Distributed System Concepts and Architectures

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Distributed System Concepts and Architectures
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Outline

• Advantages and disadvantages of distributed OS
• Goals
• Transparency
• Services
• Architecture Models
• Communication Network Protocols
• Major Design Issues
• Distributed Computing Environment (DCE)

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Distributed OS

• An integration of system services, presenting a
  transparent view of a multiple computer system
  with distributed resources and control
• A collection of independent computers that appear
  to the users of the system as a single computer
• Examples
• Personal workstations a pool of processors
  single file system
• Robots on the assembly line Robots in the parts
  department
• A large bank with hundreds of branch offices all
  over the world

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Advantages of Distributed Systems Over
Centralized Systems

• Economics microprocessors offer a better
  price/performance than mainframes
• Speed a distributed system may have more total
  computing power than a mainframe
• Inherent distribution some applications involve
  spatially separated machines
• Reliability if one machine crashes, the system
  as whole can still survive
• Incremental growth computing power can be added
  in small increments

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Advantages of Distributed Systems Over Isolated
Computers

• Data sharing allow many users access to a
  common data base
• Device sharing allow many users to share
  expensive peripherals like color printers
• Communication make human-to-human communication
  easier, for example, by E-mail
• Flexibility spread the workload over the
  available machines in the most cost effective way

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Disadvantages of Distributed Systems

• Software complex software
• Networking the network can saturate or cause
  other problems
• Security easy access also applies to secret data

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Goals (I)

• Provide a high-performance and robust computing
  environment with least awareness of the
  management and control of distributed system
  resources
• Efficiency -  difficult due to communication
  delays
• Propagation delay nothing can be done
• Protocol overhead
• Effective communication primitives, good
  protocols
• Load distribution bottleneck or congestions in
  Network/SW
• Balance and overlap computation and communication
• Distributed processing and load sharing

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Goals (II)

• Flexibility
• User view friendly system and freedom in using
  the system
• Friendliness user interface, consistency,
  reliability ? use OO
• Freedom
• No unreasonable restrictions in using systems
• Easy to build additional tools or services
• System view
• Ability to evolve and migrate
• Modularity, scalability, portability and
  interoperability
• Difficult to achieve
• Heterogeneous HW/SW components

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Goals (III)

• Consistency -  Lack of global information,
  replication and partitioning of data, component
  failures, complexity of interaction among
  components
• User needs uniformity in using the system and
  predictable system behavior
• System needs proper concurrency control
  mechanisms and failure handling and recovery
  procedure
• Robustness - problem with failures in
  communication links, processing nodes and
  client/server processes
• System must reinitialize itself to a state where
  integrity preserved and only small loss in
  performance
• Handle exceptions and errors, changes to
  topology, long message delays, inability to
  locate server
• Security reliability, protection, and access
  control

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Transparency

• Transparency
• Hide all irrelevant system-dependent details from
  users
• Create an illusion of the model users are
  supposed to see
• Trade-off between simplicity and effectiveness
• Objective
• Provide a logical view of a physical system and
  at the same time reduce the effect and awareness
  of the physical system to a minimum

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Type of Transparency (I)

• Access access local and remote system objects in
  same way
• Phone (local) VS. letter (remote)
• Location (name) No awareness of object location
  - use logical names
• Area code for other cities
• Migration object can be moved to different
  locations without changing names
• Local numbers are changed if one moves to other
  cities
• Need universal name (symbolic or numerical)
• Concurrency sharing of objects without
  interference

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Type of Transparency (II)

• Relocation a resource may be moved to another
  location when in use
• Replication consistency of multiple instances of
  files and data
• Parallelism permit parallel activities without
  users knowing how, where, and when these
  activities are carried out by the system
• Failure fault tolerance, graceful performance
  degradation, minimum damages to the user
• Performance consistent and predictable
  performance level even if changes in structure or
  load distribution
• Size modularity and scalability Incremental
  growth in HW without user awareness
• Persistence (software) resource may be in memory
  or on disk
• Revision SW revisions not visible (vertical
  growth)        

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Categorization of Transparency Based on System
Goals

• Efficiency
• Concurrency
• Parallelism
• Performance
• Flexibility
• Access
• Location
• Relocation
• Migration
• Size
• Revision

• Consistency
• Access
• Replication
• Performance
• Persistence
• Robustness
• Failure
• Replication
• Size
• Revision

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Distributed System Issues and Transparencies
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Services (I)

• Primitive services - most fundamental, in kernel
• Must implemented in the kernel of each node in
  the system
• Communication message passing (send/receive
  primitives)
• Synchronous or asynchronous
• Inter-node, inter-process Synchronization
  synchronous communication
• Synchronous semantics of communication or
  synchronization server
• Processor multiplexing -- Process server (for
  transparency reason)
• Creation, deletion, tracking for memory and
  processing time

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Service (II)

• Services by System Servers fundamental, not
  need in kernel
• Provide fundamental services for managing
  processes, files, and process communication
• Can be implemented anywhere in the system, and
  still perform functions basic to the operation of
  a distributed system
• Mapping logical names to physical addresses
• Name server locate processes, users, machines
• Directory server locate files, communication
  ports
• Translate addresses and locations into
  communication paths network server
• Broadcast messages broadcast or multicast
  servers
• Clocks for synchronization - impossible to agree
  on global clock information
• Time server  physical clocks and logical clocks
  (for event ordering)
• File servers, print servers, migration server,
  authentication server

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Service (III)

• Value-added Services - not essential in
  implementation of system but useful, higher-level
  or special purpose services (such as user
  applications)
• Increase computational performance, enhance fault
  tolerance, cooperative activities
• Example is Web server
• Groups of interacting processes
• Group server membership (add/remove), admission
  policies, privileges
• Distributed conferencing server and concurrent
  editing server

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System Architecture Models

• System Architectures
• Workstation-server model
• Client workstations
• Local processing capability and interface to the
  network
• Server workstations
• Dedicated for special services
• Processor pool model - collect all processing
  power in one place, users use terminals only
• Terminal remote booting, remote file mounting,
  virtual terminal handling, packet assembling and
  disassembling (PAD)
• File and processor allocation done by system
• Integrated hybrid model

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Workstation-Server Model
File Server
Printer Server
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Processor-Pool Model
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Communication Network Architecture Models

• HW interconnection inter-node inter-process
  communication protocols
• Hardware interconnection
• Point-to-point links direct connections between
  pairs of nodes
• Multipoint links allow connection of nodes into
  clusters
• Common bus time shared
• IEEE 802 LAN Standard Ethernet, Token Bus/Ring,
  FDDI
• Switch space/time multiplexing at higher HW
  cost/complexity
• Private switches for multiprocessor systems
  cross-bar
• Public switches ISDN, SMDS, ATM
• LAN, MAN, WAN
• Ratio of propagation delay to transmission delay
• LAN small. Close components, more suitable for
  distributed processing
• MAN/WAN large. More communication oriented

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WAN, MAN, LAN
Point-to-Point
Point-to-Point
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Communication Network Protocols

• Communication Protocol set of rules that
  regulate the exchange of messages to provide a
  reliable and orderly flow of information among
  communicating processes
• Connection-oriented communication service Phone
• Need explicit set up of a connection channel
  before communication
• Messages are delivered reliably and in sequence
• Virtual circuit (logical) or circuit switching
  (physical)
• Connectionless communication service postal
  service
• No initial connection establishment is necessary
• Messages are delivered on a best-effort basis in
  timing and route and may arrive in arbitrary
  order
• Datagram (logical) or packet switching (physical)

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OSI Protocol Suite

• Seven-layer protocol suite
• OSI focuses on interconnecting computers
• A process communicates with a remote process by
  passing data through the seven layers, then the
  physical network, and finally through the remote
  layers in reverse order
• Segmenting/reassembling
• Transparency between layers encapsulation
• Add header for protocol data unit (PDU) from
  upper layer
• The remote corresponding layer strip off the
  header
• A gateway or intermediate node only stores and
  forwards messages at the three lower network
  dependent layers

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OSI Protocol Suite (Cont.)
Peer-to-Peer Protocols
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Intermediate Node
Network
Network
Network
Network
Data Link
Data Link
Data Link
Data Link
Physical
Physical
Physical
Physical
Communication Link
Communication Link
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OSI Protocol Suite (Cont.) -- Physical Layer

• Specify the electrical and mechanical
  characteristics of the physical communication
  link standardize
• Coding method, modulation technique,
  wire/connector specification
• Sharing of common bus needs interface standards
  for the medium access control in the data link
  layer
• Reliable mapping of signals to bits need bit
  synchronization
• Bit synchronization
• Detection of the beginning of a bit and a
  sequence of bits
• Bit synchronous large blocks of bits transmitted
  at a regular rate
• Offer higher data transfer speed and better link
  utilization
• Character asynchronous small fixed-size bit
  sequences transmitted asynchronously
• Low-speed character-oriented terminals

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OSI Protocol Suite (Cont.) -- Data Link Control
(DLC) Layer

• Ensure reliable data transfer of groups of bits
  (frames)
• Configuration setup
• Establishment and termination of a connection
• Full- or half-duplex, synchronous or asynchronous
  connection?
• Error controls
• Transmission errors and loss or replication of
  data frames
• Detected by checksum or time-out mechanisms
• Recovered by retransmissions or forward error
  corrections
• Sequencing
• Maintain an orderly delivery of frames by
  sequence numbers
• Sequence number can assist error control and flow
  control of data frames
• Flow control of data frames
• Permit the transmission of a frame only if it
  falls into an allowed windows of buffers for the
  send and the receiver
• Multipoint configuration DLC sublayer MAC
  sublayer Physical layer
• Resolve the access contention of the multiple
  access channel

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OSI Protocol Suite (Cont.) Network Layer

• Address issues of sending packets across the
  network through several link segments
• Routing function
• Which link should be selected for forwarding a
  packet, based on its destination address
• Static or dynamic routing centralized or
  distributed
• Routing decision can be made at the time when a
  connection is requested and is being established
  (connection-oriented) or packet-by-packet basis
  (connectionless, multiple path routing)
• Error, sequencing, and flow control function
• Reassemble packets and discard duplicate ones
• Congestion control for favorable routing nodes

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OSI Protocol Suite (Cont.) Transport Layer

• The most important layer from the OS view
• The only interface between the communication
  sub-network layers and network-independent layers
• Provide a reliable end-to-end communication
  between peers processes
• All network-dependent faults or problems are to
  be shielded from the communicating processes
• Message ??packets (breaking/reassembling)
• Multiple sessions can be multiplexed on one
  transport connection
• One session may occupy multiple transport
  connection
• Five classes (TP0 to TP4) of transport services
  to support sessions
• Depend on application and network quality
• TP4 multiplexing, error detection, and
  retransmission

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OSI Protocol Suite (Cont.) Session,
Presentation, Application Layers

• Session layer add additional dialog and
  synchronization services to transport layer
• Dialog establishment of sessions
• Synchronization allow processes to insert
  checkpoints for efficient recovery from system
  crashes
• Presentation layer data encryption, compression,
  and code conversion for messages that use
  different coding schemes
• Application layer standard is completely left to
  the designer of the application

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TCP/IP Protocol Suite

• Address inter-process and inter-node
  communication
• How is communication between a pair of processes
  maintained?
• Transport Layer ? TCP (TP4 in OSI)
• Connection-oriented (TCP) or Connectionless (UDP)
• How are messages routed through the network
  nodes?
• Network Layer ? IP (a little more than the OSI
  network Layer)
• Virtual circuit or datagram
• TCPI/IP focuses on interconnecting networks
• (TCP, UDP) (Virtual Circuit, Datagram IP)
• Shift burden of maintaining reliable
  communication from network to OS
• Port and Socket (more in Chapter 4)
• Port inter-process communication endpoints
• Socket interface to port

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TCP/IP Protocol Suite (Cont.)
Peer to Peer Protocols
Applicationprocesses
Applicationprocesses
message
Transportlayer
Transportlayer
packet
Gateway
Internetlayer
Internetlayer
Internetlayer
datagram
Data link andphysical Layer
Data link andphysical Layer
Data link andphysical Layer
Frame in bits
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Major Design Issues

• A distributed system consists of concurrent
  processes accessing distributed resources (which
  may be shared or replicated) through message
  passing in a network environment that may be
  unreliable and contain un-trusted components
• How to model and identify objects
• How to coordinate the interaction among objects
• How to achieve objects communication
• How to manage shared or replicated objects
• How to protect objects and system security
• How to support transparency

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Major Design Issues Object Models and Naming
Schemes

• Objects  processes, data files, memory, devices,
  processors, networks
• Assume all objects can be represented uniformly
• An object is represented abstractly by the
  allowable operations
• The physical details of the object are
  transparent to other objects
• To identify a server
• By name - map name to logical address
• Physical or logical address - done by network
  service, port for logical
• By service - needed by CAS

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Major Design Issues Distributed Coordination

• Coordinate interacting concurrent processes to
  achieve synchronization
• Requirements
• Barrier synchronization a set of processes (or
  events) must reach a common synchronization point
  before they can continue
• Condition coordination a set of processes (or
  events) must wait for an asynchronously condition
  set by other processes to maintain some ordering
  of execution
• Mutual exclusion - concurrent processes must have
  mutual exclusion when accessing a critical shared
  resource
• Need knowledge of state information about other
  processes
• Through messages ? inaccurate or incomplete
  (unreliable network)
• Centralized coordinator (leader election) or
  distributed resolution
• Deadlock handling detect and recover
• Assimilate partial global state information and
  use it for decision making
• Exchange local knowledge among cooperating sites

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Major Design Issues (Cont.)

• IPC - Use high-level methods for transparency in
  communication
• Message passing low level and physical
• Client/Server Model - system interactions through
  message exchanges request/reply
• RPC - request/reply like procedure call, built on
  top of client/server model
• RPC assumes point to point, but need groups
  (multicast, broadcast)
• Distributed Resources - data processing capacity
• Multiprocessor scheduling - static load
  distribution vs. dynamic load sharing
• Process migration, real-time scheduling
• Distributed file system and distributed shared
  memory
• Sharing and replication of data

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Major Design Issues (Cont.)

• Fault tolerance and security
• Failure - unintentional intrusion - redundancy
  alleviates it
• Security violation - intentional intrusion - need
  secure communication processes, integrity of
  messages
• Need to authenticate clients/severs, messages 

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Distributed Computing Environment (DCE)

• Proposed by Open Software Foundation (OSF)
• Develop and standardize an open Unix environment
  that is free from the influence of ATT and Sun
• DEC an integrated package of software and tools
  for developing distributed applications on an
  existing OS
• Hierarchically layered architecture

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DCE Architecture