Applying Architectural Styles and Patterns

Outline  Defining Architectural Patterns and Style The activation model Styles and Quality Attributes  Common Architectural Styles Dataflow systems Call-and-return systems Independent components Virtual machines Respositories

Introduction  Relying solely on design operators to design an architecture can be a slow process.  By utilizing existing design experience from the software engineering community design productivity can be improved.  There are two types of reusable architectural products: Product families Architectural styles and patterns

Architectural Style  It is a type of metamodel that prescribes a set of elements and their relationships that characterize a system based on the style.  These elements are expressed as components and connectors.  Their relationships are the constraints on how components and connectors may be combined.

Architectural Patterns  A pattern is a solution that strikes some balance between competing forces.  However, what makes it a pattern is the fact that it is recurring.  If a design solution in its essential distilled form is found in multiple systems and is a successful design, then this solution is a pattern.

Architectural Patterns (Cont’d)  A pattern language is a set of patterns, together with the rules for describing how patterns are created and how they must be arranged with respect to other patterns.  The patterns are the vocabulary of the language, and the rules for their implementation and combination are the grammar.  Patterns in a pattern language can be constructed from other patterns.

Architectural Styles  An architectural style is a framework for a solution and not really a solution itself.  The selection of a style limits the scope of the solution space.  According to Ivar Jacobson, the architectural style of a system is “the denotation of the modeling languages used when modeling the system.”  It includes the guiding principles and prescriptions that the other development model elements must satisfy.

Architectural Styles (Cont’d)  It is the foundation of a philosophy of building.  An architectural style defines a vocabulary of components and connector types and a set of constraints on how they can be combined.  Most applications, however, are a combination of styles, i.e., they have a heterogeneous architectural style.  There are many parameters that we can use to characterize software architecture: The way in which control is passed between components (activation model) The quality attribute emphasis (e.g., maintainability and performance)

Activation Model  This model describes how components are activated and how information is passed between them (the control principle of the architecture).  In classic procedural languages and their object-oriented derivatives, method invocation and method execution are bound, i.e., method execution follows immediately from method invocation.  In languages like Smalltalk method invocation and method execution are decoupled.

Activation Model (Cont’d)  At the design level you can simulate these activation models in the language level.  Synchronous communication means that the client component activates a server component and then waits for a response.  A variation of this is the balking model in which a component may reject a request for any reason.

Activation Model (Cont’d)  Another variation is the timeout in which a client component assumes that the server has failed to complete a request after a certain amount of time.  Asynchronous communication means that the client doesn’t wait on a response from the server before continuing.  In the delegating pattern, a client component invokes a server component and provides an address to which the response is to be sent.

Activation Model – Patterns of Information Passing  Data elements can be passed directly between processing elements (pipes and filters).  Data elements may also be shared in a central store and accessed by different processing elements (repository).

Uses of the Activation Model  To compare different architectural styles.  To identify the appropriate architectural style for the entire system or the best combination of architectural styles.

Styles and Quality Attributes  Each style emphasizes a set of quality attributes while de-emphasizing another set. Pipes and filters emphasizes reusability and adaptability but sacrifices maintainability with respect to changes in data representation. Object-oriented systems emphasize maintainability but are not easily reused

Styles and Quality Attributes (Cont’d) Repository-based systems emphasize adaptability and performance but are not highly reusable or maintainable in terms of changes to algorithms or data representations  Styles can be combined in order to exhibit different quality attributes.

Common Architectural Styles (Adapted from Shaw and Garlan)  Dataflow systems Batch sequential Pipes and filters  Call-and-return systems Main program and subroutine Object-oriented systems Hierarchical layers

Common Architectural Styles (Cont’d))  Independent components Communicating processes Event systems  Virtual machines Interpretors Rule-based systems  Repositories Databases Hypertext systems Blackboards