A Two-phase Process for Software Architecture Improvement

　　CreateUnit transformation

　　This basic transformation creates the files for a new and empty unit U. Since the unit is not yet used anywhere, creating it does not have an impact on the build description. In the programming language C, a new body file U.c and header file U.h are created.

　　DeleteUnit transformation

　　This basic transformation deletes the files of a unit U given the precondition that the functions in unit U are no longer being used. The build description need not be altered. For the programming language C, the body file U.c and the header file U.h are deleted.

　　RenameUnit transformation

　　This basic transformation renames the files of a unit U to a unit V given the precondition that unit V does not yet exist. If unit V does exist, it will be overwritten. Every unit X that uses functions of unit U needs to import unit V instead. In the programming language C, the #include"U.h" is replaced by #include"V.h" in the files of unit X. In the build description, every occurrence of unit U is replaced by unit V.
　　IsolateFunction transformation

　　This basic transformation isolates one function F from unit U and relocates it to an existing empty unit V. Every unit X that uses function F needs to import unit V. If unit X does not use any other functions of unit U, the import of unit U can be removed. In the programming language C, the statement #include"V.h" is added to the files of unit X. In the build description, a dependency with unit V is added to unit X.

　　CombineUnits transformation

　　This basic transformation combines two units U and V into an existing empty unit W (not equal to U and V ). Every unit X that uses functions from either unit U or V needs to replace the imports with an import of unit W. In the programming language C, the statements #include"U.h" and #include"V.h" are removed from the files of unit X, and a statement #include"W.h" is added. In the build description, every occurrence of unit U and unit V is replaced by unit W.

　　MoveFunction transformation

　　This composite transformation moves a function F from unit U to unit V. It can be constructed using the previously defined basic transformations.

　　1.
　　CreateUnit T1
　　2.
　　IsolateFunction F from unit U in unit T1
　　3.
　　CreateUnit T2
　　4.
　　CombineUnits T1 and V into unit T2
　　5.
　　DeleteUnit T1
　　6.
　　DeleteUnit V
　　7.
　　RenameUnit T2 to unit V
　　

　　Once the ideas have been evaluated and turned into recipes, they can be expressed in architecture transformations as we have seen above. So in principle it is now clear what has to be done in the software. Implementing the transformations is another issue. In order to make automatic changes in the complete software, we implement a software renovation factory. This is an architecture that enables rapid implementation of tools that are typical of code changes. The construction of such factories is beyond the scope of this paper. The interested reader is referred to [BKV96,KV98,BKV98,DKV99,SV99b,SV99a,BD99].

　　The systems we are dealing with are mixed-language applications. We have to construct a reengineering grammar that understands all applied languages. [SV99a] describes a process and tools generating a reengineering grammar from the source code of compilers. In that paper, 20 sublanguages play a role. When the grammar has been defined, we can generate software that we call a software renovation factory; see [SV99b] for an overview and [BD99] for applications. Setting up such a factory takes time for systems containing dozens of different languages. The individual grammars of the compilers and scripting facilities need to be reverse engineered. The actual generation of the software factories is automatic.

　　To be able to carry out basic architecture transformations we can annotate some of the software with so-called scaffolding information ([SV99c]). For example, when a function is moved to another file, we can annotate all calls to this function in the source files so that we know in a later stage that we can turn those calls into other calls. It will be clear that the architecture transformations are implemented as complete assembly lines containing many small steps.

　　To give an example of architecture transformations using scaffolding, when we generate a software renovation factory from a modular grammar, the entire system consists of grammar files, and the transformation is called Gen-Factory. The implementation parses the entire grammar system using scaffolding and the transformation is effected throughout the entire system in about 200 different steps. Similar principles apply to the architecture transformations for systems other than those consisting of grammar files.