("S001",111)
Temperature received: Temperature("S001",120)

You can import the completed solution for this lesson:

In the Apama Samples Git repository, navigate to the tutorials/EPL_Fundamentals_Completed directory.