## 5 Extending the Mediator-Wrapper Architecture
### The Mask's Effect on the System Schema Hierarchy  
To show that the addition of masks affects only the mediators in the higher layers and decreases these mediators' responsibilities, in Figure 9, the assignment of schemas from the system-wide schema hierarchy from Figure 4 is shown. As in Figures 5 and 6, Figure 9 shows components and their assigned schemas adjacent to them in white rectangles.  
The wrappers themselves and their schemas have remained unchanged, but there is a significant difference above the first mediator layer. It is important to note that the placement of prior existing components has not been changed--all the mediators still connect to the same wrappers, and the mediators all operate over the same GCSs. Analogous to the examples shown in Figures 5 and 6, the mediator components of the (now only existing) mediator layer operate over their respective GCSs. The mask components have been assigned all the GESs.  
There is a noteworthy schema rename in the example of Figure 9, for what was originally GCS123. As the GCS123 itself was an exported schema in prior examples, in this example, the schema might be in a fundamentally different format. Hence, the schema operated over in the mask cannot be named the same as the schema in the mediator. To mark this change, what was once GCS123 used for exporting is now GES123--a fully fledged exported schema.  
A similar effect can be seen in the case of the mediator that in Figures 5 and 6 is operated over the LES41. As this mediator's schema is not directly exported, it is renamed GCS41, although it currently only incorporates LCS4. The mask component above this mediator has taken the responsibility of representation (exporting) and is consequently assigned LES41.  
Figure 8: Stylistic view of the MMW architecture.  
There is also an interesting case in Figure 9 concerning the translation of schema LCS\({}_{4}\) to the upper layers (via LCS\({}_{4}\), GCS\({}_{41}\), LES\({}_{41}\)) and the components used for this task. The previously exporting mediator of LES\({}_{4}\) from Figures 5 and 6 is preserved. This mediator's schema is also renamed to GCS\({}_{41}\), as stated earlier. For the moment ignoring the RMa2, it can be questioned whether this purely translational mediator is even required--rightly so, if it is additionally considered that the system probably uses standardized interfaces for inter-component communication. It could be concluded that the mask with LES\({}_{41}\) could be connected directly to the wrapper with LCS4.  
However, this is not the case, as the mediator with GCS\({}_{4}\) (formerly LES\({}_{4}\)) must be preserved. The reason for this statement is two-fold from the angle of system design. Firstly, the mediator is not only used for translation, but also enables transformations within the schema itself (as is stated by RMe2). Connecting the mask directly to the wrapper, although feasible, would disable the system to apply further transformations on schema LCS\({}_{4}\). Secondly, the benefit of using a MW architecture, and by extension our own, is the ability to append data sources after the system has been set up. Connecting the mask directly to the wrapper leaves the system without a mediator to mediate between the wrapper with LCS\({}_{4}\) and any additional to-be-connected wrapper. Because of this, the system would lose the beneficial property of being (completely) appendable.  
This is an example of how the RMa2 preserves not only the component hierarchy of the architecture, but also the properties of the system itself.