Introduction Chapter 1
Agenda • -Definition • -Hardware concept • -Software concept • -The Client-Server model
Definition of a Distributed System (1) • A distributed system is: • A collection of independent computers that appears to its users as a single coherent system.
Definition of a Distributed System (2) 1.1 A distributed system organized as middleware.Note that the middleware layer extends over multiple machines.
Goal of Distributed system • Connecting users to resources • Openness • Transparency • Scalability
Transparency in a Distributed System Different forms of transparency in a distributed system.
Scalability Problems Examples of scalability limitations.
Scaling Techniques (1) 1.4 • The difference between letting: • a server or • a client check forms as they are being filled
Scaling Techniques (2) 1.5 An example of dividing the DNS name space into zones.
Hardware Concepts 1.6 Different basic organizations and memories in distributed computer systems
Multiprocessors (1) 1.7 • A bus-based multiprocessor.
Multiprocessors (2) 1.8 • A crossbar switch • An omega switching network
Multicomputers • Lower traffic than multiprocessors • CPU – CPU versus CPU – Memory traffic • Homogeneous or Heterogeneous • Each node is an autonomous machine • Private memory
Homogeneous Multicomputer Systems (1) • Homogeneous access to network • Single network • Bus-based or point-to-point communication • Similar nodes • Same processors and memory space
Homogeneous Multicomputer Systems (2) 1-9 • Grid • Hypercube
Heterogeneous Multicomputers • None-homogeneous access to network • Different network • Distributed systems are commonly built on this H/W category • Need S/W to make it transparent • Different nodes • Nodes can be complex system
Software Concepts • An overview between • DOS (Distributed Operating Systems) • NOS (Network Operating Systems) • Middleware
Uniprocessor Operating Systems 1.11 • Separating applications from operating system code through a microkernel.
Multicomputer Operating Systems (1) 1.14 • General structure of a multicomputer operating system
Multicomputer Operating Systems (2) 1.15 • Alternatives for blocking and buffering in message passing.
Distributed Shared Memory Systems (1) • Pages of address space distributed among four machines • Situation after CPU 1 references page 10 • Situation if page 10 is read only and replication is used
Distributed Shared Memory Systems (2) 1.18 • False sharing of a page between two independent processes.
Network Operating System (1) 1-19 • General structure of a network operating system.
Network Operating System (2) 1-20 • Two clients and a server in a network operating system.
Network Operating System (3) 1.21 • Different clients may mount the servers in different places.
DOS and NOS v.s DS • NOS qualifies as DS? • No single coherent view • Scalable and open • DOS qualifies as DS? • Computers are not independent • Easy to use and transparent
Positioning Middleware 1-22 • General structure of a distributed system as middleware.
Middleware and Openness 1.23 • In an open middleware-based distributed system, the protocols used by each middleware layer should be the same, as well as the interfaces they offer to applications.
Comparison between Systems • A comparison between multiprocessor operating systems, multicomputer operating systems, network operating systems, and middleware based distributed systems.
Client and Server General interaction between a client and a server. • -Clients request services. • -Servers provide services by replying to the requests.
An Example Client and Server (1) • The header.h file used by the client and server. • Client and server need to agree on a couple things first!
An Example Client and Server (2) • A sample server (A basic server loop) • -The server continually listens for new requests (receive). • -Extracting the request and simply executes it. • -It then prepares a response message and sends it back to the requesting client.
An Example Client and Server (3) 1-27 b • Sample Client: using the server to copy a file.
Application Layering • A client-server application typically consists of three layers: • 1.User-Interface level: • +Consists of the programs that allow end users to interact with application. • 2.Processing level: • +Implements the application logic (core functionality) • +Typically implemented at the server side • 3.Data level • +Maintain the actual data on which the application operate. • +Also keep data consistent across different application
Web browser 1-28 Application server Database server Application Layering • The general organization of an Internet search engine into three different layers
Client-Server Architectures Two-tiers Architectures 1-29 • Alternative client-server organizations (a) – (e).
Client-Server Architectures (con.t) Three-tiers Architectures 1-30 • An example of a server acting as a client.
Modern Architectures -Vertical distribution Model : where the tiers corresponded directly with the logical organization of the apps (The previous architectures) -Horizontaldistribution Model: where the application layers are physically split up into logically equivalent part -> This is how high-performance web services (e.g., Google, Amazon, E-bay..) • An example of horizontal distribution of a Web service.