Topics

General

In general, the system appears to be reasonable:

  • The architecture appears to be stable.

    The need for stability is dictated by the nature of the Construction phase: in Construction the project typically expands, adding developers who will work in parallel, communicating loosely with other developers as they produce the product. The degree of independence and parallelism needed in Construction simply cannot be achieved if the architecture is not stable.

    The importance of a stable architecture cannot be overstated. Do not be deceived into thinking that 'pretty close is good enough' - unstable is unstable, and it is better to get the architecture right and delay the onset of Construction rather than proceed. The coordination problems involved in trying to repair the architecture while developers are trying to build upon its foundation will easily erase any apparent benefits of accelerating the schedule. Changes to architecture during Construction have broad impact: they tend to be expensive, disruptive and demoralizing.

    The real difficulty of assessing architectural stability is that "you don't know what you don't know"; stability is measured relative to expected change. As a result, stability is essentially a subjective measure. We can, however, base this subjectivity on more than just conjecture. The architecture itself is developed by considering 'architecturally significant' scenarios - sub-sets of use cases which represent the most technologically challenging behavior the system must support. Assessing the stability of the architecture involves ensuring that the architecture has broad coverage, to ensure that there will be no 'surprises' in the architecture going forward.

    Past experience with the architecture can also be a good indicator: if the rate of change in the architecture is low, and remains low as new scenarios are covered, there is good reason to believe that the architecture is stabilizing. Conversely, if each new scenario causes changes in the architecture, it is still evolving and baselining is not yet warranted.

  • The complexity of the system matches the functionality it provides.
  • The conceptual complexity is appropriate given the skill and experience of its:
    • users
    • operators
    • developers
  • The system has a single consistent, coherent architecture
  • The number and types of component is reasonable
  • The system has a consistent system-wide security facility.  All the security components work together to safeguard the system.
  • The system will meet its availability targets.
  • The architecture will permit the system to be recovered in the event of a failure within the required amount of time.
  • The products and techniques on which the system is based match its expected life?
    • An interim (tactical) system with a short life can safely be built using old technology because it will soon be discarded.
    • A system with a long life expectancy (most systems) should be built on up-to-date technology and methods so it can be maintained and expanded to support future requirements.
  • The architecture provides defines clear interfaces to enable partitioning for parallel team development.
  • The designer of a model element can understand enough from the architecture to successfully design and develop the model element.
  • The packaging approach reduces complexity and improves understanding.
  • Packages have been defined to be highly cohesive within the package, while the packages themselves are loosely coupled.
  • Similar solutions within the common application domain have been considered.
  • The proposed solution can be easily understood by someone generally knowledgeable in the problem domain.
  • All people on the team share the same view of the architecture as the one presented by the architect(s).
  • The Software Architecture Document is current.
  • The Design Guidelines have been followed.
  • All technical risks been either mitigated or have been addressed in a contingency plan. New risk discovered have been documented and analyzed for their potential impact.
  • The key performance requirements (established budgets) have been satisfied.
  • Test cases, test harnesses, and test configurations have been identified.
  • The architecture does not appear to be "over-designed".
  •  
    • The mechanisms in place appear to be simple enough to use.
    • The number of mechanisms is modest and consistent with the scope of the system and the demands of the problem domain.
  • All use-case realizations defined for the current iteration can be executed by the architecture, as demonstrated by diagrams depicting:
  •  
    • Interactions between objects,
    • Interactions between tasks and processes,
    • Interaction between physical nodes.

Models Top

    • The model is at an appropriate level of detail given the model objectives.
    • For the business model, requirements model or the design model during the elaboration phase, there is not an over-emphasis on implementation issues.
    • For the design model in the construction phase, there is a good balance of functionality across the model elements, using composition of relatively simple elements to build a more complex design.
    • The model demonstrates familiarity and competence with the full breadth of modeling concepts applicable to the problem domain; modeling techniques are used appropriately for the problem at hand.
    • Concepts are modeled in the simplest way possible.
    • The model is easily evolved; expected changes can be easily accommodated.
    • At the same time, the model has not been overly structured to handle unlikely change, at the expense of simplicity and comprehensibility.
    • The key assumptions behind the model are documented and visible to reviewers of the model. If the assumptions are applicable to a given iteration, then the model should be able to be evolved within those assumptions, but not necessarily outside of those assumptions. Documenting assumptions is a way of indemnifying designers from not looking at "all" possible requirements. In an iterative process, it is impossible to analyze all possible requirements, and to define a model which will handle every future requirement.

Diagrams Top

    • The purpose of the diagram is clearly stated and easily understood.
    • The graphical layout is clean and clearly conveys the intended information.
    • The diagram conveys just enough to accomplish its objective, but no more.
    • Encapsulation is effectively used to hide detail and improve clarity.
    • Abstraction is effectively used to hide detail and improve clarity.
    • Placement of model elements effectively conveys relationships; similar or closely coupled elements are grouped together.
    • Relationships among model elements are easy to understand.
    • Labeling of model elements contributes to understanding.

Documentation Top

    • Each model element has a distinct purpose.
    • There are no superfluous model elements; each one plays an essential role in the system.

Error recovery Top

    • For each error or exception, a policy defines how the system is restored to a "normal" state.
    • For each possible type of input error from the user or wrong data from external systems, a policy defines how the system is restored to a "normal" state.
    • There is a consistently applied policy for handling exceptional situations.
    • There is a consistently applied policy for handling data corruption in the database.
    • There is a consistently applied policy for handling database unavailability, including whether data can still be entered into the system and stored later.
    • If data is exchanged between systems, there is a policy for how systems synchronize their views of the data.
    • In the system utilizes redundant processors or nodes to provide fault tolerance or high availability, there is a strategy for ensuring that no two processors or nodes can æthinkÆ that they are primary, or that no processor or node is primary.
    • The failure modes for a distributed system have been identified and strategies defined for handling the failures.

Transition and Installation Top

    • The process for upgrading an existing system without loss of data or operational capability is defined and has been tested.
    • The process for converting data used by previous releases is defined and has been tested.
    • The amount of time and resources required to upgrade or install the product is well-understood and documented.
    • The functionality of the system can be activated one use case at a time.

Administration Top

    • Disk space can be reorganized or recovered while the system is running.
    • The responsibilities and procedures for system configuration have been identified and documented.
    • Access to the operating system or administration functions is restricted.
    • Licensing requirements are satisfied.
    • Diagnostics routines can be run while the system is running.
    • The system monitors operational performance itself (e.g. capacity threshold, critical performance threshold, resource exhaustion).
      • The actions taken when thresholds are reached are defined.
      • The alarm handling policy is defined.
      • The alarm handling mechanism is defined and has been prototyped and tested.
      • The alarm handling mechanism can be ætunedÆ to prevent false or redundant alarms.
    • The policies and procedures for network (LAN, WAN) monitoring and administration are defined.
    • Faults on the network can be isolated.
    • There is an event tracing facility that can enabled to aid in troubleshooting.
      • The overhead of the facility is understood.
      • The administration staff possesses the knowledge to use the facility effectively.
    • It is not possible for a malicious user to:
      • enter the system.
      • destroy critical data.
      • consume all resources.

Performance Top

See Checkpoints: Workload Analysis Document

Memory Utilization Top

    • Memory budgets for the application have been defined.
    • Actions have been taken to detect and prevent memory leaks.
    • There is a consistently applied policy defining how the virtual memory system is used, monitored and tuned.

Cost and Schedule Top

    • The actual number of lines of code developed thus far agrees with the estimated lines of code at the current milestone.
    • The estimation assumptions have been reviewed and remain valid.
    • Cost and schedule estimates have been re-computed using the most recent actual project experience and productivity performance.

Portability Top

    • Portability requirements have been met.
    • Programming Guidelines provide specific guidance on creating portable code.
    • Design Guidelines provide specific guidance on designing portable applications.
    • A 'test port' has been done to verify portability claims.

Reliability Top

    • Measures of quality (MTBF, number of outstanding defects, etc.) have been met.
    • The architecture provides for recovery in the event of disaster or system failure

Security Top

    • Security requirements have been met.

Organizational Issues Top

    • Are the teams well-structured? Are responsibilities well-partitioned between teams?
    • Are there political, organizational or administrative issues that restrict the effectiveness of the teams?
    • Are there personality conflicts?

The Logical View Top

The Logical View section of the Software Architecture Document:

    • accurately and completely presents an overview of the architecturally significant elements of the design.
    • presents the complete set of architectural mechanisms used in the design along with the rationale used in their selection.
    • presents the layering of the design, along with the rationale used to partition the layers.
    • presents any frameworks or patterns used in the design, along with the rationale used to select the patterns or frameworks.
    • The number of architecturally significant model elements is proportionate to the size and scope of the system, and is of a size which still renders the major concepts at work in the system understandable.
 

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