d istributed s ystem unit-1 prepared by: g.s.mishra
TRANSCRIPT
DISTRIBUTED SYSTEMUNIT-1
Prepared By: G.S.Mishra
WHAT IS A DISTRIBUTED SYSTEM?
WHAT IS A DISTRIBUTED SYSTEM? CONTD….
• “ A distributed system in which hardware or, software components located at networked computers communicate & coordinate their actions only by passing messages.”
Consequences:
Concurrency No global clock Independent failure
EXAMPLES OF DISTRIBUTED SYSTEM
The Internet
The Intranet
Mobile & Ubiquitous Computing
MOBILE SYSTEMS Cellular Concept Frequency Reuse
CLUSTERS
ARCHITECTURE
TERMINOLOGIES USED
OTHER EXAMPLES
Web server ( Centralized )
MP3.com : a number of MP3 files stored in the Web
site.
Failure of this site cause unavailability of services
Napster: stores the MP3 files on the actual users’
machines
napster.com is used as a massive index (or meeting
place) for connecting users
EXAMPLES CONTD….
Users connect to Napster to search for the files
they desire
Thereafter connect to users directly to download
the file.
MP3 file is distributed across a number of servers
making it more reliable against failure.
But search is centralized.
Gnutella does not have a centralized search facility nor a central storage facility for the files.
EXAMPLE CONTD…
Each user in the network runs a servent (a client
and a server),
which allows him/her to act as both a provider and
consumer of information (as in Napster)
but acts as a search facility also.
if any of the servents are unavailable, users can
almost certainly still reach the file they require
DIFFERENCE BETWEEN CENTRALIZED & DECENTRAIZED SYSTEMS
Broad Areas:1. Resource Discovery2. Resource Availability3. Resource Communication
1-Resource Discovery:
A two-stage process The discovery service needs to be located The relevant information is retrieved. E.g. DNS
DIFFERENCE….. CONTD…
Resource Availability:
Web servers fall into the centralized category here because there is only one IP address that hosts a particular site.
If that machine goes down then the Web site is unavailable
machines could be made fault tolerant by replicating
the web site employ some internal switching mechanisms but
the availability of the IP address remains the same.
In a more decentralized approach; by offering many duplicate services that can perform the same functionality.
Resource availability is tied in closely to resource discovery.
3- Resource Communication
1. Brokered Communication: where the communication is always passed through
a central server and therefore a resource does not have to reference the other resource directly
2. Point-to-Point (or Peer-to-Peer) Communication:
this involves a direct connection (although this connection may be multi-hop) betweenthe sender and the receiver. In this case, the sender is aware of the receiver’s location.
GOALS OF DISTRIBUTED SYSTEM
Advantages of distributed system over centralized system
GOALS OF DISTRIBUTED SYSTEM CONTD…
Advantages of Distributed System over Independent PCs
GOALS OF DISTRIBUTED SYSTEM CONTD…
Disadvantages of Distributed System
HARDWARE CONCEPTS : TAXONOMY OF PARALLEL & DISTRIBUTED COMPUTER SYSTEMS
BUS BASED MULTIPROCESSOR
MULTICOMPUTERS WORKSTATION ON LAN
SWITCHED MULTIPROCESSORS
SWITCHED MULTICOMPUTERS
SOFTWARE CONCEPTS
SOFTWARE CONCEPTS
SOFTWARE CONCEPTS
SOFTWARE CONCEPTS
SOFTWARE CONCEPTS
SOFTWARE CONCEPTS
RESOURCE SHARING hardware resources like printers or disks to
reduce costs sharing databases, a set of web pages, search
engine
Service: file services - read, write, delete. We buy goods by electronic payment service
Server: a running program (process) on a networked computer that accepts requests
Remote invocation: The complete interaction between the client and the server from sending request to receiving server’s response
THE WORLD WIDE WEB
CHALLENGES
Heterogenity :
Networks, computer hardware, Operating systems, Programming languages
Openness
Security : Denial of Service attack
Scalability
CHALLENGES CONTD…
Failure Handling:
Detecting failures, Masking failures, tolerating failures, Recovery from failures, Redundancy
Concurrency
Transparency: Access transparency- Enables local & remote
resources to be accessed using identical operations.
CHALLENGES CONTD…
Location transparency- Enables resources to be accessed without knowledge of their location.
Concurrency transparency- Enables processes to operate concurrently using shared resources without interference between them.
Replication transparency- enables multiple instances of resources to be used to increase reliability and performance without knowledge of replicas by users.
CHALLENGES CONTD…Failure transparency- enables the concealment of
faults, allowing users and application programs to complete their tasks despite the failure of hardware or software components.
Mobility transparency- allow the movement of resources and clients within a system without affecting the operation of users or programs.
Performance transparency- allows the system to be reconfigured to improve performance as loads vary.
Scaling transparency- allows the system and applications to expand in scale without change to the system structure.
WEB SERVERS AND WEB BROWSERS
SHARED MEMORY MULTI-PROCESSOR
SYSTEM MODELS Architectural Model: Concerned with the
placement of parts and the relationships between them.
Software Layers System Architecture
Fundamental model: Concerned with a more formal description of the properties that are common in all architectural models.
Interaction Model Failure Model Security Model
SOFTWARE LAYERS
SYSTEM ARCHITECTURES
The working of Search engines
CLIENT SERVER MODEL
SERVICES PROVIDED BY MULTIPLE SERVERS
For Replicated data
WEB PROXY SERVER
Proxy servers are used as a cache to store recently used data objects, sharing other web resources, to increase availability and performance, reducing load on a network & web servers.
A DISTRIBUTED APPLICATION BASED ON PEER PROCESSES
All of the processes play similar roles, interacting cooperatively as peers to perform a distributed activity without any distinction between clients and servers.
WEB APPLELTS
FUNDAMENTAL MODELS
Fundamental model: Concerned with a more formal description of the properties that are common in all architectural models.
Interaction Model Failure Model Security Model
INTERACTION MODEL
Performance of communication channel
Latency: delay between the start of a messages transmission from one process and the beginning of its receipt by another process.
Bandwidth: Total amount of information transmitted in a given time.
Jitter: The time taken to deliver a series of messages.
INTERACTION MODEL CONTD…
o Two variants of the interaction model Synchronous Distributed system Asynchronous Distributed system
o Computer clocks & Timing Events
o Event ordering
REAL TIME ORDERING OF EVENT
FAILURE MODEL
Omission failure
Arbitrary failure
Masking failures
Reliability
SECURITY MODEL
Securing processes and their interactions
The enemy
Protecting objects
Defeating Security Threats
PROTECTING OBJECTS
THE ENEMY
SECURE CHANNELS
THIN CLIENTS AND COMPUTE SERVERS
OBJECTS AND PRINCIPALS
THE ENEMY
SECURE CHANNELS
THE WEB
SYSTEM MODELS Architectural Model:
Concerned with the placement of parts and the relationships between them.
Fundamental model:
Concerned with a more formal description of the properties that are common in all architectural models.
SYSTEM MODELSArchitectural Models:
The placements of its parts & relationship between them:
The partition of data or, replication
Requirements to add & remove mobile devices
conveniently
Use of mobile code & agents
Caching of data
Software Layers:
Applications, services Middleware Operating System Computer & Network Hardware
(OS & CN Hardware referd to as Platform)
ARCHITECTURAL MODELS
SYSTEM MODELSArchitectural Models:
The placements of its parts & relationship between them:
The partition of data or, replication
Requirements to add & remove mobile devices
conveniently
Use of mobile code & agents
Caching of data
ORDERING OF EVENTSLamport’s Happened Before relationship:
For two events a and b, a → b if a and b are events in the same process and a occurred
before b a is an event of sending a message m and b is the
corresponding receive event at the destination process If a → c and c → b for some event c, then a → c (transitive relation )
ORDERING OF EVENTSLamport’s Happened Before relationship:
For two events a and b, a → b if a and b are events in the same process and a occurred
before b a is an event of sending a message m and b is the
corresponding receive event at the destination process If a → c and c → b for some event c, then a → c (transitive relation )
ORDERING OF EVENTSLamport’s Happened Before relationship:
For two events a and b, a → b if a and b are events in the same process and a occurred
before b a is an event of sending a message m and b is the
corresponding receive event at the destination process If a → c and c → b for some event c, then a → c (transitive relation )
ORDERING OF EVENTSLamport’s Happened Before relationship:
For two events a and b, a → b if a and b are events in the same process and a occurred
before b a is an event of sending a message m and b is the
corresponding receive event at the destination process If a → c and c → b for some event c, then a → c (transitive relation )
ORDERING OF EVENTSLamport’s Happened Before relationship:
For two events a and b, a → b if a and b are events in the same process and a occurred
before b a is an event of sending a message m and b is the
corresponding receive event at the destination process If a → c and c → b for some event c, then a → c (transitive relation )
ORDERING OF EVENTSLamport’s Happened Before relationship:
For two events a and b, a → b if a and b are events in the same process and a occurred
before b a is an event of sending a message m and b is the
corresponding receive event at the destination process If a → c and c → b for some event c, then a → c (transitive relation )