Reliable Distributed Systems

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Reliable Distributed Systems

• Fundamentals
• Chapter 1

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Some terminology

• A program is the code you type in
• A process is what you get when you run it
• A message is used to communicate between
  processes. Arbitrary size.
• A packet is a fragment of a message that might
  travel on the wire. Variable size but limited,
  usually to 1400 bytes or less.
• A protocol is an algorithm by which processes
  cooperate to do something using message
  exchanges.

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More terminology

• A network is the infrastructure that links the
  computers, workstations, terminals, servers, etc.
• It consists of routers
• They are connected by communication links
• A network application is one that fetches needed
  data from servers over the network
• A distributed system is a more complex
  application designed to run on a network. Such a
  system has multiple processes that cooperate to
  do something.

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A network is like a mostly reliable post office
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Why isnt it totally reliable?

• Links can corrupt messages
• Rare in the high quality ones on the Internet
  backbone
• More common with wireless connections, cable
  modems
• Routers can get overloaded
• When this happens they drop messages
• This is very common
• But protocols that retransmit lost packets can
  increase reliability

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How do distributed systems differ from network
applications?

• Distributed systems may have many components but
  are often designed to mimic a single,
  non-distributed process running at a single
  place.
• State is spread around in a distributed system
• Networked application is free-standing and
  centered around the user or computer where it
  runs. (E.g. web browser.)
• Distributed system is spread out, decentralized.
  (E.g. air traffic control system)

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Web connectivity

• Browser is independent fetches data you request
  when you ask for it.
• Web servers do not keep track of who is using
  them. Each request is self-contained and treated
  independently of all others.
• Cookies do not count they are stored on client
  machine
• And the database of account info doesnt count
  either this is ancient history, nothing recent
• ... So the web has two network applications that
  talk to each other
• The browser on your machine
• The web server it happens to connect with which
  has a database behind it

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Web (contd.)
Cookie identifies this user, encodes past
preferences
Database
HTTP request
Web browser with stashed cookies
Web servers are kept current by the database but
usually dont talk to it when your request comes
in
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Web (contd.)
Web servers immediately forget the interaction
Reply updates cookie
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Web (contd.)
Web servers have no memory of the interaction
Purchase is a transaction on the database
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Web (contd.)

• The data center that serves your request may be a
  complex distributed system
• Many servers and perhaps multiple physical sites
• Opinions about which clients should talk to which
  servers
• Data replicated for load balancing and high
  availability
• Complex security and administration policies
• So we have a networked application talking to
  a distributed system

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Other examples of distributed systems

• Air traffic control system with workstations for
  the controllers
• Banking/brokerage trading system that coordinates
  trading (risk management) at multiple locations
• Factory floor control system that monitors
  devices and re-plans work as they go on/offline

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Is the Web reliable?

• We want to build distributed systems that can be
  relied upon to do the correct thing and to
  provide services according to the users
  expectations
• Not all systems need reliability
• If a web site doesnt respond, you just try again
  later
• Reliability is a growing requirement in
  critical settings but these remain a small
  percentage of the overall market for networked
  computers

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Reliability is a broad term

• Fault-Tolerance remains correct despite failures
• High or continuous availability resumes service
  after failures, doesnt wait for repairs
• Performance provides desired responsiveness
• Recoverability can restart failed components
• Consistency coordinates actions by multiple
  components, so they mimic a single one
• Security authenticates access to data, services
• Privacy protects identity, locations of users

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Failure also has many meanings

• Halting failures component simply stops
• Fail-stop halting failures with notifications
• Omission failures failure to send/recv. message
• Network failures network link breaks
• Network partition network fragments into two or
  more disjoint subnetworks
• Timing failures action early/late clock fails,
  etc.
• Byzantine failures arbitrary malicious behavior

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Examples of failures

• My PC suddenly freezes up while running a text
  processing program. No damage is done. This is
  a halting failure
• A network file server tells its clients that it
  is about to shut down, then goes offline. This
  is a failstop failure. (The notification can be
  trusted)
• An intruder hacks the network and replaces some
  parts with fakes. This is a Byzantine failure.

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More terminology

• A real-world network is what we work on. It has
  computers, links that can fail, and some problems
  synchronizing time. But this is hard to model in
  a formal way.
• An asynchronous distributed system is a
  theoretical model of a network with no notion of
  time
• A synchronous distributed system, in contrast,
  has perfect clocks and bounds all events, like
  message passing.

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ISO protocol layers Oft-cited Standard

• ISO is tied to a TCP-style of connection
• Match with modern protocols is poor
• We are mostly at layer 4 session

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Internet protocol suite

• Can be understood in terms of ISO
• Defines addressing standard, basic network
  layer (IP packets, limited to 1400 bytes), and
  session protocols (TCP, UDP, UDP-multicast)
• For example, TCP is a session protocol
• Includes standard domain name service that maps
  host names to IP addresses
• DNS itself is tree-structured and caches data

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Typical hardware options

• Ethernet 10Mbit CSMA technology, limited to 1400
  byte packets. Uses single coax cable.
• FDDI twisted pair, self-repairing if cable
  breaks
• Bridged Ethernet common in big LANs, ring with
  multiple ethernet segments
• Fast Ethernet 100Mbit version of ethernet
• ATM switching technology for fiber optic paths.
  Can run at 155Mbits/second or more. Very
  reliable, but mostly used in telephone systems.

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Implications for reliability?

• Protocol designers have problems predicting the
  properties of local-area networks
• Latencies and throughput may vary widely even in
  a single installation
• Hardware properties differ widely often, must
  assume the least-common-denominator
• Packet loss a minor problem in hardware itself

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Technology trends
Did the sudden growth inin LAN speed give us the
Web?
Source Scientific American, Sept. 1995
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Typical latencies (milliseconds)
WAN, disk latencies are fairly constant due to
physical limitations
Note dramatic drop in LAN latencies over
ATM This is the hardware usedtelephone systems
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O/S latency the most expensive overhead on LAN
communication!
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Broad observations

• A discontinuity is currently occurring in WAN
  communication speeds!
• Especially in military systems, where ATM
  networking hardware has been deployed widely
• Other performance curves are all similar
• Disks have maxed out and hence are looking
  slower and slower
• Memory of remote computers looks closer and
  closer
• O/S imposed communication latencies has risen in
  relative terms over past decade!

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Implications?

• The revolution in WAN communication we are now
  seeing is not surprising and will continue
• Look for a shift from disk storage towards more
  use of access to remote objects over the
  network
• O/S overhead is already by far the main obstacle
  to low latency and this problem will seem worse
  and worse unless O/S communication architectures
  evolve in major ways.

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More Implications

• Look for full motion video to the workstation by
  around 2010 or 2015 today we already see this in
  bits and pieces but not as a routine option
• Low LAN latencies an unexploited niche
• One puzzle what to do with extremely high data
  throughput but relatively high WAN latencies
• O/S architecture and whole concept of O/S must
  change to better exploit the pool of memory of
  a cluster of machines otherwise, disk latencies
  will loom higher and higher

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Discovery

• Consider the problem of discovering the right
  server to connect with
• Your computer needs current map data for some
  place, perhaps an amusement park
• Can think of it in terms of layers the basic
  park layout, overlaid with extra data from
  various services, such as length of the line for
  the Cyclone Coaster or options for vegetarian
  dining near here

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Why is discovery hard?

• Client has opinions
• You happen to like vegetarian food, but not spicy
  food. So your search is partly controlled by
  client goals
• But a given service might have multiple servers
  (e.g. Amazon might have data centers in Europe
  and in the US) and may want your request to go
  to a particular one
• Once we find the server name we need to map it to
  an IP address
• And the Internet itself has routing opinions too

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Fundamental terms Protocol

• Protocol is a set of rules that end points in a
  telecommunication system use when exchanging
  information.
• IP Internet protocol defines an unreliable
  packet transfer protocol.
• TCP Transmission Control Protocol builds on IP
  to define a reliable data delivery protocol.
• LDAP Lightweight Directory Access Protocol
  builds on TCP to define a query-response protocol
  for querying the state of a remote database.
• HTTP Hyper Text Transfer Protocol builds on TCP
  to facilitate hyper-text document exchange.

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Fundamental terms Service

• Service is a network-enabled entity that provides
  a specific capability.
• Service Protocol Behavior
• A service definition permits many
  implementations.
• Examples ability to move files, create
  processes, verify access rights
• An FTP server speaks File Transfer Protocol and
  supports remote read and write access to a
  collection of files.

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Fundamental terms API

• Application Program Interface (API) defines a
  standard interface for invoking a specified set
  of functionality.
• Examples The Generic Security Service (GSS) API
  defines standard functions for verifying identity
  of communicating parties, encrypting messages and
  so forth.

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Client/Server

• Server refers to a process on a networked
  computer that accepts requests from other (local
  or remote) processes to perform a service and
  responds appropriately.
• Client requesting process in the above is
  referred to as the client.
• Request and response are in the form of messages.
• Client is said to invoke an operation on the
  server.
• Many distributed systems today are constructed
  out of interacting clients/servers.

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Middleware (as defined by NSF)

• Middleware refers to the software which is common
  to multiple applications and builds on the
  network transport services to enable ready
  development of new applications and network
  services.
• Middleware typically includes a set of components
  such as resources and services that can be
  utilized by applications either individually or
  in various subsets.
• Examples of services Security, Directory and
  naming, end-to-end quality of service, support
  for mobile code.
• OMGs CORBA defines a middleware standard.
• OMG Object Management Group
• CORBA Common Object Request Broker Architecture

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Middleware
BR
server
server
client
client
desktop
middleware
middleware
network
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Summary

• In this course, we will study distributed systems
  at the middleware level how to define, design
  and implement services, how to use the middleware
  services in a distributed application.
• We will study Java RMI as a case study for simple
  distributed system.
• We will also introduce webservices standard.