Chapter 2 Architectures. Software Architectures

Transcription

1 Chapter 2 Architectures Software architectures of distributed systems System architectures of distributed systems 1 Software Architectures Software architecture describes how the software components are logically organized and how they interact We will study four different software architectures Layered architecture Object based architecture Event based architecture Shared data space 2 1

2 Layered Architectures Software components are organized into layers A component in layer i uses the services provided by the components in layer i 1 and provides services to the components in layer i+1. Example: network protocols are organized into layers 3 Layered Architecture of the Internet Application layer HTTP, SMTP, FTP Transport layer Network layer Data link layer TCP, UDP IP Ethernet, Wi-Fi, PPP Physical layer 4 2

3 Object Based Architectures Each object corresponds to a software component An object contains a set of data and a set of methods Objects interact via remote method invocation (RMI) A calling object can invoke a method in a remote project (i.e., an object in a different process) Example middleware: CORBA, Java RMI 5 Event Based Architectures (1) Components communicate through the propagation of events Events are information items of interests (e.g., drop in a stock price) Two types of components: publishers and subscribers Publishers publish events Subscribers express interest in particular events through subscriptions 6 3

4 Event Based Architectures (2) Events are delivered to subscribers by a middleware called an event based system (also known as a publish/subscribe system) In an event based system, components are Referentially decoupled: processes need not explicitly refer to each other Temporally coupled: sender and receiver must both be active when communication takes place Example middleware: CORBA Event Service, TIBCO Rendezvous, Java Message Service (JMS) 7 Shared Data Spaces (1) Components communicate through a common repository of data items A component can write data items to shared data space; other components can read or remove data items from the shared data space 8 4

5 Shared Data Spaces (2) Components in shared data spaces are Referentially decoupled: processes need not explicitly refer to each other This is the same as event based systems Temporally decoupled: sender and receiver need not both be active in order for communication to take place This is different from event based systems Example middleware: Sun s JavaSpaces, IBM s TSpaces 9 System Architectures System architecture describes the division of responsibilities between software components and the placement of the components on computers We will study three different system architectures Client server architecture Peer to peer architecture Hybrid architecture 10 5

6 Client Server Architecture Processes take on the roles of being clients or servers A server is a process that offers a service A client is a process that requests a service from a server Client server architecture is the most widely employed architecture E.g., the Web, DNS Interaction between a client and a server 11 Application Layering Many client server applications are organized into three layers User interface layer contains application s user interface Processing layer contains the core functionality of the application Data layer contains the data that a client wants to access through the application 12 6

7 Search Engine Example 13 Two Tiered Architecture A client server application is physically distributed across two types of machines: client machine and server machine A spectrum of choices. 14 7

8 Three Tiered Architecture (1) Two physical servers: the processing layer resides on an application server and the data layer resides on a database server The application server is a client of the database server 15 Three Tiered Architecture (2) Advantages Improves scalability by splitting up a server s functionality over two computers Enhances software maintainability because each tier has a well defined role Disadvantages Added complexity of managing two separate servers Added network traffic and latency associated with each operation 16 8