ECI-M-811 Distributed Systems and Internetworking

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ECI-M-811 Distributed Systems and Internetworking

• Coordinator Dr. Zhanfang Zhao
• Office T409
• Tel [phone]
• Fax [phone]
• Email zhaoza_at_lsbu.ac.uk
• URL http//ecce3.sbu.ac.uk/staff/zhaoza/dsi/
• Blackboard http//blackboard.lsbu.ac.uk/

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Textbooks

• Core reading
• Distributed Systems Principles and Paradigms
  Tanenbaum AS. Prentice Hall 2002.
• Background reading
• Distributed Systems Concept and Design
  Coulouris G etc, Addison Wesley, 2001.

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Topics

• Introduction to DS
• Communication in DS
• Process in DS
• Naming System in DS
• Synchronization in DS
• New technologies

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Outcomes (Distributed System Part)

• At the end of this unit the student will be able
  to
• Familiar with the concepts of distributed systems
  hardware and distributed system facilities.
• Able to appreciate the capabilities and
  limitations of applications software running over
  local and wide area networks.
• Able to understand, design and implement simple
  client-server systems.
• Able to know how the World Wide Web system works

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Introduction to Distributed System

• Contents of this lesson
• Definition of the Distributed System
• The Goals when Building a Distributed System
• Basic Software Concepts
• The Middleware
• System Architecture Client-Server Model

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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.
  --Tanenbaum
• Two aspects of this definition
• Hardware The machines are autonomous.
• Software The user think they are dealing with a
  single system.
• Important characteristics of DS
• Differences between various computers and the
  ways in which they communicate are hidden from
  users
• Users and applications can interact with a DS in
  a consistent and uniform way
• Ds should also be easy to extend or scale
• To support heterogeneous computers and networks
  while offering a single system view, DS is often
  organized in certain layers called middleware

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Definition of a Distributed System (2)

• A distributed system is a collection of
  autonomous hosts that that are connected through
  a computer network. Each host executes components
  and operates a distribution middleware, which
  enables the components to coordinate their
  activities in such a way that users perceive the
  system as a single, integrated computing
  facility.
• -----------Wolfgang Emmerich, 2000

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Definition of a Distributed System (3)
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Middleware Examples

• Transaction-oriented
• IBM CICS
• BEA Tuxedo
• IBM Encina
• Microsoft Transaction Server
• Message-oriented
• Microsoft Message Queue
• Sun Tooltalk

• Procedural
• Sun ONC
• Linux RPCs
• OSF DCE
• Object-oriented
• OMG CORBA
• Sun Java/RMI
• Microsoft COM
• .net Remote
• Sun Enterprise Java Beans

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Centralised System Characteristics

• One component with non-autonomous parts
• Component shared by users all the time
• All resources accessible
• Software runs in a single process
• Single Point of control
• Single Point of failure

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Distributed System Characteristics

• Multiple autonomous components
• Components are not shared by all users
• Resources may not be accessible
• Software runs in concurrent processes on
  different processors
• Multiple Points of control
• Multiple Points of failure

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Goals

• Four important goals should be met to make it
  worth to build a distributed systems
• Connecting users and resources
• Transparency
• Openness
• Scalability

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Transparency in a Distributed System
Transparency Description
Access Hide differences in data representation and how a resource is accessed
Location Hide where a resource is located
Migration Hide that a resource may move to another location
Relocation Hide that a resource may be moved to another location while in use
Replication Hide that a resource may be replicated
Concurrency Hide that a resource may be shared by several competitive users
Failure Hide the failure and recovery of a resource
Persistence Hide whether a (software) resource is in memory or on disk
Different forms of transparency in a distributed
system. Definition A DS that is able to present
itself to users and applications as if it were
only a single computer system is said to be
transparent
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Transparency

• Distributed systems should be perceived by users
  and application programmers as a whole rather
  than as a collection of cooperating components.
• Transparency has different dimensions
• These represent various properties that
  distributed systems should have.

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Distribution Transparency
Concurrency Transparency
Access Transparency
Location Transparency
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Access Transparency

• Enables local and remote information objects to
  be accessed using identical operations.
• Example File system operations in NFS.
• Example Navigation in the Web.
• Example SQL Queries

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Location Transparency

• Enables information objects to be accessed
  without knowledge of their location.
• Example File system operations in NFS
• Example Pages in the Web
• Example Tables in distributed databases

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Concurrency Transparency

• Enables several processes to operate concurrently
  using shared information objects without
  interference between them.
• Example Automatic teller machine network
• Example Database management system

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Replication Transparency

• Enables multiple instances of information objects
  to be used to increase reliability and
  performance without knowledge of the replicas by
  users or application programs
• Example Distributed DBMS
• Example Mirroring Web Pages.

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Failure Transparency

• Enables the concealment of faults
• Allows users and applications to complete their
  tasks despite the failure of other components.
• Example Database Management System

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Migration Transparency

• Allows the movement of information objects within
  a system without affecting the operations of
  users or application programs
• Example NFS
• Example Web Pages

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Performance Transparency

• Allows the system to be reconfigured to improve
  performance as loads vary.
• Example Distributed make.

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Scaling Transparency

• Allows the system and applications to expand in
  scale without change to the system structure or
  the application algorithms.
• Example World-Wide-Web
• Example Distributed Database

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Openness

• An open distributed system is a system that
  offers services according to standard rules that
  describe the syntax and semantics of those
  services.
• Openness means that the system can easily be
  extended and modified.
• To facilitate the openness, the system should
  have a well defined and well-documented
  interfaces. These interface must declare the
  services that a component offers. And these
  interfaces are often described in an Interface
  Define Language (IDL)
• A service is an operation that a component
  performs on behalf of a user or another
  component.
• The interface definition should be complete and
  neutral.
• Complete means that everything that is necessary
  to make an implementation has indeed been
  specified.

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Scalability

• Adaption of distributed systems to
• accomodate more users
• respond faster (this is the hard one)
• Usually done by adding more and/or faster
  processors.
• Components should not need to be changed when
  scale of a system increases.
• Design components to be scalable!

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Scalability Problems
Concept Example
Centralized services A single server for all users
Centralized data A single on-line telephone book
Centralized algorithms Doing routing based on complete information
Examples of scalability limitations.
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Software Concepts
System Description Main Goal
DOS Tightly-coupled operating system for multi-processors and homogeneous multicomputers Hide and manage hardware resources
NOS Loosely-coupled operating system for heterogeneous multicomputers (LAN and WAN) Offer local services to remote clients
Middleware Additional layer atop of NOS implementing general-purpose services Provide distribution transparency

• An overview between
• DOS (Distributed Operating Systems)
• NOS (Network Operating Systems)
• Middleware

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Uniprocessor Operating Systems

• Separating applications from operating system
  code through a microkernel.

1.11
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Multiprocessor Operating Systems (1)

• An important extension of MOS is that it supports
  multiple processors having access to a shared
  memory. These data in the shared memory must be
  protected against the concurrent access to
  guarantee consistency.
• Two synchronized primitives are used, one is
  semaphore, another is monitor

monitor Counter private int count
0 public int value() return count void
incr () count count 1 void decr()
count count 1

• A monitor to protect an integer against
  concurrent access.

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Multiprocessor Operating Systems (2)
monitor Counter private int count 0 int
blocked_procs 0 condition unblocked public
int value () return count void incr ()
if (blocked_procs 0) count
count 1 else signal
(unblocked)
void decr() if (count 0) blocked_procs
blocked_procs 1 wait (unblocked)
blocked_procs blocked_procs 1 else
count count 1

• A monitor to protect an integer against
  concurrent access, but blocking a process.

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Multicomputer Operating Systems (1)
1.14

• General structure of a multicomputer operating
  system

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Multicomputer Operating Systems (2)

• Alternatives for blocking and buffering in
  message passing.

1.15
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Network Operating System (1)

• General structure of a network operating system.

1-19
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Network Operating System (2)

• Two clients and a server in a network operating
  system.
• File systems is supported by one or more machines
  called file servers

1-20
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Positioning Middleware

• General structure of a distributed system as
  middleware.

1-22
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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.

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System Architecture Client Server Model

• Important styles of architecture for distributed
  systems
• Layered architectures
• Object-based architectures
• Data-centered architectures
• Event-based architectures

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Architectural Styles (1)

• The layered architectural style and

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Architectural Styles (2)

• The object-based architectural style.

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Architectural Styles (3)

• The event-based architectural style (
  Publish/Subscribe System)

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Architectural Styles (4)

• The shared data-space architectural style.

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Clients and Servers
1.25

• General interaction between a client and a
  server.
• In the basic client server model, the process in
  a computing system are divided into two groups. A
  server is a process implementing a specific
  service, for example database service. A client
  is a process that requests a service from a
  server by sending it a request and subsequently
  waiting or the servers reply

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An Example Client and Server (1)
Used for parameters

• The header.h file used by the client and server.

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An Example Client and Server (2)

• A sample server.

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An Example Client and Server (3)
1-27 b

• A client using the server to copy a file.

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Application Layering

• For the web applications, the CS applications
  generally been organized into three levels

1-28
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Multitiered Architectures (1)

• Alternative client-server organizations (a) (e).

1-29
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Multitiered Architectures (2)

• An example of a server acting as a client.

1-30
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Modern Architectures

• An example of horizontal distribution of a Web
  service.

1-31
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Distributed System Paradigms

• World Wide Web Architecture Model
• Traditional Web-based Systems
• Overall organization
• Web Documents
• Programming Languages HTML, XML MIME
  (Multipurpose Internet Mail Exchange)
• Multi-tiered Architectures
• Web Services

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Traditional Web-Based Systems

• Figure 12-1. The overall organization of a
  traditional Web site.

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Web Documents

• Six top-level MIME types and some common subtypes.

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Multitiered Architectures

• The principle of using server-side CGI programs.

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Web Services Fundamentals

• The principle of a Web service
• WSDL (Web Services Definition Language) is a
  formal language very much the same as the
  interface definition languages of RPC
• SOAP ( Simple Object Access Protocol) is
  essentially a framework in which much of the
  communication between two process can be
  standardized
• UDDI ( Universal Description, Discovery and
  Integration)