Multithreaded and Distributed Programming – How Distributed Programs Communicate

ECEN5053 Software Engineering of

Distributed Systems

University of ColoradoDistributed Systems – Concepts and Design, 3rd ed. Coulouris, Dollimore and Kindberg, Addison-Wesley, 2001

Examples of Distributed SystemsATMWeb-based travel siteStock transaction processing systemSearch service

ATMClient-serverSimple reply-responseTransactions requiredTightly controlled distributed system

Web-Based Travel SiteMulti-tiered

Client Travel site Vendor reservation systems Referred to as n-tiered

Organization responsible for site has little control over other tiers

Session-oriented

Stock Transaction ProcessingPeer-to-peer communicationPublish/subscribe model for many

interactions

Google Search ServiceLanguage-neutral serviceEasy to write program againstFreely availableLow support overhead

What They All NeedCommunication infrastructureRemote references to objects and

methodsRemote method invocation or remote

procedure call

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

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

Communication Infrastructure

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

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

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)

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)

Messaging-Oriented Middleware IBM MQ SeriesOracle AQSJMS – 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

Messaging-Oriented Middleware IBM MQ SeriesOracle AQSJMS – 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

External Data Representation Behavior vs State

An object’s behavior is defined independently of its identity

An object’s 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

External Data Representation Behavior vs State

An object’s behavior is defined independently of its identity

An object’s 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

External Data Representation Behavior vs State

An object’s behavior is defined independently of its identity

An object’s 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

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

Data Representation Approaches External representation is defined by the

language (e.g., Java) Sender marshalls to receiver’s representation All processes subscribe to common, pre-

compiled external representation (e.g., CORBA)

External representation is self-descriptive (e.g., XML, Web Services)

Data Representation Approaches External representation is defined by the

language (e.g., Java) Sender marshalls to receiver’s representation All processes subscribe to common, pre-

compiled external representation (e.g., CORBA)

External representation is self-descriptive (e.g., XML, Web Services)

Java Marshalling and unmarshalling are built into the

language Called serialization and deserialization

Huge advantage: it’s 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

Java (cont.)Referenced objects are also serializedRedundant references and circularities

are handledStatic attributes are not automatically

serialized

CORBAObject 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 vendor’s CORBA tools are interoperable

Language neutrality has its costs…

Web ServicesUses Simple Object Access Protocol

(SOAP) as common representationSOAP is expressed in an XML dialectAll information is transmitted as stringsUses HTTP as the request-reply

protocol

XMLeXtensible 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

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

Self-Descriptive DataConsider data used in a pizza business to

describe a pizza:StyleToppingsSizePrice

What do you think the following data record should mean?“fishy”, 12, “cheese”, “anchovies”, 12.5

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.

XML is Self-DescriptiveEach datum is enclosed and tagged

with a descriptor that tells us about its semantics

Possible XML representation for a pizza:<pizza style=“fishy”>

<toppings>

<topping>cheese</topping>

<topping>anchovies</topping>

</toppings>

<size>12</size>

<price>12.5</price>

</pizza>

XML is Platform & Language NeutralUsually sent in a character format:

Usually ASCII StringsCould be Unicode, although this is still less

commonDocuments are human-readableDrawbacks

Uses more bytes than other representationsDocuments can get hard to readWriting documents can be error-proneThis 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.

XML SchemaSchema documents define the structure

and semantics of an XML dialectPeople who want to define an XML

dialect (say for standard medical records), would define a schema

Distributed Objects

ATMATM “Teller” object works with Account

objectTeller object is local to the ATMAccount object is in some process back

at the bankHow do they talk to each other?

Talking to Remote ObjectsFinding the object you wantGetting a reference to the object Invoking methods on the objectReceiving resultsDifferences from talking to local objects

Finding The Object You WantRequires a naming or directory serviceFinding hosts on the Internet – Domain

Name System (DNS) You give it a URL DNS gives you an IP address

Getting The Object You WantAsk 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 stubThe stub supports the same interface as

the actual remote object

Getting The Object You WantAsk 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 stubThe stub supports the same interface as

the actual remote object

Invoking Methods on the Object Client talks to the stub Stub’s job

Specifies the method to execute Marshalls the parameters Sends method ID and parameters to the skeleton

Skeleton’s job Unmarshalls parameters Calls the actual object

Receiving Results Skeleton’s job

Receives results from the actual object Marshalls the results Sends results to the stub

Stub’s job Unmarshalls the results Passes them back to the client

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?

Maintaining a Shopping SessionHttp is a stateless protocolSession 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

Talking to Distributed DatabasesN-tier architecture

Client Web Server Application server Database server

Use standard database abstractions Connections ODBC and JDBC

Talking to Distributed DatabasesN-tier architecture

Client Web Server Application server Database server

Use standard database abstractions Connections ODBC and JDBC

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?

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

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

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”

Messaging – Point to PointOne consumer

Usually owns the queueMany producersProcess that wants to send a message

to the receiver puts a message on the receivers input queue

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

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

Google Search ServiceMust be easy to program clients forClients can be written in one of many

languagesLow support overhead for GoogleHow do you make the service simple,

easy, and universal?

Web ServicesFinding Services - UDDIDescribing Services - WSDL Invoking Services – SOAPBoth RPC and messaging

Finding Services - UDDIUniversal Description, Discovery and

IntegrationA way for clients to find web servicesNot being used very much, yet

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)

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

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

Web Services Reading Listhttp://www.oreillynet.com/pub/a/webservices/2002/04/12/

execreport.htmlA brief, high-level overview of some of the important business

motivations for web services.http://www.oreillynet.com/pub/a/webservices/2002/02/12/

webservicefaqs.htmlA 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.htmlAn interesting and opinionated editorial that raises some of the

important political and business-related issues around web services, XML and the Internet.

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 what’s going on