Problems with CBSE
There are a number of problems with CBSE that have limited its impact and which mean that many companies are adopting service-oriented architectures rather than developing systems using components:
- Component trustworthiness Components are black-box program units and the source code of the component may not be available to component users. In such cases, how does a user know that a component is to be trusted? The component may have undocumented failure modes that compromise the system where the component is used. Its non-functional behaviour may not be as expected and, most seriously, the black-box component could be a Trojan horse, that conceals malicious code that breaches system security.
- Component certification Closely related to trustworthiness is the issue of certification. It has been proposed that independent assessors should certify components to assure users that the components could be trusted. However, it is not clear how this can be made to work. Who would pay for certification, who would be responsible if the component did not operate as certified and how could the certifiers limit their liability? In my view, the only viable solution is to certify that components conform to a formal specification. However, the industry does not appear to be willing to pay for this.
- Emergent property prediction All systems have emergent properties, and trying to predict and control these emergent properties is important in the system development process. Because components are opaque, predicting their emergent properties is particularly difficult. Consequently, you may find that when components are integrated, the resulting system has undesirable properties that limit its use.
- Requirements trade-offs You usually have to make trade-offs between ideal requirements and available components in the system specification and design process. At the moment, making these trade-offs is an intuitive process. We need a more structured, systematic trade-off analysis method to help designers select and configure components.
SEI report on component-based software engineering