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Software Plasticity: An Architectural Attribute

Modifiability is the architectural quality attribute most related to plasticity. A modifiable architecture is one for which future changes don't have far-reaching consequences. The plasticity requirements in this article are simply that we should be able to open some Java code, create some additional logging detail, and then apply a bug fix to the area.

Figure 2 illustrates the idea of two developers working on two different products. Bug reports come in from the field and both programmers have to try to deduce an appropriate fix. Let's assume that both have access to the fielded system. The two programmers have a good idea which Java class has the problem, but the Java class in question has insufficient logging data to determine the exact problem. How can the programmers determine what to do? To answer this question, let's look a little more closely at the area of in-field debugging and the code in question.

Figure 2

Figure 2 Two programmers—only one of whom is using plastic software.

In-field Debugging and AOP

Network management tends to feature transactions such as reading a block of data from the network or modifying a table in a device. For our in-field debugging mechanism to work, we need close cooperation with the end user. Let's say that a specific transaction has failed. We get the user to repeat the operation after we've dropped in our interception code. The interception code has the necessary logging detail that will tell us exactly the nature of the problem.

Peeking at the Java Code

In the Java code examples for this article, the topmost layer of Figure 1 is represented by a class called ApplicationLayerRequestData:

public class ApplicationLayerRequestData {
  public static void main(String argv[]) {
    ApplicationLayerRequestData appLayerReqData = new
    ApplicationLayerRequestData();
    appLayerReqData.readData(argv);
    appLayerReqData.writeData();
  }

public void readData(String argv[]) {
    RequestNetworkData someData = new RequestNetworkData(argv);
    someData.collectNetworkData(argv);
}

The ApplicationLayerRequestData has a main method that calls into the middle layer of Figure 1 by instantiating an object of the class RequestNetworkData and invoking its collectNetworkData() method. Let's have a look at that method:

public void collectNetworkData(String argv[])
{
    final String host = argv[0];
    final String port = argv[1];
    final int WAIT_TIME = 10000;
    final int MS_DIVIDEND = 1000;

    // Lots of Java DMK code here

    SnmpVarBindList vbList =
      dispatchRequest(list, session, host, port, WAIT_TIME);
}

The collectNetworkData() method calls into Java DMK via the dispatchRequest method to retrieve some SNMP data from a selected device. The selected device is specified on the command line of the program and must be accompanied by a UDP port number (the standard SNMP port is 161).

Once we've collected the network data, we use it for some purpose, such as updating a device in the network. The latter is handled by calling the ApplicationLayerRequestData method writeData(). This completes the transaction started when we read the network data.

Putting It All Together

In the previous section, we saw a transaction that reads data from a designated network device and uses this data to update either that device or another one. This is the network-management process called configuration, whereby some change is introduced to the network. In this case, the update method does little more than print a message to screen. However, for the moment I want to focus on the data that we've read from the device.

Very often, network managers execute complex transactional configuration procedures against their networks. Typically, this process involves a combination of steps:

  1. Reading data from the network.
  2. Supplying some additional operational data.
  3. Updating the configuration of one more devices in the network.
  4. Retrieving some network data to verify the change(s).

The first two steps dictate the success of the overall operation. If the data supplied to the update software (in step 3) is inaccurate, the configuration change may fail. Often, the failure is very difficult to track down. Dozens of data items may be involved, the value of one of which may cause a downstream failure. So we need a way of looking at the configuration data before the network update occurs. This is where we make use of the magic of AOP and AspectWerkz to intercept the code after steps 1 and 2.

Intercepting the Network Update Operation

We want to intercept the operation of the class ApplicationLayerRequestData between the following two method calls:

appLayerReqData.readData(argv);
appLayerReqData.writeData();

When we've acquired our data, we want to be able to see it prior to using it to update the network. Part of the collectNetworkData() method writes the required data to a file called networkData.txt. In an operational scheme, this data might be stored in a database table or in an internal data structure. Whatever the location, we want to be able to look at this data!

The method collectNetworkData contains code that captures the data to file as follows:

try {
BufferedWriter out = new BufferedWriter(new FileWriter("networkData.txt"));
out.write(vbList.varBindListToString(), 0, vbList.varBindListToString().length());
out.close();
} catch (IOException e)

We want to see the contents of the file networkData.txt prior to calling the method ApplicationLayerRequestData.writeData(). This can be achieved elegantly by creating an AOP aspect class called MyAspect.java. This class contains a method called beforeNetworkUpdate() that gets called prior to the update method as required:

public class MyAspect {
public void beforeNetworkUpdate(JoinPoint joinPoint) {
System.out.println("");
System.out.println("Before Network Update - fasten your seatbelt!");

To orchestrate the interception of ApplicationLayerRequestData.readData(), we employ an XML file called aop.xml:

<system id="AspectWerkzExample">
    <aspect class="MyAspect">
        <pointcut name="greetMethod" expression="execution(* ApplicationLayerRequestData.writeData(..))"/>
        <advice name="beforeNetworkUpdate" type="before" bind-to="greetMethod"/>
        <advice name="afterNetworkUpdate" type="after" bind-to="greetMethod"/>
    </aspect>
  </system>

In aop.xml, I've defined a pointcut called greetMethod that is used to intercept the ApplicationLayerRequestData.writeData() method. The pointcut includes two advice items that correspond to two methods in the MyAspect class: beforeNetworkUpdate() and afterNetworkUpdate().

Don't worry if this seems very complicated; we just want to break into the execution of the readData() method.

Building and Running the Code

The following commands are used to compile the Java files and execute the program. The program parameters are shown as myPCIPAddress and 161. You need to replace the first of these with an IP address or DNS name that matches your own network setup. You'll also need copies of Java DMK and AspectWerkz. (See the "Resources" section at the end of this article for links.)

javac RFC1213_MIBOidTable.java
javac RequestNetworkData.java
javac ApplicationLayerRequestData.java
javac MyAspect.java

aspectwerkz -Daspectwerkz.definition.file=aop.xml ApplicationLayerRequestData myPCIPAddress 161

Running the program on my PC results in the following data:

AspectWerkz - INFO - Pre-processor org.codehaus.aspectwerkz.transform.AspectWerkzPreProcessor loaded and initialized
SyncManager::dispatchRequest: Sending getRequest to SNMP agent on myLaptopPC at port 161

Result:
[Object ID : 1.3.6.1.2.1.1.1.0 (Syntax : String)
Value : Hardware: x86 Family 6 Model 6 Stepping 10 AT/AT COMPATIBLE - Software: Windows NT Version 4.0
Object ID : 1.3.6.1.2.1.2.2.1.8.1 (Syntax : Integer32)
Value : 1, Object ID : 1.3.6.1.2.1.2.2.1.10.1 (Syntax : Counter32)
Value : 653, Object ID : 1.3.6.1.2.1.2.2.1.5.1 (Syntax : Gauge32)
Value : 10000000]
Hello World from ApplicationLayerRequestData.readData()!

Before Network Update - fasten your seatbelt!  (********----- INTERCEPTION
Object ID : 1.3.6.1.2.1.1.1.0 (Syntax : String)
Value : Hardware: x86 Family 6 Model 6 Stepping 10 AT/AT COMPATIBLE - Software: Windows NT Version 4.0
Object ID : 1.3.6.1.2.1.2.2.1.8.1 (Syntax : Integer32)
Value : 1
Object ID : 1.3.6.1.2.1.2.2.1.10.1 (Syntax : Counter32)
Value : 653
Object ID : 1.3.6.1.2.1.2.2.1.5.1 (Syntax : Gauge32)
Value : 10000000

Hello World from ApplicationLayerRequestData.writeData()!
After Network Update - we've crossed the Rubicon

The line marked INTERCEPTION indicates the point at which we've intercepted the program. At this point, we echo the configuration data as required. To achieve this goal, we would have to drop in the MyAspect class file and the aop.xml file. Then the target class must be executed again.

The use of AOP has enabled us to extract data from legacy code and even to modify its operation. This is what I mean by software plasticity.

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