A key benefit of system and software modeling is the ability to explore various design alternatives to reach a fixedpoint representation of a concrete system design. Among a diverse set of configuration possibilities, a model engineer must be able to explo

is available for download at http://www.dre.vanderbilt.edu/cosmic). The scalability issues in EQAL arise when a small federation of CORBA event services must be scaled from a base model to accommodate a larger number of publishers and subscribers. As can be seen in Figure 3, scaling an EQAL model from three sites (with two gateways per site) to a larger model containing 8 sites (with 7 gateways per site) requires the addition of many new modeling elements. Our experience has shown that model transformations serving as replicators can significantly automate this manual task.

Figure 3. Replication among a federation of CORBA gateways. At the outermost level, three federation sites are replicated to eight sites, with each internal site replicated from two to seven gateways.

The expandSite strategy in Listing 2 illustrates a transformation that replicates the internal CORBA gateways for a specific site. The site and number of desired gateways are passed as parameters in line 1 of the strategy, which then finds the specific site in line 3 and calls a recursive strategy. The addGateWay_r strategy ensures that the proper number of gateways is added. The addGateWay strategy performs the actual transformation on the model by creating a new gateway (line 19), locating the site’s core event channel (line 20) and connecting the new gateway to the existing channel (line 21).

The collection of interacting strategies in Listing 2 accomplishes the transformation that is shown in the upper-right of Figure 3 (i.e., the internal expansion of Site 2). These strategies are also reusable and can be utilized by a model engineer to flexibly explore a design by applying the transformation to several different sites. Although not shown here, a different set of transformations have been specified to scale the outermost number of sites (i.e., moving from three sites to eight sites, as shown in the bottom-right of Figure 3). The parameterization of expandSite is a key to defining the alternative designs. These strategies are not an “aspect” in the sense of Listing 1, but represent a different type of model evolution that can also be specified in a transformation language like ECL.