Continuous Integration

[ Team LiB ]

Continuous Integration Having defined a BOUNDED CONTEXT, we must keep it sound. When a number of people are working in the same BOUNDED CONTEXT, there is a strong tendency for the model to fragment. The bigger the team, the bigger the problem, but as few as three or four people can encounter serious problems. Yet breaking down the system into ever-smaller CONTEXTS eventually loses a valuable level of integration and coherency. Sometimes developers do not fully understand the intent of some object or interaction modeled by someone else, and they change it in a way that makes it unusable for its original purpose. Sometimes they don't realize that the concepts they are working on are already embodied in another part of the model and they duplicate (inexactly) those concepts and behavior. Sometimes they are aware of those other expressions but are afraid to tamper with them, for fear of corrupting the existing functionality, and so they proceed to duplicate concepts and functionality. It is very hard to maintain the level of communication needed to develop a unified system of any size. We need ways of increasing communication and reducing complexity. We also need safety nets that prevent overcautious behavior, such as developers duplicating functionality because they are afraid they will break existing code. It is in this environment that Extreme Programming (XP) really comes into its own. Many XP practices are aimed at this specific problem of maintaining a coherent design that is being constantly changed by many people. XP in its purest form is a nice fit for maintaining model integrity within a single BOUNDED CONTEXT. However, whether or not XP is being used, it is essential to have some process of CONTINUOUS INTEGRATION. CONTINUOUS INTEGRATION means that all work within the context is being merged and made consistent frequently enough that when splinters happen they are caught and corrected quickly. CONTINUOUS INTEGRATION, like everything else in domain-driven design, operates at two levels: (1) the integration of model concepts and (2) the integration of the implementation. Concepts are integrated by constant communication among team members. The team must cultivate a shared understanding of the ever-changing model. Many practices help, but the most fundamental is constantly hammering out the UBIQUITOUS LANGUAGE. Meanwhile, the implementation artifacts are being integrated by a systematic merge/build/test process that exposes model splinters early. Many processes for integration are used, but most of the effective ones share these characteristics: A step-by-step, reproducible merge/build technique; Automated test suites; and Rules that set some reasonably small upper limit on the lifetime of unintegrated changes. The other side of the coin in effective processes, although it is seldom formally included, is conceptual integration. Constant exercise of the UBIQUITOUS LANGUAGE in discussions of the model and application Most Agile projects have at least daily merges of each developer's code changes. The frequency can be adjusted to the pace of change, as long as any unintegrated change would be merged before a significant amount of incompatible work could be done by other team members. In a MODEL-DRIVEN DESIGN, the integration of concepts smooths the way for the integration of the implementation, while the integration of the implementation proves the validity and consistency of the model and exposes splinters. Therefore: Institute a process of merging all code and other implementation artifacts frequently, with automated tests to flag fragmentation quickly. Relentlessly exercise the UBIQUITOUS LANGUAGE to hammer out a shared view of the model as the concepts evolve in different people's heads. Finally, do not make the job any bigger than it has to be. CONTINUOUS INTEGRATION is essential only within a BOUNDED CONTEXT. Design issues involving neighboring CONTEXTS, including translation, don't have to be dealt with at the same pace. CONTINUOUS INTEGRATION would be applied within any individual BOUNDED CONTEXT that is larger than a two-person task. It maintains the integrity of that single model. When multiple BOUNDED CONTEXTS coexist, you have to decide on their relationships and design any necessary interfaces. . . .

[ Team LiB ]