• Architecture model
  • define the way in which the components of systems interact with one another
  • define the way in which they are mapped onto the underlying network of computers
• Including
  • Client-server model
  • Peer process model
  • Variations of the client-server model
• To master the complexity of distributed systems, it is crucial that they are properly organized
• Concern the logical organization of distributed systems into:
  • Communicating entities (objects, components and web services);
  • Communication paradigms (inter-process communication, remote invocation and indirect communication);
  • Roles responsibilities and placement
• Some important architectural styles and patterns:
  • Layered architectures
  • Object-based architectures
  • Event-based architectures
  • Shared data spaces

• Layered architecture :
  • vertical organization of services
• Tiered architectures are complementary to layering
• Organize layer functionality into appropriate servers and physical nodes
  
  

• Platform :Are the lowest-level hardware and software layers
• e.g.
  • Intel x86/Windows
  • Intel x86/Linux
  • Intel x86/Solaris                                         
  • SPARC/SunOS
  • PowerPC/MacOS
    
    
• Middleware : Its purpose is to mask heterogeneity and provide a convenient programming model, e.g. OMG’s CORBA, Java RMI, DCOM
• Support of abstractions
  • Remote method invocation: Sun RPC
  • Group communication: Isis
  • Notification of events
  • The replication of shared data
  • Transmission of multimedia data

• Natural units of decomposition
• Accessed via interfaces
• Connected via RMI
• Objects can be both clients or servers
  
  

Event-based architecture

• Client-server model:
  • Known for more than 25 years, very popular in distributed system design 
• General interaction between a client and a server.
  
  

Decentralized-system  architecture

• Referred to as peer-to-peer (P2P) systems
• Every node act both as a client and server (“servant”), and “pays” for the participation by offering access to some if its resources (typically processing and storage resources, but can also be logical resources (services)
• Advantages: 
  • no single point of failure, scalability
• Disadvantages: 
  • complexity of protocols
• Many application areas
• File sharing, streaming, process sharing, collaborative and social applications, web-caching etc.

Performance Issues

• Responsiveness. e.g. the performance of web-browsing clients
  • the load and performance of the server and network 
  • delay in the client and server operating system’s communication and middleware services as well as code of the service
• Throughput
  • the rate at which computational work is done
  • It is affected by processing speeds and by data transfer rates
• Balancing computational loads
  • E.g. applets, several computers for a service
• Reliability, security, performance and adaptability
• QoS is commandeered to refer to the ability to meet time-critical data
• Qos applies to OSs as well as networks.
• Performance issues are bars to successful deploy distributed systems
• Replication and caching, with a variety of different cache consistency protocols; e.g. Web-caching
  

• Architecture models: defines the components of the system, the way they interact, and the way they are deployed in a network of computers
  • client-server models (many variants)
  • peer processes (P2P)
  • spontaneous networks (mobility)
• Student  → Do search the pictorial diagram (picture) of the three different architecture models above, and explain the difference between them.