Discover Quality Attributes
Architecture is mainly concerned with ensuring that the system's quality attributes are covered. By quality attributesalso known as "nonfunctional" requirements or "ilities"I mean requirements that have system-wide implications and cover areas such as performance, security, availability, flexibility, usability, testability, and the like.
Discovering or hatching quality attributes is extremely important because they provide the basis for modeling the architecture. I recommend using the scenarios-based approach detailed by Paul Clements et al. in Evaluating Software Architectures: Methods and Case Studies (Addison-Wesley, ISBN 020170482X). With this approach, you construct a "utility tree" that demonstrates the overall goodness of a system. The base quality attributes are refined to specific areas that are then further refined to scenarios in the application where the attributes are evident (the scenarios are used both for modeling and testing/evaluating the architecture). This is best explained by examples such as Figure 1.
Once you have more than one quality attribute nailed down, it is important to prioritize them because it is unlikely the architecture will be able to address all of them (at least not equally). Prioritization doesn't have to be performed by the architect alone. It can be wise to get other stakeholders involved in the process. Doing so helps everyone understand the trade-offs later in the game.
One quality attribute the architect should introduce is flexibility. Architectural decisions are relatively hard to change. You should try to allow even a small amount of flexibility into the design because requirements are sure to change.