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13.4 Model Solutions

We decided to skip implementing the indirect IIOP ensemble. Prior experience convinced us that this design could be implemented. Therefore we didn't feel this effort would be justified. Instead, we focused our attention on the two remaining ensembles.

MODEL SOLUTION WITH DIRECT HTTP ENSEMBLE

Figure 13-3 shows a deployment diagram for a model solution developed with the direct HTTP transfer ensemble.

Web Browser Evaluation and Risk/Misfit

One of our early interactions with the DIRS design team centered on Web browser evaluation. Prior to our involvement, the design team had a long list of what were deemed essential browser features. Unfortunately, there were no browsers available on the market that possessed all of these features, nor was it clear which features were essential and which were merely desirable. There was considerable controversy, for example, over whether the browser required an on-board JVM of version 1.1 or better, and there were the usual squabbles over whether Netscape Navigator or Microsoft Explorer or both should be supported.

To help matters along we introduced the use of Risk/Misfit, and began by defining the objective of the evaluation. We defined two objectives for the evaluation:

  • The evaluation criteria should be satisfiable by Explorer and Navigator, as these possessed (at that time) over 90% of the browser market.

  • The evaluation criteria would identify only those browser features that the DIRS design would depend on; DIRS would depend on no other browser features.

Each browser feature that had been identified as "essential" had to be jus-tified in terms of the design risk that would arise without that feature. The aim of this was twofold. First, it forced feature advocates to use constructive rather than categorical arguments to justify their advocacy. Second, it sur-faced and documented design assumptions that were held by different mem-bers of the design team. Sometimes these assumptions were in conflict, and clarifying the risk statement helped to resolve these conflicts. One other ben-efit of this approach was that, having identified design risks associated with the absence of features, the design team was provoked into imagining possi-ble mitigators to the risk. In effect, these mitigators expressed design options for DIRS that did not depend on so-called essential features. In fact, the design team discovered that the only truly essential features were those for which no risk mitigation (called "repair" in Risk/Misfit) could be conceived. Thus, building Risk/Misfit evaluation criteria had the effect of identifying those features that defined minimum satisfaction criteria for browsers to be consid-ered "feasible," while all other features expressed mere preference.

A fragment of the resulting evaluation criteria is shown in Table 13-1. Note that we had adapted Risk/Misfit for the purpose just described, and so, at this point, repair cost and residual risk were not yet a concern. Observe, though, that the criteria include something not discussed in the Risk/Misfit chapter: a specification of how a misfit will be detected. Such modifications to Risk/Misfit or any other technique described in this book are encouraged, provided they are sound, in addition to being expedient.

—shissam

Table 13-1 Web Browser

Support Feature

Risk

Mitigator

Assessment Technique

Feature Description

HTTP/1.0

persistent connections

performance overhead

Use HTTP/1.0 and "cookies"

Product LiteratureThe server can keep a connection with an HTTP/1.1 client open for more than a single request.

Plug-in support for editors.

Loss of support for 45 million stored images.

NONE

Model Problem

Specify other data and object formats that are supported by the browser.

Object Signing

loss of authentication and integrity checking

Use 3rd party security COTS and added integration effort

Model Problem

Support of object (applet or ActiveX) signatures with manual and/or automatic acceptance/denial of download of object based on signature.

SSL v3.0

unencrypted data can be disclosed; also no indication of loss of data integrity

Revert to SSL v2.0

Model Problem

The server can communicate using the SSL version 3 protocol.


Figure 13-3 Deployment View of Direct HTTP Ensemble

The UML sequence diagram shown in Figure 13-4 provides a detailed description of the interactions between components in the solution. As depicted in this figure, the end user first opens the main DIRS HTML page using a browser. The browser contacts the HTTP server that downloads the page content including a Java applet. The applet is loaded into the Java Virtual Machine in the browser and passes control to the applet. The applet uses the CORBA bind operation to bind to the BRI, which in turn binds to the SM. Control returns to the end user who can now request the location of the image by interacting directly with the applet running within the browser. The applet contacts the BRI with this request, which is forwarded to the storage manager running on the back-end server. The storage manager returns a URL for the image to the BRI that constructs a new HTML page containing a link to this URL. The URL for this newly constructed page is returned to the applet. The applet sends a request to the browser to display the new HTML page (effectively terminating the applet's existence). The new HTML page containing a link to the image on the storage manager is displayed in the browser. The end user can now select the link to view the image.

Figure 13-4 Direct HTTP Ensemble sequence diagram.

The implementation went well until we needed to generate a request to the browser to open the new page. At this time, we did not have any experience communicating from the applet to a browser. Luckily, we acquired a copy of the book Java Network Programming by Elliotte Rusty Harold [44] and learned about the showDocument() method in the java.applet.AppletContext class. On page 131 of this book we discovered that:

This method shows the document at URL u in the AppletContext's window. It is not supported by all web browsers and applet viewers, but it is supported by Netscape and HotJava.

We added a call to the showDocument() method from the applet to display the HTML page in the browser. As promised, this worked with Netscape Navigator, but it was not known to work with Internet Explorer. By using Netscape Navigator to implement the browser component, we had, at a minimum, proven that one implementation of the ensemble was possible. However, we had not satisfied the evaluation criterion that the solution work with both Navigator and Explorer. This was an important criterion since the DIRS user community used both browsers. After implementing the model solution we modified our blackboard as shown in Figure 13-5.

Figure 13-5 Revised Direct HTTP.

The first and most obvious change is that technologies have been replaced with components. CORBA has been replaced with Orbix 2.2, used to enable communication with the legacy C++ servers and OrbixWeb 2.0.1, used to enable communication with the Java applets within the browser. The interaction between applet and BRI is associated with both ORBs since it involves different implementations of the CORBA standard, albeit by the same vendor. The diagram also includes a credential for the show document method. Its equivalent on Explorer was only a postulate. 1

Without the show document method we would have to abandon this ensemble. When dealing with emerging technologies, the critical role played by low-level component features may not be apparent until implementation time, which is what makes early prototyping so important. In our experience, model problems is the most efficient way of implementing these prototypes.

MODEL SOLUTION WITH DIRECT IIOP ENSEMBLE

The second model solution we implemented used the direct IIOP ensemble. This model solution used the same components employed in the direct HTTP transfer solution except it did not require an HTTP server on the back-end. We originally intended to deploy this model solution as depicted in Figure 13-6.

Figure 13-6 Deployment view of Direct IIOP Ensemble.

We did have some apprehensions about implementing this ensemble. The model solution requires that a Java applet, running within a browser on the client platform, directly call remote methods on both the BRI server running in the middle tier and on the SM server running on the back-end. However, for reasons of security, the JVMs within both the Netscape and IE browsers prevent an applet from connecting to a second machine. To circumvent this restriction a little creativity was required. We created a second "helper" applet that is loaded with the HTML page generated by the BRI and served up from a Web server on the back-end platform. So we needed the Web server after all! Figure 13-7 shows the sequence diagram for this model solution. (The second Web server is not shown to make the diagram easier to read.)

Figure 13-7 Direct IIOP transfer sequence diagram.

As in the direct HTTP transfer ensemble, the end user opens the main DIRS page with the browser. The browser contacts the server that downloads the page content including a Java applet. The applet is loaded into the browser JVM and control is passed to the applet. The applet uses the CORBA bind operation to bind to the BRI. Unlike the direct HTTP transfer solution, the BRI no longer binds directly to the SM. Control is returned to the end user who sends a request for the image location to the applet. The applet forwards this request to the BRI that constructs an HTML page containing a second, helper applet. The location of the image is included in the new page as a parameter to the helper applet. When control is returned to the applet, it calls the show document method as in the direct HTTP transfer solution. The new page, including the helper applet, is loaded by the browser and control is passed to the new helper applet that binds to the storage manager. The end user now sends a request to the helper applet to retrieve the image, which is forwarded to the storage manager. The storage manager converts the requested image into a sequence of bytes, or octets in CORBA terminology, and returns it via IIOP to the helper applet running within the browser.

Figure 13-8 Revised direct HTTP transfer abstract model solution.

Figure 13-8 shows the revised deployment diagram for this model solution. This is a good example of how actual experience gained from implementing model problems can alter a design. As suggested by Moltke, in the epigram that opened this chapter, it is impossible to predict the utility of a plan or design beyond the initial phases. This makes it incumbent on the designer to insist on increasingly stringent proofs of design feasibility.

EXTENDING THE SANDBOX

We succeeded in transferring an image from the storage manager to the client in the direct IIOP transfer ensemble, and, by logical extension, in the indirect IIOP ensemble as well. However, we have come short of a full solution in both cases. The image data is stored as a byte array within the Java Virtual Machine (JVM) and cannot be directly displayed. We still need to get the image data out of the JVM and into the 3rd-party image viewing component. Although other commercial image viewers were available, they did not support some of the non-standard image formats used by DIRS. The image viewer is invoked on a command with an argument specifying the location of the file containing the image to be viewed. To invoke this image viewer from our thin client we had to:

  1. Write the image data to a temporary file on the client platform.

  2. Launch an external application on the client platform.

Unfortunately, these are both privileged operations under Java. To prevent downloaded applets from malicious behavior, version 1.1 of the JDK, operating in a browser environment, enforces a security mechanism known as the sandbox. An applet running in a sandbox is restricted from performing operations that might compromise the security of the platform. The sandbox, for example, restricts reading or writing files on the local file system.

The options available in 1997 for allowing applets to operate outside of the browser sandbox were extremely limited. Netscape provided the Netscape capabilities classes that allowed an applet to operate outside of the sandbox. The capabilities classes added facilities to refine the control provided by the standard Java security manager class. These classes could then be used to exercise fine-grained control over an applet's activities outside of the sandbox.

A full discussion of the use of digital certificates and security is contained in the following chapter, but a brief introduction is required here. Access control decisions boil down to who is allowed to do what. In the capabilities model, a principal represents the "who," a target represents the "what," and the privileges associated with a principal represent the authorization for a principal to access a specific target. Using digital certificates, the principal is represented by a signing certificate while the target is one or more system resources, such as files stored on a local disk. The capabilities classes make it possible to determine whether any given principal (i.e., signing certificate) is allowed to access the local system resources represented by a given target. The answer is expressed by a privilege, which states whether access is allowed and, if so, for how long.

We extended our IIOP transfer solutions to use the Netscape capabilities classes to write image data out to a temporary file and invoke an external application to view them. Additionally, we used the capabilities classes to circumvent the restriction that a Java client could only communicate with the host from which it was downloaded. The capability of connecting to multiple hosts also eliminated the need for an additional helper applet to communicate with the Storage Manager.

Unfortunately, the capability classes also had drawbacks. Since it is proprietary, we could not use it in non-Netscape browsers e.g., Microsoft Internet Explorer. The revised blackboard for the Direct IIOP ensemble is illustrated in

Figure 13-9 Revised Direct IIOP.

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