Understanding Distributed Systems: Networking, Communication Protocols, and Coordination, Slides of Operating Systems

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Lecture 23

Chapter 16/18:

Distributed Systems

Chapter 16: Distributed System Structures

• Motivation
• Types of Network-Based Operating Systems
• Network Structure
• Network Topology
• Communication Structure
• Communication Protocols
• Robustness
• Design Issues
• An Example: Networking

Motivation

• Distributed system is collection of loosely coupled processors interconnected by a communications network
• Processors variously called nodes, computers, machines, hosts - Site is location of the processor
• Reasons for distributed systems
  • Resource sharing
    • sharing and printing files at remote sites
    • processing information in a distributed database
    • using remote specialized hardware devices
  • Computation speedup – load sharing
  • Reliability – detect and recover from site failure, function transfer, reintegrate failed site
  • Communication – message passing

A Distributed System

Distributed-Operating Systems

• Users not aware of multiplicity of machines
  • Access to remote resources similar to access to local resources
• Data Migration
  • transfer data by transferring entire file, or
  • transferring only those portions of the file necessary for the immediate task
• Computation Migration
  • transfer the computation, rather than the data, across the system

Distributed-Operating Systems (Cont.)

• Process Migration
  • execute an entire process, or parts of it, at different sites - Load balancing - distribute processes across network to even the workload - Computation speedup - subprocesses can run concurrently on different sites - Hardware preference - process execution may require specialized processor - Software preference - required software may be available at only a particular site - Data access - run process remotely, rather than transfer all data locally

Network Types (Cont.)

• Wide-Area Network (WAN) – links geographically separated sites - Point-to-point connections over long-haul lines - often leased from a phone company - Speed ≈ 1.544 – 45 megbits/second - Broadcast usually requires multiple messages - Nodes: - usually a high percentage of mainframes

Network Topology

• Sites in the system can be physically connected in a variety of ways; - They are compared with respect to the following criteria: - Installation cost - How expensive is it to link the various sites in the system? - Communication cost - How long does it take to send a message from site A to site B? - Reliability - If a link or a site in the system fails, can the remaining sites stillcommunicate with each other?
• The various topologies are depicted as graphs whose nodes correspond to sites - An edge from node A to node B corresponds to a direct connection between the two sites

Communication Structure

• Naming and name resolution
  • How do two processes locate each other to communicate?
• Routing strategies
  • How are messages sent through the network?
• Connection strategies
  • How do two processes send a sequence of messages?

The design of a communication network must address four basic issues:

Naming and Name Resolution

• Name systems in the network
• Address messages with the process-id
• Identify processes on remote systems by <host-name, identifier> pair
• Domain name service (DNS)
  • specifies the naming structure of the hosts, as well as name to address resolution (Internet)

Routing Strategies (Cont.)

• Dynamic routing: The path used to send a message form site A to site B is chosen only when a message is sent - Usually a site sends a message to another site on the link least used at that particular time - Adapts to load changes by avoiding routing messages on heavily used path - Messages may arrive out of order - This problem can be remedied by appending a sequence number to each message

Connection Strategies

• Circuit switching : A permanent physical link is established for the duration of the communication - i.e., telephone system
• Message switching : A temporary link is established for the duration of one message transfer - i.e., post-office mailing system
• Packet switching: Messages of variable length are divided into fixed-length packets which are sent to the destination Docsity.com

Contention (Cont.)

• Token passing : A unique message type, known as a token, continuously circulates in the system (usually a ring structure) - A site that wants to transmit information must wait until the token arrives - When the site completes its round of message passing, it retransmits the token - A token-passing scheme is used by some IBM and HP/Apollo systems
• Message slots: A number of fixed-length message slots continuously circulate in theDocsity.com

Communication Via ISO Network

Model