introduction
DESCRIPTION
Introduction. Outline. Definitions Examples Hardware concepts Software concepts Readings: Chapter 1. Quotable quote. “I think there is a world market for maybe five computers” Thomas J. Watson Chairman of IBM, 1943. Definition of a Distributed System (Tannenbaum and van Steen). - PowerPoint PPT PresentationTRANSCRIPT
Introduction
Outline
Definitions Examples Hardware concepts Software concepts Readings: Chapter 1
Quotable quote
“I think there is a world market for maybe five computers”
Thomas J. WatsonChairman of IBM, 1943
Definition of a Distributed System (Tannenbaum and van Steen)
A distributed system is a piece of software that ensures that
A collection of independent computers that appears to its users as a single coherent system.
Definition of a Distributed System (Colouris)
A distributed system is
In which hardware and software components located at networked computers communicate and coordinate their actions only by passing messages.
Definition of a Distributed System (Lamport)
A distributed system is one in which I cannot get something done because a machine I've never heard of is down.
Primary Characteristics of a Distributed System
Multiple computers Concurrent execution Independent operation and failures
Communications Ability to communicate No tight synchronization
Relatively easy to expand or scale Transparency
Example: A typical intranet (Coulouris)
the rest of
email server
Web server
Desktopcomputers
File server
router/firewall
print and other servers
other servers
Local areanetwork
email server
the Internet
intranet
ISP
desktop computer:
backbone
satellite link
server:
network link:
Example: A typical portion of the Internet (Coulouris)
Example: Portable and handheld devices in a distributed system
(Coulouris)
Laptop
Mobile
PrinterCamera
Internet
Host intranet Home intranetWAP
Wireless LAN
phone
gateway
Host site
Why Build Distributed Systems?
Economics Share resources Relatively easy to expand or scale Speed – A distributed system may have more total
computing power then a mainframe. Reliability
If a machine crashes, the system as a whole can survive. People are distributed, information is distributed.
Key Design Goals
Connectivity Transparency Reliability Consistency Security Openness Scalability
Connectivity
It should be easy for users to access remote resources and to share them with other users in a controlled fashion.
Resources that can be shared include printers, storage facilities, data, files, web pages, etc; Why? Economical
Connecting users and resources makes collaboration and the exchange of information easier. Just look at e-mail
Transparency
A distributed system that is able to present itself to users and applications as if it were only a single computer system is said to be transparent.
Very difficult to make distributed systems completely transparent.
You may not want to, since transparency often comes at the cost of performance.
Transparency in a Distributed System
Different forms of transparency in a distributed system.
Transparency Description
AccessHide 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
RelocationHide that a resource may be moved to another location while in use
ReplicationHide that a resource may be shared by several competitive users
ConcurrencyHide 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
Degree of Transparency
The goal of full transparency is not always desirable. Users may be located in different continents; distribution is apparent and
not something you want to hide. Completely hiding failures of networks and nodes is (theoretically and
practically) impossible: You cannot distinguish a slow compuer from a failing one. You can never be sure that a server actually performed an operation
before a crash. Full transparency will cost in performance.
Keeping Web caches exactly up-to-date with the master copy Immediately flushing write operations to disk for fault tolerance.
Openness
An open distributed system allows for interaction with services from other open systems, irrespectively of the underlying environment. Systems should conform to well-defined interfaces. Systems should support portability of applications. Systems should easily interoperate. Interoperability is
characterized by the extent by which two implementations of systems or components from different manufacturers can co-exist and work together.
Example: In computer networks there are rules that govern the format, contents and meaning of messages send and received.
Scalability
There are three dimensions to scalability: The number of users and processes (size scalability) The maximum distance between nodes (geographical
scalability) The number of administrative domains (administrative
scalability) Most systems focus on administrative size scalability
Well sort of – the typical solution is to buy powerful servers.
Techniques for Scaling
Partition data and computations across multiple machines Move computations to clients (Java applets) Decentralized naming services (DNS) Decentralized information systems (WWW)
Make copies of data available at different machines Replicated file servers (for fault tolerance) Replicated databases Mirrored web sites
Allow client processes to access local copies Web caches (browser/Web proxy) File caching (at server and client)
Scaling – The problem
Applying scaling techniques is easy, except for the following: Having multiple copies (cached or replicated) leads to
inconsistencies – modifying one copy makes that copy different from the rest.
Always keeping copies consistent requires global synchronization.
Global synchronization is expensive with respect to performance.
We have learned to tolerate some inconsistencies.
Challenges
Heterogeneity Networks Hardware Operating systems Programming languages
Challenges
Failure Handling Partial failures
Can non-failed components continue operation? Can the failed components easily recover?
Detecting failures Recovery Replication
Hardware Concepts
Multiprocessors Multicomputers Networks of computers
Multiprocessors and Multicomputers
1.6
Different basic organizations and memories in distributed computer systems
Multiprocessors Coherent memory
Each CPU writes though, reflected at other immediately Note the use of cache memory for efficiency Limited to small number of processors
Homogeneous Multicomputer Systems
• Grid• Hypercube
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Multiprocessor Usage
• Scientific and engineering applications often require loops over large vectors e.g., matrix elements or points in a grid or 3D mesh. Applications include
• computational fluid dynamics• dynamic structures• scheduling (airline)• health and biological modeling• economics and financial modelling (e.g., option pricing)
• Other applications include:• Transaction processing• Video on demand
Networks of Computers
High degree of node heterogeneity Nodes include PCs, workstations, multimedia
workstations, palmtops, laptops High degree of network heterogeneity
This includes local-area ethernet, Atm and wireless connections.
A distributed system should try to hide these differences. In this course, the focus really is in networks of computers.
Software Concepts
An overview between DOS (Distributed Operating Systems) NOS (Network Operating Systems) Middleware
System Description Main Goal
DOSTightly-coupled operating system for multi-processors and homogeneous multicomputers
Hide and manage hardware resources
NOSLoosely-coupled operating system for heterogeneous multicomputers (LAN and WAN)
Offer local services to remote clients
MiddlewareAdditional layer atop of NOS implementing general-purpose services
Provide distribution transparency
Distributed Operating System
OS on each computer knows about the other computers
OS on different computer is generally the same
Services are generally (transparently) distributed across computers.
1.14
Distributed Operating System
This is harder to implement then a traditional operating system. Why? Memory is not shared No simple global communication No simple systemwide synchronization mechanisms May require that OS maintain global memory map in
software. No central point where resource allocation decisions can
be made. Only very few truly multicomputer operating systems exist.
Network Operating System
Each computer has its own operating system with networking facilities
Computers work independently i.e., they may even have different operating systems
Services are tied to individual nodes (ftp, telnet, www) Highly file oriented
Middleware
OS on each computer need not know about the other computers
OS on different computers may be different
Services are generally (transparently) distributed across computers.
Middleware and Openness
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.
1.23
Middleware Services
Communication Services Hide “primitive” socket programming
Data management in a distributed system Naming services Directory services (e.g., LDAP, search engines) Location services for tracking mobile objects Persistent storage facilities Data caching and replication
Middleware Services
Services giving applications control over when, where and how they access data. Distributed transaction processing Code migration
Services for securing processing and communication: Authentication and authorization services Simple encryption services Auditing services
There are varying levels of success in being able to provide these types of middleware services.