ECEN 5053, Paradigms & Patterns, Wk 81 Paradigms & Patterns - 3 ECEN 5053 SW Eng of Distributed Systems University of Colorado, Boulder

ECEN 5053, Paradigms & Patterns, Wk 82 Token-Passing Algorithms A token is a special kind of message used to convey information synchronize access to replicated files achieve conflict resolution that is fair determine global states in distributed computations Using two tokens in opposite directions Solve distributed mutual exclusion Can bypass a failed node Can regenerate a lost token

ECEN 5053, Paradigms & Patterns, Wk 83 Benefits of Token- Passing Algorithms Easy to generalize to synchronization solutions Compared to distributed semaphores Decentralized and fair, too Token-passing requires far fewer messages

ECEN 5053, Paradigms & Patterns, Wk 84 Microkernel Pattern Found in POSA 1, pp Also found in Real-Time Design Patterns: Robust Scalable Architecture for Real-Time Systems, Bruce Powel Douglass, Addison Wesley, The Microkernel architectural pattern applies to software systems that must be able to adapt to changing system requirements. It separates a minimal functional core from extended functionality and customer-specific parts. The microkernel also serves as a socket for plugging in these extensions and coordinating their collaboration.

ECEN 5053, Paradigms & Patterns, Wk 85 Context & Problem Context: The development of several applications that use similar programming interfaces that build on the same core functionality. Problem: Want to develop software for an application domain that needs to cope with a broad spectrum of similar standards and technologies. Systems like operating systems and GUI’s often have a long life-span. New technologies emerge; old ones change.

ECEN 5053, Paradigms & Patterns, Wk 86 Forces Application platform must cope with continuous hw and sw evolution Application platform should be portable, extensible, and adaptable to allow easy integration of emerging technologies Applications in the domain need to support different but similar application platforms. The applications may be categorized into groups use the same functional core in different ways require the underlying application platform to emulate existing standards. Functional core  separate component with minimal memory size and services that consume as little processing as possible

ECEN 5053, Paradigms & Patterns, Wk 87 Solution Encapsulate fundamental services in a microkernel component. Internal servers contain core functionality that cannot be implemented in the microkernel without causing an unnecessary increase in size or complexity. External servers implement their own view of the underlying microkernel using mechanisms available through the interfaces of the microkernel. Every external server is a separate process that represents an application platform. A microkernel system is an application platform that integrates other application platforms. Clients communicate with ext. servers using comm facilities provided by microkernel

ECEN 5053, Paradigms & Patterns, Wk 88 CRC Cards ClassCollaborators  MicrokernelInternal server Responsibility  Provides core mechanisms  Offers communi- cation facilities  Encapsulates sys- tem dependencies  Manages and controls resources ClassCollaborators  Internal serverMicrokernel Responsibility  Implements additional services  Encapsulates some system depen- dencies Class Collaborators  External server Microkernel Responsibility Provides program- ming interfaces for its clients.

ECEN 5053, Paradigms & Patterns, Wk 89 More CRC cards Class Collaborators  Client Adapter Responsibility  Represents an application Class Collaborators  Adapter External server Microkernel Responsibility Hides system depen- dencies such as communication facilities from the client. Invokes methods of external servers on behalf of clients

ECEN 5053, Paradigms & Patterns, Wk 810 Static Relationships External Server receiveRequest dispatchRequest executeService Microkernel executeMechanism initCommunication findReceiver createHandle sendMessage callInternalServer Adapter createRequest callService Client doTask Internal Server receiveRequest executeService initializes communication sends request calls service

ECEN 5053, Paradigms & Patterns, Wk 811 Scenario I

ECEN 5053, Paradigms & Patterns, Wk 812 Scenario II

ECEN 5053, Paradigms & Patterns, Wk 813 Variant Microkernel System with indirect Client-Server connections. A Client and Server communicate with each other indirectly using the microkernel as a message backbone. All requests pass through the microkernel. Useful when security requirements force the system to control all communication between participants.

ECEN 5053, Paradigms & Patterns, Wk 814 Another Variant – Distributed Microkernel System A microkernel acts as a messaging backbone responsible for sending messages to remote machines or receiving messages from them. Every machine in the distributed system uses its own microkernel implementation. From the user’s viewpoint, the whole system appears as a single Microkernel system – distribution is transparent. A Distributed Microkernel System allows you to distribute servers and clients across a network of machines or microprocessors. The microkernels in a distributed implementation must include additional services for communicating with each other.

ECEN 5053, Paradigms & Patterns, Wk 815 Examples Windows NT and 4 others in POSA Douglass says: an RTOS is common subsystem Core set of services include create and delete a task, allocate and deallocate memory, provide task event & message queues, and schedule/execute a task set. Plus, a developer can link in additional components to provide more services. Common service components for RTOS include bus communications, file services, networking services, and middleware services. Thus an RTOS becomes usable in a much wider set of application problems from tiny, highly memory- constrained systems to systems consisting of sets of high-powered networked CPUs.

ECEN 5053, Paradigms & Patterns, Wk 816 Consequences Benefits portability flexibility and extensibility separation of policy and mechanism scalability reliability transparency Liabilities performance complexity of design and implementation.