ECE 151: Goals, Transparency, Scalability, and Techniques in Distributed Systems, Study notes of Electrical and Electronics Engineering

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ECE151 - Lecture 1

Chapter 1

Introduction

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.

Goals of Distributed Systems

 Connecting resources and users

 Distribution transparency

 Openness

 Scalability

Transparency in a Distributed System

Different forms of transparency in a distributed system.

Hide whether a (software) resource is in memory or on disk Persistence Failure Hide the failure and recovery of a resource Hide that a resource may be shared by several competitive users Concurrency Hide that a resource may be available on several distinct computers Replication Hide that a resource may be moved to another location Relocation Hide that a resource may be moved to another location while in use Migration Location Hide where a resource is located Hide differences in data representation and how a resource is accessed Access Transparency Description

Openness of Distributed Systems

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

Policies versus Mechanisms

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?

Implementing openness: Ideally, distributed operating systems and

middleware provide 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

Scale in Distributed Systems

Observation: Many developers of modern distributed system

easily use the adjective “scalable” without making clear why

their system actually scales.

Scalability: At least three components:

Number of users and/or processes (size scalability)

Maximum distance between nodes (geographical scalability)

Number of administrative domains (administrative scalability)

Most systems account only, to a certain extent, for size

scalability. The (non)solution: powerful servers.

Today, the challenge lies in geographical and administrative

scalability.

Techniques for Scaling

Distribution: Partition data and computations 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 tolerance)  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 (1)

The difference between letting:

a) a server or

b) a client check forms as they are being filled

Scaling Techniques (2)

An example of dividing the DNS name space into zones.

Multiprocessors (1)

A bus-based multiprocessor.

Multiprocessors (2)

a) A crossbar switch b) An omega switching network

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 connections  Wide-area switched megabit connections

Observation: Ideally, a distributed system hides these differences

Software Concepts

An overview between

• DOS (Distributed Operating Systems)
• NOS (Network Operating Systems)
• Middleware Provide distribution transparency Additional layer atop of NOS implementing general-purpose services Middleware Offer local services to remote clients Loosely-coupled operating system for heterogeneous multicomputers (LAN and WAN) NOS Hide and manage hardware resources Tightly-coupled operating system for multi- processors and homogeneous multicomputers DOS System Description Main Goal