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Architectural Pattern: Broker

Architectural Pattern: Broker Used to structure distributed systems decouple components that interact by remote service invocations Responsible for coordinating communication: forwarding of requests from client to server transmission of results and exceptions

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Architectural Pattern: Broker

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  1. Architectural Pattern: Broker • Used to structure distributed systems • decouple components that interact by remote service invocations • Responsible for coordinating communication: • forwarding of requests from client to server • transmission of results and exceptions • Context: distributed, heterogeneous, with independent components environment Software Architecture

  2. Problem characteristics • Building a system as a set of decoupled interacting components gains in: • flexibility • mantainability • changeability • distributability • scalability Software Architecture

  3. Problem characteristics • Distributed components need inter-process communication • A communication solution: • Components handle communication : • Positive points: • Easier to build • Can use same programming language Software Architecture

  4. Problem characteristics • Negative points • system is dependent on communication method used • clients need to know location of servers • new components have to be written using same language • components need to know such communication protocol • Furthermore, need component services for adding, removing, exchanging, activating, locating services • these services cannot depend on detail specifics to guarantee: portability and interoperability Software Architecture

  5. Developer’s Hint • There should be no essential difference between developing software for centralized systems and for distributed systems • OO applications should : • use only interface offered by objects • OO applications should not need to know: • implementation details • object’s physical location Software Architecture

  6. Forces to balance • Components should be able to access services provided through remote, location transparent service invocations • Need to exchange, add, remove components at run-time • architecture should hide system and implementation specific details from users of components and services Software Architecture

  7. Broker Pattern solution • Design broker component to decouple clients from servers • Servers: • Register with broker • present method interfaces to clients • Clients • access server’s methods via broker • uses same form to call server’s methods Software Architecture

  8. Broker Pattern Solution • Broker’s tasks • locating appropriate server • forwarding requests to server • transmitting results and exceptions to client Software Architecture

  9. Broker Pattern Solution • Applications access distributed services: • sending message calls to appropriate object as if in same memory space • no need to focus on low-level inter-process communication protocol • Broker architecture flexibility: dynamic change, addition, deletion, relocation of objects Software Architecture

  10. Broker characteristics • Makes distribution transparent to developer • How: Introduces distributed OO model encapsulated within the objects • Integrates two core technologies: • distributed systems • Object technology • An added plus: components can be written in different programming languages Software Architecture

  11. Broker Architectural Structure • Six types of participating components: • Clients • Servers • Brokers • Bridges • Client-side proxies • Server-side proxies Software Architecture

  12. Broker Architectural Structure • Servers kinds • library-type: offer services to many applications • application specific servers • Server’s Objects interface: • written using an IDL or • through a binary standard* Software Architecture

  13. Broker Architectural Structure • Clients: are applications that access servers • To call remote service: • client forward requests to broker • broker forwards response or exception to client • client and server model of interaction: • Dynamic: servers may also act as clients • Contrast traditional client-server model: Static Software Architecture

  14. Broker Architectural Structure • Broker’s role: messenger • transmits requests from clients to servers • transmits response and exceptions to client • Broker must have means to locate server of a request based on server’s unique ID • Brokers presents API to client and servers • to registering services (of server) • invoking servers methods (by client) Software Architecture

  15. Broker Architectural Structure • Each client and server is hosted by a broker • if a client makes request to local server: • broker forwards request directly to server • if client makes request to remote server • client’s broker finds route to remote broker • forwards request on this route • Conclusion: brokers need to interoperate Software Architecture

  16. Broker Architectural Structure • Bridges: layer between two brokers, used to hide each side implementation details • In particular when a Broker system is run on a heterogeneous network • two brokers have to communicate independently of network and OS in use • Bridges encapsulate these system-specific details Software Architecture

  17. Broker Architectural Structure • Client-side proxies • represent layer between client and broker • layer provides transparency: • remote objects appear local to client • there is no dependence between a client and broker • proxies allow implementation hiding: • inter-process communication between clients and brokers • creation and deletion of memory blocks • marshalling of parameters to broker • receives message, unmarshals results and exceptions from broker and forwards to client Software Architecture

  18. Broker Architectural Structure • Server-side proxies • responsible for receiving requests: • unpacking messages • unmarshalling parameters • calling appropriate service • marshalling results and exceptions to client Software Architecture

  19. Two definitions • Marshalling • The semantic invariant conversion of data into a machine independent format (ASN or XDR) • Unmarshalling • performs reverse transformation Software Architecture

  20. Another definition • Name service: • provides association between names and objects • A name service determines which server is associated with a given name. Software Architecture

  21. Broker Architecture: Static Diagram Broker Main_event_loop update_repository register-service acknoledgment find_server find_client forward_request forward_response Client-side Proxy Server-side Proxy Pack_data unpack_data send request return Pack_data unpack_data call_service send_response Client Server Bridge Initialize register_service enter_main_loop run_service use_broker_API Call_server start_task use_broker_API Pack_data unpack_data forward_message transmit_message Software Architecture

  22. Dynamics • Scenario I: server registers with local Broker system • broker is started in inialization phase of system. Broker enters event loop and waits for messages • user, or some other entity, starts server application. Server executes initialization code. Server registers with broker • Broker receives registration request. Extracts information from message and stores in repository. Acknoledgment is sent • Server enters main loop waiting for client requests Software Architecture

  23. Dynamics • Scenario II: client sends synchronous request to local server. • Client app started. Client invokes remote server’s method. • Client-side proxy packs parameters and other information in message to forward to local broker • broker looks up location of server in repository. Server local, broker forwards message to server-side proxy. • Server-side proxy unpacks parameters and other information. Server-side proxy invokes appropriate message on server • after completion, server returns results to server-side proxy which packages it to broker • broker forwards message to client-side proxy • client-side proxy unpacks result and returns to client. Software Architecture

  24. Dynamics • Scenario III: interaction of different brokers via bridge. • Broker receives request. Locates server in remote node. Broker forwards request to remote broker. • Message is passed from Broker A to Bridge A. Bridge A converts message to common protocol understood by both bridges. Bridge A transmit message to bridge B. • Bridge B maps common protocol to Broker B format. • Broker B performs all actions necessary when request arrives, as in scenario I. Software Architecture

  25. Implementation Issues • Object model • existing one • define one • Characteristics of object model • object names, objects, requests, values, exceptions, supported types, type exceptions, interfaces, operations • Component interoperability to offer • binary standard (OLE) • IDL (CORBA) • Combination ( IBM’s SOM) Software Architecture

  26. Implementation Issues • Specify Broker’s API for clients and servers • Use proxy objects to hide implementation details from clients and servers • Client-side proxy: represents server object • Server-side proxy: represents client Client Client-side proxy Broker Server-side Proxy Server Software Architecture

  27. Implementation Issues • Simultaneously design Broker • Design it into layers • Lots of more issues to consider here • Design IDL compilers one per PL language to support. Software Architecture

  28. Implementations Available: CORBA • CORBA defined by OMG • OO technology for distribution on heterogeneous systems • It’s an abstract specification , not constraining underlying implementations • CORBA basic components • Interface definition language • Language independent • Has a rich set of data types, and ways to define value objects • A wire protocol: IIOP (internet interoperability object protocol) based on TCP/IP, and based on RPC • Set of language mappings Software Architecture

  29. Implementations Available: CORBA • CORBA components (cont.) • Portable object adapter: • pulls request of wire, demarshalls data, forwards to proxy • More flexible than RMI run-time and more programmer process control • Services: • Naming • Event service • Transaction service Software Architecture

  30. Implementations Available: RMI/IIOP • Uses much of RMI • Replaces RMI native protocol with IIOP protocol • Interfaces are define as in RMI: • They use serializable objects • Extend remote interface • Throw remote exceptions • Server is implemented differently • Does not extend UnicastRemoteObject, or Activatable, it extends PortableRemoteObject • Proxies are generated with rmic • with th –iiop flag to use IIOP as communication protocol • Naming service is CORBA’s Software Architecture

  31. Implementations Available • IBM SOM/DSOM • iteroperability: IDL and binary • subclassing from binary patern allows to do mix-language inheritance • Microsoft’s • OLE • DCOM • The Web • Browsers act as brokers, and a client uses a web-browser • WWW servers act as service providers Software Architecture

  32. Broker Benefits • Location transparency • components changeability and extensibility • Broker system portability • Broker systems interoperability • Reusability Software Architecture

  33. Broker Liabilities • Restricted efficiency • due to indirection • Lower fault tolerance • may need object replication for higher fault tolerance Software Architecture

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