Multithreaded and Distributed Programming

1
Multithreaded and Distributed Programming How
Distributed Programs Communicate

• ECEN5053 Software Engineering of
• Distributed Systems
• University of Colorado

Distributed Systems Concepts and Design, 3rd
ed. Coulouris, Dollimore and Kindberg,
Addison-Wesley, 2001
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Examples of Distributed Systems

• ATM
• Web-based travel site
• Stock transaction processing system
• Search service

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ATM

• Client-server
• Simple reply-response
• Transactions required
• Tightly controlled distributed system

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Web-Based Travel Site

• Multi-tiered
• Client
• Travel site
• Vendor reservation systems
• Referred to as n-tiered
• Organization responsible for site has little
  control over other tiers
• Session-oriented

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Stock Transaction Processing

• Peer-to-peer communication
• Publish/subscribe model for many interactions

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Google Search Service

• Language-neutral service
• Easy to write program against
• Freely available
• Low support overhead

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What They All Need

• Communication infrastructure
• Remote references to objects and methods
• Remote method invocation or remote procedure call

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Special Needs

• Transactions
• ATM
• Stock transaction processing
• Asynchronous Messaging
• Stock transaction processing
• Sessions
• Travel site
• Language neutrality standard interfaces
• Travel site
• Stock transaction processing
• Google search service

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Special Needs

• Transactions
• ATM
• Stock transaction processing
• Asynchronous Messaging
• Stock transaction processing
• Sessions
• Travel site
• Language neutrality standard interfaces
• Travel site
• Stock transaction processing
• Google search service

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Communication Infrastructure
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Synchronous communication

• Remote procedure call (RPC)
• Remote method invocation (RMI)
• Client waits until server responds, or request
  times out
• Most distributed processing falls into this model
• Is much like normal, non-distributed programming,
  but
• Pass by reference is not practical
• Not all data types may be available
• Platform neutrality may be hard to achieve

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Synchronous communication

• Remote procedure call (RPC)
• Remote method invocation (RMI)
• Client waits until server responds, or request
  times out
• Most distributed processing falls into this model
• Is much like normal, non-distributed programming,
  but
• Pass by reference is not practical
• Not all data types may be available
• Platform neutrality may be hard to achieve

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Asynchronous communication

• Messaging
• Client sends a message and moves on
• If a response is needed, the client has a
  mechanism that listens for it
• Point-to-point
• Used in publish/subscribe applications, e.g.
• Uses message-oriented middleware (MOM)

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Asynchronous communication

• Messaging
• Client sends a message and moves on
• If a response is needed, the client has a
  mechanism that listens for it
• Point-to-point
• Used in publish/subscribe applications, e.g.
• Uses message-oriented middleware (MOM)

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Messaging-Oriented Middleware

• IBM MQ Series
• Oracle AQS
• JMS Java Messaging Service
• Part of the J2EE standard
• A set of standard interfaces, not a defined
  implementation
• Many vendors provide JMS wrappers or adapters for
  their MOM

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Messaging-Oriented Middleware

• IBM MQ Series
• Oracle AQS
• JMS Java Messaging Service
• Part of the J2EE standard
• A set of standard interfaces, not a defined
  implementation
• Many vendors provide JMS wrappers or adapters for
  their MOM

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External Data Representation

• Behavior vs State
• An objects behavior is defined independently of
  its identity
• An objects state is intimately associated with
  its identity
• To send an object between remote processes, only
  its state needs to be transmitted
• Marshalling and Unmarshalling
• The mechanisms by which state is packaged for
  transmission, and unpackaged on the other end

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External Data Representation

• Behavior vs State
• An objects behavior is defined independently of
  its identity
• An objects state is intimately associated with
  its identity
• To send an object between remote processes, only
  its state needs to be transmitted
• Marshalling and Unmarshalling
• The mechanisms by which state is packaged for
  transmission, and unpackaged on the other end

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External Data Representation

• Behavior vs State
• An objects behavior is defined independently of
  its identity
• An objects state is intimately associated with
  its identity
• To send an object between remote processes, only
  its state needs to be transmitted
• Marshalling and Unmarshalling
• The mechanisms by which state is packaged for
  transmission, and unpackaged on the other end

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Data Representation Issues

• Hardware specifics
• Endian issues how numbers are stored
• OS specifics
• Character sets
• Integer sizes
• Language specifics
• String representations
• Floating point number representations
• Date/time representations
• Object definitions

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Data Representation Approaches

• External representation is defined by the
  language (e.g., Java)
• Sender marshalls to receivers representation
• All processes subscribe to common, pre-compiled
  external representation (e.g., CORBA)
• External representation is self-descriptive
  (e.g., XML, Web Services)

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Data Representation Approaches

• External representation is defined by the
  language (e.g., Java)
• Sender marshalls to receivers representation
• All processes subscribe to common, pre-compiled
  external representation (e.g., CORBA)
• External representation is self-descriptive
  (e.g., XML, Web Services)

23
Java

• Marshalling and unmarshalling are built into the
  language
• Called serialization and deserialization
• Huge advantage its part of the language
• Only works for Java-to-Java
• Encompasses a very wide range of Java types
• Widely used in Java
• Simple persistence
• Distributed computing

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Java (cont.)

• Referenced objects are also serialized
• Redundant references and circularities are
  handled
• Static attributes are not automatically serialized

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CORBA

• Object interfaces are defined in CORBA Interface
  Description Language (IDL)
• CORBA interface compiler for a specific
  language/OS/platform creates code to marshall and
  unmarshall objects
• Not all vendors CORBA tools are interoperable
• Language neutrality has its costs

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Web Services

• Uses Simple Object Access Protocol (SOAP) as
  common representation
• SOAP is expressed in an XML dialect
• All information is transmitted as strings
• Uses HTTP as the request-reply protocol

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XML

• eXtensible Markup Language
• Structured data in a text file
• XML looks a bit like HTML but isn't HTML
• XML is text, but isn't meant to be read
• XML is new, but not that new
• XML is not really a markup language itself, but a
  meta-language for defining markup languages
• HTML can be defined using XML
• Groups are attempting to define standard
  domain-specific XML dialects
• Home-grown XML dialects are common for single
  applications, too

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Why is XML a Big Deal?

• Self-descriptive
• Open standard
• Platform and language neutral
• License-free
• Widely supported with free tools
• A great way for applications to communicate with
  each other
• The basis for a wide array of emerging
  technologies

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Self-Descriptive Data

• Consider data used in a pizza business to
  describe a pizza
• Style
• Toppings
• Size
• Price
• What do you think the following data record
  should mean?

fishy, 12, cheese, anchovies, 12.5
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Self-Descriptive Data (cont.)

• Is 12 an integer or a float?
• Is 12.5 the size or the price?
• If 12.5 were sent as a binary number,
  marshalling/unmarshalling would require knowledge
  of endpoint platforms
• If the data came from a database, we need the
  database schema, and probably the database engine
  itself, to interpret the data.
• If the data came from a file, we need to write
  code to interpret the data.
• Adding or removing fields causes major problems.

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XML is Self-Descriptive

• Each datum is enclosed and tagged with a
  descriptor that tells us about its semantics
• Possible XML representation for a pizza

ltpizza stylefishygt lttoppingsgt lttoppinggtchees
elt/toppinggt lttoppinggtanchovieslt/toppinggt lt/topp
ingsgt ltsizegt12lt/sizegt ltpricegt12.5lt/pricegt lt/pizz
agt
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XML is Platform Language Neutral

• Usually sent in a character format
• Usually ASCII Strings
• Could be Unicode, although this is still less
  common
• Documents are human-readable
• Drawbacks
• Uses more bytes than other representations
• Documents can get hard to read
• Writing documents can be error-prone
• This format can be awkward when dealing with
  binary (non-character) data.
• Binary data can be sent in encoded form, but the
  programs at either end of the conversation must
  understand the encoding.

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XML Schema

• Schema documents define the structure and
  semantics of an XML dialect
• People who want to define an XML dialect (say for
  standard medical records), would define a schema

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Distributed Objects
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ATM

• ATM Teller object works with Account object
• Teller object is local to the ATM
• Account object is in some process back at the
  bank
• How do they talk to each other?

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Talking to Remote Objects

• Finding the object you want
• Getting a reference to the object
• Invoking methods on the object
• Receiving results
• Differences from talking to local objects

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Finding The Object You Want

• Requires a naming or directory service
• Finding hosts on the Internet Domain Name
  System (DNS)
• You give it a URL
• DNS gives you an IP address

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Getting The Object You Want

• Ask the server that has the object to send it to
  you
• Identify it by some sort of key
• The server sends you a proxy or stub
• The stub supports the same interface as the
  actual remote object

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Getting The Object You Want

• Ask the server that has the object to send it to
  you
• Identify it by some sort of key
• The server sends you a proxy or stub
• The stub supports the same interface as the
  actual remote object

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Invoking Methods on the Object

• Client talks to the stub
• Stubs job
• Specifies the method to execute
• Marshalls the parameters
• Sends method ID and parameters to the skeleton
• Skeletons job
• Unmarshalls parameters
• Calls the actual object

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Receiving Results

• Skeletons job
• Receives results from the actual object
• Marshalls the results
• Sends results to the stub
• Stubs job
• Unmarshalls the results
• Passes them back to the client

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Web-Based Travel Site

• The site must maintain a shopping cart for the
  client, remember his preferences, etc.
• One remote service, the site, must collaborate
  with several other remote services, the airline
  and hotel reservation systems
• How does the site avoid treating each as a
  special case?
• How does the site perform adequately?

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Maintaining a Shopping Session

• Http is a stateless protocol
• Session state must be implemented somehow
• Cookies
• Hunks of information the server leaves on the
  client machine
• URL rewriting
• Session information is encoded in the URL string
  sent between the client and server

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Talking to Distributed Databases

• N-tier architecture
• Client
• Web Server
• Application server
• Database server
• Use standard database abstractions
• Connections
• ODBC and JDBC

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Talking to Distributed Databases

• N-tier architecture
• Client
• Web Server
• Application server
• Database server
• Use standard database abstractions
• Connections
• ODBC and JDBC

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Stock Transaction Processing System

• Client processes must be able to subscribe to
  trades and changes in price of specific
  securities
• Subscriptions are maintained when client or
  server system is restarted
• If client is not running, notices to which it
  subscribes are saved for it
• System sends asynchronous notices to subscribers
• How does the client know what happens without
  poling?

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Messaging

• Asynchronous interaction between clients and
  servers
• Messages are passed between them on queues
• Queues are provided by message-oriented
  middleware
• Producers put message on queues
• Consumers get messages off queues
• Can poll for messages (not common)
• Can be notified when messages arrive
• Assign listeners to the queue

48
Messaging

• Asynchronous interaction between clients and
  servers
• Messages are passed between them on queues
• Queues are provided by message-oriented
  middleware
• Producers put message on queues
• Consumers get messages off queues
• Can poll for messages (not common)
• Can be notified when messages arrive
• Assign listeners to the queue

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Messaging (cont.)

• Durable subscriptions
• If the receiver is not running, the queue will
  deliver the messages when the receiver comes back
• Persistent queues
• All messages that are put on the queue are also
  saved to a database
• Listeners
• Methods that wait to receive messages from queues
• Separate thread owned by the receiver process
• When a message arrives, the listener is woken
  up

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Messaging Point to Point

• One consumer
• Usually owns the queue
• Many producers
• Process that wants to send a message to the
  receiver puts a message on the receivers input
  queue

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Messaging Publish/Subscribe

• One or more producer
• One of them, or central service, owns the queue
• Many consumers
• Consumers subscribe to topic queues
• Consumer is usually not interested in every
  message published on the topic queue
• Specifies filters on messages
• Specify the local listener to call when a
  qualifying message is published

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Messaging Publish/Subscribe

• One or more producer
• One of them, or central service, owns the queue
• Many consumers
• Consumers subscribe to topic queues
• Consumer is usually not interested in every
  message published on the topic queue
• Specifies filters on messages
• Specify the local listener to call when a
  qualifying message is published

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Google Search Service

• Must be easy to program clients for
• Clients can be written in one of many languages
• Low support overhead for Google
• How do you make the service simple, easy, and
  universal?

54
Web Services

• Finding Services - UDDI
• Describing Services - WSDL
• Invoking Services SOAP
• Both RPC and messaging

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Finding Services - UDDI

• Universal Description, Discovery and Integration
• A way for clients to find web services
• Not being used very much, yet

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Describing Services - WSDL

• Web Services Definition Language
• A language for defining the interfaces for a web
  service
• Names of methods
• Types and order of parameters
• Types of return values
• URL of service
• Writing WSDL
• Do it once, when service is defined
• Annoying and error-prone to do by hand
• Several free generating packages available
• GLUE (The Mind Electric)
• IBM Web Services Toolkit (IBM Alphaworks)

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Invoking Services - SOAP

• Simple Object Access Protocol
• Runs over HTTP
• Defines how objects are passed back and forth
• Writing SOAP
• Do it every time a service is requested
• REALLY annoying and error-prone to write by hand
• Good tools are available
• Apache SOAP
• AXIS
• Visual Studio .NET

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Invoking Services - SOAP

• Simple Object Access Protocol
• Runs over HTTP
• Defines how objects are passed back and forth
• Writing SOAP
• Do it every time a service is requested
• REALLY annoying and error-prone to write by hand
• Good tools are available
• Apache SOAP
• AXIS
• Visual Studio .NET

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Web Services Reading List

• http//www.oreillynet.com/pub/a/webservices/2002/0
  4/12/execreport.html
• A brief, high-level overview of some of the
  important business motivations for web services.
• http//www.oreillynet.com/pub/a/webservices/2002/0
  2/12/webservicefaqs.html
• A overview of the technologies involved in web
  services, and why they make web services
  different from other distributed computing
  technologies.
• http//www.xml.com/pub/a/2002/04/24/taglines.html
• An interesting and opinionated editorial that
  raises some of the important political and
  business-related issues around web services, XML
  and the Internet.

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Comments on homework
What kinds of clients What kinds of
servers exchange data base messaging What
kinds of interfaces putting something up for
auction submitting bids ask questions about
whats going on