Copyright © Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy. All rights reserved. Software Connectors in Practice Software Architecture

Foundations, Theory, and Practice Software Architecture 2 What is a Software Connector? Architectural element that models u Interactions among components u Rules that govern those interactions Simple interactions u Procedure calls u Shared variable access Complex & semantically rich interactions u Client-server protocols u Database access protocols u Asynchronous event multicast Each connector provides u Interaction duct(s) u Transfer of control and/or data

Foundations, Theory, and Practice Software Architecture 3 Where are Connectors in Software Systems? Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Foundations, Theory, and Practice Software Architecture 4 Implemented vs. Conceptual Connectors Connectors in software system implementations u Frequently no dedicated code u Frequently no identity u Typically do not correspond to compilation units u Distributed implementation Across multiple modules Across interaction mechanisms

Foundations, Theory, and Practice Software Architecture 5 Implemented vs. Conceptual Connectors (cont’d) Connectors in software architectures u First-class entities u Have identity u Describe all system interaction u Entitled to their own specifications & abstractions

Foundations, Theory, and Practice Software Architecture 6 Reasons for Treating Connectors Independently Connector  Component u Components provide application-specific functionality u Connectors provide application-independent interaction mechanisms Interaction abstraction and/or parameterization Specification of complex interactions u Binary vs. N-ary u Asymmetric vs. Symmetric u Interaction protocols

Foundations, Theory, and Practice Software Architecture 7 Treating Connectors Independently (cont’d) Localization of interaction definition Extra-component system (interaction) information Component independence Component interaction flexibility

Foundations, Theory, and Practice Software Architecture 8 Benefits of First-Class Connectors Separate computation from interaction Minimize component interdependencies Support software evolution u At component-, connector-, & system-level Potential for supporting dynamism Facilitate heterogeneity Become points of distribution Aid system analysis & testing

Foundations, Theory, and Practice Software Architecture 9 An Example of Explicit Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Foundations, Theory, and Practice Software Architecture 10 An Example of Explicit Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Foundations, Theory, and Practice Software Architecture 11 Software Connector Roles Locus of interaction among set of components Protocol specification (sometimes implicit) that defines its properties u Types of interfaces it is able to mediate u Assurances about interaction properties u Rules about interaction ordering u Interaction commitments (e.g., performance) Roles u Communication u Coordination u Conversion u Facilitation

Foundations, Theory, and Practice Software Architecture 12 Connectors as Communicators Main role associated with connectors Supports u Different communication mechanisms e.g. procedure call, RPC, shared data access, message passing u Constraints on communication structure/direction e.g. pipes u Constraints on quality of service e.g. persistence Separates communication from computation May influence non-functional system characteristics u e.g. performance, scalability, security

Foundations, Theory, and Practice Software Architecture 13 Connectors as Coordinators Determine computation control Control delivery of data Separates control from computation Orthogonal to communication, conversion, and facilitation u Elements of control are in communication, conversion and facilitation

Foundations, Theory, and Practice Software Architecture 14 Connectors as Converters Enable interaction of independently developed, mismatched components Mismatches based on interaction u Type u Number u Frequency u Order Examples of converters u Adaptors u Wrappers

Foundations, Theory, and Practice Software Architecture 15 Connectors as Facilitators Enable interaction of components intended to interoperate u Mediate and streamline interaction Govern access to shared information Ensure proper performance profiles u e.g., load balancing Provide synchronization mechanisms u Critical sections u Monitors

Foundations, Theory, and Practice Software Architecture 16 Connector Types Procedure call Data access Event Stream Linkage Distributor Arbitrator Adaptor

Foundations, Theory, and Practice Software Architecture 17 A Framework for Classifying Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Foundations, Theory, and Practice Software Architecture 18 Procedure Call Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Foundations, Theory, and Practice Software Architecture 19 Event Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Foundations, Theory, and Practice Software Architecture 20 Data Access Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Foundations, Theory, and Practice Software Architecture 21 Linkage Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Foundations, Theory, and Practice Software Architecture 22 Stream Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Foundations, Theory, and Practice Software Architecture 23 Arbitrator Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Foundations, Theory, and Practice Software Architecture 24 Adaptor Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Foundations, Theory, and Practice Software Architecture 25 Distributor Connectors Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Foundations, Theory, and Practice Software Architecture 26 How Does One Select a Connector? Determine a system’s interconnection and interaction needs u Software interconnection models can help Determine roles to be fulfilled by the system’s connectors u Communication, coordination, conversion, facilitation For each connector u Determine its appropriate type(s) u Determine its dimensions of interest u Select appropriate values for each dimension For multi-type, i.e., composite connectors u Determine the atomic connector compatibilities

Foundations, Theory, and Practice Software Architecture 27 Simple Example System components will execute in two processes on the same host u Mostly intra-process u Occasionally inter-process The interaction among the components is synchronous The components are primarily computation-intensive u There are some data storage needs, but those are secondary

Foundations, Theory, and Practice Software Architecture 28 Simple Example (cont’d)  Select procedure call connectors for intra-process interaction  Combine procedure call connectors with distributor connectors for inter-process interaction  RPC  Select the values for the different connector dimensions  What are the appropriate values?  What values are imposed by your favorite programming language(s)?

Foundations, Theory, and Practice Software Architecture 29 Two Connector Types in Tandem Select the appropriate values for PC and RPC! Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Foundations, Theory, and Practice Software Architecture 30 Discussion Connectors allow modeling of arbitrarily complex interactions Connector flexibility aids system evolution u Component addition, removal, replacement, reconnection, migration Support for connector interchange is desired u Aids system evolution u May not affect system functionality

Foundations, Theory, and Practice Software Architecture 31 Discussion Libraries of OTS connector implementations allow developers to focus on application-specific issues Difficulties u Rigid connectors u Connector “dispersion” in implementations Key issue u Performance vs. flexibility