CS603 Basics of underlying platforms

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CS603Basics of underlying platforms

• January 9, 2002

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Basics of Underlying Platforms

• Why do we want distributed systems?
• Basic hardware concepts
• Software concepts
• System models

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Distributed System Goals

• Connect users and resources
• Why not place resource and user together?
• Multiple users share same resource
• We dont know where the user will be (ATM)
• Mobile users
• Transparency
• User shouldnt be aware system is distributed
• Is this always true?

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Distributed System Goals (continued)

• Distributed systems should be open
• Allow user to choose platform
• Separate development of resource/client
• Distributed systems should scale
• Number of users
• Size of resource
• Number of types of clients/servers

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Hardware

• What machines are used (nodes)?
• Homogenous
• Client/server
• All clients identical
• Multiple types of clients
• Multi-server
• Replicated servers
• Different resources

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Hardware

• How are things connected?
• Is a multiprocessor a distributed system?
• Homogeneous networks (e.g., LAN)
• Different types of networks (wireless, dial-up,
  LAN)
• How does this affect system?
• Build to lowest common denominator?
• Different views for different paths?
• Violates transparency!

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Software concepts

• Distributed operating systems
• Conceptually the ultimate in transparency
• User/programmer view is single system
• OS handles all mapping
• Resource request is system call
• Response comes automatically
• Same code for single site or distributed
• In practice, programmer can see distribution
• Enables concurrent processing
• Shared memory view
• OS provides constructs to support sharing

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Software ConceptsConcurrency Primitives

• Semaphore
• Value with operations Increment and Decrement
• Decrement blocks if value 0
• Alternative view Test and Set
• Key Semaphore operations atomic
• Monitor
• Object (package of values and procedures)
• Atomicity Only one process may execute object
  at any time
• Thought exercise Implement Semaphore with
  Monitors

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Software ConceptsConcurrency Primitives

• What went wrong? Blocked processes
• Solution Additional primitives
• Wait
• Signal
• How does this affect only one process at a
  time?
• Wait effectively exits monitor
• Resume after wait

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Software Concepts

• Problem How to implement shared memory across a
  network?
• Similar to paging, but paged out is on a
  different machine, not disk
• Need global page table
• Distributed OS handles the details
• What does this do to performance?
• Alternative Expose distribution
• Message passing
• Remote procedure call

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Software Concepts

• Network Operating System
• No single system view
• Primitives/commands handle distribution
• rsh/rcp Command level remote procedure call and
  remote file transfer
• Primitives for cross-machine concurrency control
  (e.g., flock on NFS)
• Big problem User must be location-aware!

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Software ConceptsMiddleware approach

• What does middleware do?
• Doesnt hide distribution
• Hides location
• Hides heterogeneity
• Examples
• Unix (and especially Plan 9) Everything is a
  file
• Remote Procedure Call
• Distributed Objects
• Distributed Documents

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CS603Basics of underlying platforms

• January 11, 2002

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Distributed System Models

• Client-Server
• Resource sits at a server
• Application sits at a client
• Server waits for client to request resource
• Three-tier Architecture
• Client-(Client-Server)
• Sometimes tighter coupling than client-server
  between lower levels
• Peer to Peer
• Everyone is client and server
• What do these have in common?

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Distributed System ModelsWhat is next?

• Push models
• Not just client driven
• Otherwise looks like client-server (or
  multi-tier)
• Agent-based models
• Is this anything new?
• Any other ideas?