Lec1 - ECE151 Lecture 1 Chapter 1 Introduction ECE 151...

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ECE 151 – Lecture 1 1 ECE151 - Lecture 1 Chapter 1 Introduction
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ECE 151 – Lecture 1 2 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.
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ECE 151 – Lecture 1 3 Definition of a Distributed System (2) A distributed system organized as middleware. Note that the middleware layer extends over multiple machines. 1.1
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ECE 151 – Lecture 1 4 Goals of Distributed Systems Connecting resources and users Distribution transparency Openness Scalability
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ECE 151 – Lecture 1 5 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 Hide the failure and recovery of a resource Failure 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 Hide where a resource is located Location Hide differences in data representation and how a resource is accessed Access Description Transparency
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ECE 151 – Lecture 1 6 Degree of Transparency Observation: Aiming at full distribution transparency may be too much: 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 computer from a failing one You can never be sure that a server actually performed an operation before a crash Full transparency will cost performance, exposing distribution of the system Keeping Web caches exactly up-to-date with the master copy Immediately flushing write operations to disk for fault tolerance
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ECE 151 – Lecture 1 7 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
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ECE 151 – Lecture 1 8 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?
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