Chapter 1

transcript

Introduction

Chapter 1

Definition of a Distributed System (1)

Distributed System is:

A collection of independent computers that appears to its users

as a single coherent system

Definition of a Distributed System (2)

• A distributed system organized as a middleware

• Note that the middleware layer extends over multiple machines

Distributed Systems: Examples (1)

Network of workstations in a company departmentsuch a system have a single file system, all files accessible

from all machines in the same way and using the same pathname

when a user types a command, the system could look for the best place to execute that command

if the system as a whole looks and acts like a classical single-processor time-sharing system

A workflow information systemsupports the automatic processing of orders

used by people from several departments, possibly at different locations

people from sales department may be spread across an entire country

orders are placed by means of laptop computers that are connected to the system through cellular phones

Internet

A typical Intrarnet

Portable / handheld devices in Distributed Systems

Key Design Challenges

•Connecting resources and users

•Distribution transparency

•Heterogeneity

•Openness

•Scalability

•Concurrency

•Security

•Failure handling

•Mobility and location dependency

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

Replication Hide that a resource is replicated

ConcurrencyHide that a resource may be shared by several competitive users

Failure Hide the failure and recovery of a resource

PersistenceHide whether a (software) resource is in memory or on disk

Openness (1/3)Open distributed system: Be able to interact with services from other open systems, irrespective of the underlying environment:• systems should conform to well-defined interfaces

• systems should support portability of applications

• systems should easily interoperate

Achieving openness: At least make the distributed system independent from heterogeneity of the underlying environment:• hardware

• platforms

• languages

Openness (2/3)

Implementing Openness: Requires support for different policies specified by applications and users:

• What level of consistency do we require for client cached data?

• Which operations do we allow downloaded code to perform?

• Which QoS requirements do we adjust in the face of varying bandwidth?

• What level of secrecy do we require for communication?

Openness (3/3)Implementing Openness: Ideally, a distributed system provides only mechanisms;

• Allow (dynamic) setting of caching policies, preferably per cachable item

• Support different levels of trust for mobile code

• Provide adjustable QoS parameters per data stream

• Offer different encryption algorithms

Scalability

•Size scalabilitynumber of users and/or processes

•Geographical scalabilitymaximum distance between nodes

•Administrative scalabilitynumber of administrative domains

Today, the challenge lies in geographical and administrative scalability

Scalability Problems

Examples of scalability limitations

Concept Example

Centralized services A single server for all users

Centralized data A single on-line telephone book

Centralized algorithmsDoing routing based on complete information

Scaling Techniques (1/3)Distribution: Partition data and computation across multiple machines:• Move computations to clients (Java applets)• Decentralized naming services (DNS)• Decentralized information systems (WWW)

Replication: Make copies of data available at different machines:• Replicated file servers (mainly for fault toleranc e)• Replicated databases• Mirrored Web sites• Large-scale distributed shared memory systems

Caching: Allow client processes to access local copies:• Web caches (browser/Web proxy)• File caching (at server and client)

Scaling Techniques (2/3)

The difference between letting:

a) a server or

b) a client check forms as they are being filled

Scaling Techniques (3/3)

An example of dividing the DNS name space into zones

Mobility and Location Dependency

Moving changes everything.

Location management

Tracking moving objects

Location-dependent data management

Location Based Services (LBS)

Hardware Concepts

Different basic organizations and memories in distributed computer systems

Multiprocessors (1)

A bus-based multiprocessor

Multiprocessors (2)

a) A crossbar switchb) An omega switching network

Homogeneous Multicomputer Systems

a) Gridb) Hypercube

Heterogeneous Multicomputer Systems: Networks of Computers

High degree of node heterogeneity• High-performance parallel systems (multiprocessors as well as

multicomputers)• High-end PCs and workstations (servers)• Simple network computers (offer users only network access)• Mobile computers (palmtops, laptops)• Multimedia workstations

High degree of network heterogeneity

• Local-area gigabit networks• Wireless connections• Long-haul, high-latency POTS (Plain Old Telephone Service)

connections• Wide-area switched megabit connections

Observation: Ideally, a distributed system hides these differencies

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 a top of NOS implementing general-purpose services

Provide distribution transparency

Uniprocessor Operating Systems

Separating applications from operating system code through a microkernel

• Built to manage computers with only a single CPU

• Allow users and applications an easy way of sharing resources such as CPU, main memory, disks and peripheral devices

Multiprocessor Operating Systems (1)

A monitor to protect an integer against concurrent access

monitor Counter {

private:

int count = 0;

public:

int value() { return count;}

void incr () { count = count + 1;}

void decr() { count = count – 1;}

•Main difference from the uniprocessor OS is that data are now accessible by multiple processors

•So, they have to be protected against concurrent access to guarantee consistency semaphore , monitor

Multiprocessor Operating Systems (2)

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

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;

signal (unblocked);

void decr() {

if (count ==0) {

blocked_procs = blocked_procs + 1;

wait (unblocked);

blocked_procs = blocked_procs – 1;

count = count – 1;

Multicomputer Operating Systems (1)Some characteristics:• OS on each computer knows about the other computers• OS on different computers generally the same

• Services are generally (transparently) distributed across computers

1.1Distributed operating system services

Kernel Kernel Kernel

Multicomputer Operating Systems (2)Harder than traditional (multiprocessor) OS: Because memory is not shared, processor communication is done by message passing:

• Often no single global communication

• No simple system wide synchronization mechanisms such as semaphores and monitors

• Virtual (distributed) shared memory requires OS to maintain global memory map in software

• Inherent distributed resource management: no central point where allocation decision can be made

Practice: Only very few truly multicomputer operating systems exist (example: Amoeba)

Multicomputer Operating Systems (3)

Alternatives for blocking and buffering in message passing

Distributed Shared Memory Systems (1)

• Practice shows that programming multicomputers is much harder than programming multiprocessors

• Communication by accessing shared data and using semaphores or monitors is much easier than message-passing facilities

• There has been considerable research in emulating shared memory on multicomputers

• The goal is to provide a distributed (virtual) shared memory machine, running on multicomputer

a) Pages of address space distributed among four machines

b) Situation after CPU 1 references page 10

c) Situation if page 10 is read only and replication is used

d) Replicate not only read-only pages, but all pages applying data consistency issues

False sharing of a page between two independent processes

• Another issue is deciding how large pages should be?

• Having large pages may possibly reduce the amount of data that is transferred

• But, if a page contains data of two independent processes on different processors, OS may need to repeatedly transfer the page between two processors

Network Operating Systems (1)Some characteristics: • Each computer has its own operating system with networking

facilities• Computers work independently (they may even have different

operating systems)• Services are tied to individual computers (ftp, telnet, WWW)• Highly file oriented (basically, processors share only files)

Network Operating System (2)

Two clients and a server in a network operating system

•There is a global file system accessible from all the workstations

•File system is supported one or more machines called file servers

Positioning Middleware

Some characteristics:• OS on each computer need not know about the other computers• OS on different computers need not generally be the same• Services are generally (transparently) distributed across

computers

Need for MiddlewareMotivation: Too many networked applications were hard or difficult to

integrate:

•Departments are running different NOSs

•Integration and interoperability only at level of primitive NOS services

•Need for federated information systems:–Combining different databases, but providing a single view to applications

–Setting up enterprise-wide Internet services, making use of existing information systems

–Allow transactions across different databases

–Allow extensibility for future services (e.g., mobility, teleworking, collaborative applications)

Constraint: use the existing operating systems, and treat them as the underlying environment (they provided the basic functionality anyway)

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

Middleware Services (1/3)

•Remote procedure calls across networks

•Remote object-based method invocation

•Message-queuing systems

•Advanced communication streams

•Event notification services

Communication services: Abandon primitive socket-based message passing in favor of :

•Large-scale, system-wide naming services (DNS)

•Advanced directory services (search engines)

•Location services for tracking mobile objects

•Data caching and replication

•Persistent storage facilities

Information system services: Services that help manage data in a distributed system:

•Distributed transaction processing

•Code migration

Control services: Services giving applications control over, when, where, and how they access data:

•Authentication and authorization services

•Simple encryption services

Security services: Services for secure processing and communication:

Comparison Between Systems

ItemDistributed OS

Network OS

Middleware-based OSMultipro

c.Multicomp

Degree of transparency

Very High High Low High

Same OS on all nodes

Yes Yes No No

Number of copies of OS

1 N N N

Basis for communication

Shared memory

Messages FilesModel

specific

Resource management

Global, central

Global, distributed

Per node Per node

Scalability No Moderately Yes Varies

Openness Closed Closed Open Open

Clients and Servers

Characteristics:

•There are processes offering services (servers)

•There are processes that use services (clients)

•Clients and servers can be distributed across different machines

•Clients follow request/reply model with respect to using services

Application Layering (1)

•Client-server applications are usually constructed with a distinction between three levels:

•User-interface level

•Processing level

•Data level

•Clients implement the user-interface level.

•Servers implement the rest !!

C/S Example: Processing Level (1)

The general organization of an Internet search engine into three different layers

C/S Examples: Multitiered Architectures (2)

Alternative client-server organizations (a) – (e)

C/S Examples: Multitiered Architectures (3)

An example of a server acting as a client

C/S Examples: Modern Architectures (4)

An example of horizontal distribution of a Web service

Multiple Server Model

Proxy Servers and Cache Model

Peer Process Model

The Applet Model

Mobile Code and Mobile Agent

Thin Clients

Spontaneous Networking (1)

Spontaneous Networking (2)

Key characteristics•easy connection to a local network•easy integration with local services•limited connectivity•security and privacy

Discovery services•registration service•lookup service

Design Requirements (1)

Performance Issues•Responsiveness - fast and consistent response•Throughput - high work completion rate•Computational Load Balancing

Quality of Service (QoS)•Reliability•Security•Performance•Adaptability

Design Requirements (2)

Caching and Replication•Admission policy and replica

placement•Replacement policy•Consistency and validation•Caching and replication protocols

Dependability•Correctness•Security•Fault tolerance

Chapter 1

Documents