Activity:  Capsule Design

Capsules are used to define concurrent threads of execution in the system. Capsules may be nested to an arbitrary depth, as well as having associations to design (passive) classes. This activity is performed once for each capsule, including new capsules identified within the scope of this activity.

Create Ports and Bind to Protocols

Consider the responsibilities of the capsule, creating an initial set of port classes. These port classes represent the 'interfaces' to the capsule. Port classes represent the realization of an Artifact: Protocol, which in turn represents a set of in and out signals used to communicate with capsules.

In creating ports, consider the Checkpoints: Protocol to determine whether the Protocol is appropriate. The port should reflect a singular set of related responsibilities; having a similarly scoped protocol enables its re-use across a number of capsules. Once the appropriate protocol is selected, bind the port to the appropriate protocol.

Validate Capsule Interactions

Once the ports are bound to protocols, the external behavior of the capsule must be evaluated and validated. Using either manual walk-through techniques or automated simulation tools, the behavior of the capsule should be tested by simulating the events that will exercise the capsule behavior. Validation will also consider the capsules which interact with the capsule under design. Using automated tools, stub code will need to be written within the capsule to allow the ports to be tested. Errors in protocol or port definition, or in capsule responsibilities, should be detected and appropriate changes to capsule, port and protocol definitions should be made.

Define Capsule State Machine

Once the capsule ports and protocols have been validated, the internal behavior of the capsule must be defined. The behavior of the capsule is defined using a statechart diagram, refer to the Guidelines: Statechart Diagram. Other general capsule information can be obtained from Guidelines: Capsule and Checkpoints: Capsules.

Define States

First, identify the states in which the capsule can exist. The states must be unique (a capsule cannot be in two states simultaneously) and descriptive. See the appropriate guidelines and checkpoints for more information.

Define State Transitions

Once states are defined, consider the transitions between states. Transition code should read like high level application pseudo-code, it should consist primarily of real-time operating system service calls e.g., frame services, time services, port operations, capsule operations and passive class operations.

When adding detail code to a Capsule transition:

• If the code would be useful in other transitions consider delegating it to a Capsule operation.
• Consider if the code implements capabilities which conform to the Capsule’s responsibility.

When defining a Capsule operation:

• Consider if the function would be useable at any time from any transition in the Capsule, and if whether any of the work being done would ever be useful elsewhere in the system. If it is consider delegating it to a passive class function.
• If the code is too application-specific to be stored in a particular Data class, consider creating an additional Data class as an abstraction for that code.
• If the code handles data structure manipulation (e.g., maintaining lists), or performs complex (more than 1 line) computations then it should be pushed into a data class.

Define Requirements on Passive Classes

Based on the capsule state machines, examine the passive classes referenced by the capsule. If there are new requirements on these classes, change requests need to be generated to effect the required changes. If new classes have been identified, the requirements on these classes (most specifically the required operations on them) should be gathered together and the classes should be created. These classes will be further described in the Activity: Class Design.

Introduce Capsule Inheritance

Capsule inheritance is used to implement generalization-specialization, to make use of polymorphism, to reuse implementation. The key word here is 'implementation' - it is a technique that is used primarily to re-use the internal structure of capsules, not the external behavior of capsules. The primary purpose of Artifact: Protocols is to re-use behavioral definitions, so capsule inheritance should not be used for this purpose.

Inheritance is often misapplied to achieve something that could more easily have been achieved using simpler design techniques.

Using inheritance for generalization-specialization

There are three kinds of inheritance. Listed from lowest complexity (most desirable) to most complex (least desirable), they are:

• Interface inheritance — just inherits ports and protocols, this is the type of inheritance that is most desirable
• Structural inheritance — inherits interface plus structural containment hierarchies (useful for frameworks)
• Behavioral inheritance — in addition to above also reuses behavioral code and state machines

Structural and behavioral inheritance pose some problems:

• The very strong degree of coupling provided by inheritance causes changes to cascade to subclasses when changes are made to superclasses.
• The need to override and delete superclass behavior and structure in subclasses indicates inappropriate use of inheritance (usually for tactical code re-use). Re-factoring classes and capsules and appropriate use of delegation is a more appropriate strategy.
• Inheritance means moving design decisions up the class hierarchy, causing undesirable design and compilation dependencies. 

Other problems include:

• Decisions may not be appropriate in all usage situations.
• Introducing inheritance actually makes things harder to reuse, since design elements are more tightly coupled.
• The design becomes more fragile because any new requirement that invalidates the decision causes large problems.
• The design has to be made extremely flexible to compensate, which is often difficult. This is why designing reusable frameworks is hard!

All designs containing structure/behavior have decisions and assumptions built in (either explicit or implicit). The critical question to ask is: are you absolutely sure that decision/assumption will always be valid? If not, what can you do to remove it or make it possible to change?

Validate Capsule Behavior

As a final step, the behavior of the capsule must be evaluated and validated. Using either manual walk-through techniques or automated simulation tools, the behavior of the capsule should be tested by simulating the events that will exercise the capsule behavior. In addition, the internal structure of the capsule should be validated, ensuring that not only the external behavior but also the internal implementation of that behavior is validated. Using automated tools, stub code may need to be written to simulate the implementation of passive data classes and external capsules with which the capsule interacts. Defects detected should be documented and appropriate changes to capsule definitions should be made.

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