Designing Distributed Object Systems

Ian Gorton, Paul Greenfield

Advanced Distributed Software Arhitectures and Technologies

CSIRO, Sydney

Who is CMIS?• CSIRO is Australia’s largest scientific R&D

organisation and has around 7000 staff

• CSIRO Mathematical and Information Sciences

is – Australia’s premier group of mathematicians and Information

Technology specialists working to improve performance in

Australian industry.

ADSaT • 11 person R&D group, Sydney and

Canberra

• Involved in software engineering R&D

• Specifically looking at the problems of distributed systems, eg

• architecture, components, technologies

• testing and analysis

• performance, scalability, reliability

Architecture Issues

Solution Space

Physical constraints

Performance Requirements/Scalability

Security

Technologies, Components

Testing

Fault Tolerance

Trade-offs between competing forces can be extremely

complex

Management

Transactions

ADSaT capabilities• Designing software architectures

• Expertise in enterprise distributed technologies (eg COM, ORBs, DCE, Java), transactions, databases

• Research based around software architectures

• In-depth knowledge of Internet and e-commerce issues

Week1 Distributed Systems Technology

Overview

Acronym frenzy!!

What is Middleware?

Software plumbing and

associated services for building

distributed systems

Basic distributed systems blocks

• Provides high(ish) level facilities (RPC, RMI) for building distributed system communication infrastructures

• Hides low level issues from the programmer through abstraction Network/OS

TCP/IP

CORBA/RMI/COM

Component Technologies

The Next Big Thing??

Associated Services• Basic middleware

add-ons• Fulfill common

application needs in distributed systems

• See CORBA Services spec for an example and comprehensive list Middleware Plumbing

Secu

rity

Tra

nsa

ction

s

Dire

ctory

Messa

gin

g

Middleware Technologies• CORBA

• COM

• Java

• DCE

• Proprietary, ie– Forte, Tuxedo, CICS, etc..– TIBCO, MQ, etc..

So What is it Good For?• Geek answer…

• Middleware makes it feasible ‘for mere mortals’ to build distributed software systems that are:

– high-performance, scalable

– reliable, high availability

• Don’t be fooled. Distributed systems are inherently difficult to build.

Er..try me again?

• The bottom line answer...

• Businesses need a WWW presence• secure, transactional accesses to business systems

• Integration• mergers, acquisitions, virtual enterprises

• Internet competition and time-to-market

Some Gartner Survey Figures

• To develop a currently competetive e-commerce site:

• 5 months average duration (some 1 year+)

• uses more than 1 consulting organization

• cost average US$1 million (~$350K-$2m+)

• 79% costs were labor, 10% s/w

• No enterprise WWW site was ‘on budget’

Gartner Group Predictions

• Simple site - $300k-$1 million

• Competetive site - $1-5 million

• Market leader - $5-20 million

• Site costs will increase 25% annually in next few years

Solving Business Problems with Middleware

• Legacy system access

• Data integrity

• Scalability

• Availability

• Client access

Legacy System Access• Ease legacy system

integration by providing standard interfaces (wrappers)

• Hides details of legacy systems from clients

CICS\VSAMAS400

Middleware Wrappers

Clients Clients

MQ CICSGateway

Server Server

Data Integrity• Ensuring data integrity

with distributed databases requires transactions

• Transactions ensure ACID properties

SQL ServerOracle

Clients Clients

SQL ODBC

Server Server

TP Service

Scalability• Middleware

supports scaling system performance by service replication

• This isn’t free - it requires good architecture! Clients Clients

Server Server

Availability• Replication enhances

fault tolerance and improves availability

• Graceful degradation, lower performance but it keeps running

Clients Clients

Server Server

Client Access• Client access

enhanced due to:– physical distribution– variety of access

technologies (ie Internet protocols, IIOP, COM) and available bridges.

Clients Clients

ServerWWWServer

IIOP

Clients

IIOP

COM Bridge

HTTP

Enterprise Middleware Products

• Some terminology first:• Middleware - basic products

• Orbix, Visibroker, DCE, Java RMI, COM

• Object Transaction Monitor (OTM)• middleware plus enterprise systems capabilities,

OrbixOTM, BEA WebLogic

• Application Server• cynical answer - new name for OTMs with Java 2

Enterprise Edition support :-}

N-Tier System ArchitectureClient layer (browser, applets, apps)

Business Logic (CORBA objects, EJBs, COM+)

Data Access (DBMSs)

WWW server (WWW server, JSPs, ASPs)

http, IIOP, COM, RMI, XML

IIOP, COM, RMI, XML

SQL, ODBC, JDBC, XA

Standards (wot standards?)

• CORBA - OMG

• Java 2 Enterprise Edition (EJB+JMS+JNDI+…) - Sun

• COM+ - Microsoft

• MQ Series - IBM

• TIBCO multicast protocol..

• ???

CORBA

• OMG is vendor consortium

• CORBA Interface Definition Language (IDL)

• Internet Inter-ORB Protocol (IIOP) (GIOP over TCP/IP)

• CORBA services, eg transactions, naming, events, security

Java

• Remote Method Invocation (RMI)• Sun’s JRMP or RMI-over-IIOP

• Enterprise Java Beans (EJBs)

• Java Transaction Service (JTS)• Java mapping to CORBA OTS

• Java Naming and Directory Interface (JNDI)

• Java Messaging Service (JMS)

EJBs• EJBs are server-side components

• Separate business logic from infrastructure code

EJBEJB EJB

EJB Container

EJBs

• EJBs capture design patterns:• Stateless Session Bean

• Stateful Session Bean

• Entity Bean (bean or container managed)

• Containers/EJB Servers provide:• transaction policies

• security policies

• database connection pooling

• multi-threading

Transaction Service

• Essential for distributed transaction processing

• Manages consistent updates to multiple databases

• CORBA OTS, Java JTS, M’soft MTS

• X/Open DTP standards - XA

ACID properties

• A set of operations that have ACID properties:– Atomic– Consistent– Isolated– Durable

• Classic example - a bank account transfer

Java Transaction Example

try {// Create a transaction.transaction.begin();// Invoke operations in transactionsavingsAccount.makeWithdrawal(50)chequeAccount.makeDeposit(50);// Commit the transaction.transaction.commit(true);

} catch (APP_ERROR ex) {transaction.rollback()

} catch (TRANSACTION_ROLLEDBACK ex) {// handle rollback}

Two-Phase CommitApplication Program

Transaction Manager

Resource Manager (RM)

commit

Prepare tocommit

Response(commitof abort)

commit orabort

Acknowledge

X/Open Distributed TP Reference Model

Application Program

Transaction Manager

Resource Manager(RM)

CommunicationsResource

Manager(CRM)

TX

XA

TxRPCXATMICPI - C

XA+

XA Interface details• Database must implement an XA library

compatible with transaction service product

• embedded SQL

• ODBC/DTC for SQL Server

• OCI for Oracle

• ??? Others...

• JDBC-2 incorporates XA support

• No XA - no distributed transactions

Directory Service

• Clients need to get references to server objects

• Server objects advertise their reference in a directory service

• Clients query the directory service to retrieve desired reference

• CORBA Naming service, JNDI, LDAP

Example Name Space

Bank

AccountsLoansStocks

QueryStock

NewStockService

HomeLoan

BusLoan

AccountService

Security

• Provide:• user authentication

• user/service authorization

• encryption

• Secure Socket Layer (SSL)• public key infrastructure

• asymmetric private and public keys

• X.509 digital certificates

Messaging

• Many apps require asynchronous communications

• CORBA event/notifcation/messaging service

• Java Messaging Service

• TIBCO/Rendezvous

• MQ Series

• others…

• More on these later...

WWW Server Tier

• Need to accept HTML requests via HTTP post or get

• CGI scripts first used to add processing capability:

• spawn new process for each request

• slow, doesn’t scale

• Proprietary improvents• Netscape’s NSAPI, Microsoft’s ISAPI

Java Servlets/Server Pages

• Java platforms includes specs for:• servlets

• Java Server Pages (JSPs)

• Java servlets launched by WWW Server in response to URL request

• JSP’s allow code (eg Java beans) for formatting dynamic content to be embedded in HTML page

Java Servlets/Server Pages

WWWServer

URL JSP

App.Server

Component

Servlet

results

query

query

results

html

html

invoke

invoke

HTML

Extensible Markup Language (XML)

• Subset of SGML

• Standard textual format for structured documents

• Document Type Definition (DTD) defines document structure with tags for each entry in the document

• XML document instance contains both tags and document data

• Text format is easy to process

Sample DTD<?xml encoding="US-ASCII"?>

<!ELEMENT orders (order)*>

<!ELEMENT order (header,item+,total)>

<!ELEMENT header (billing_info,shipping_info)>

<!ELEMENT billing_info (name,address,credit_card)>

<!ELEMENT shipping_info (name,address)>

<!ELEMENT name (given,family)>

<!ELEMENT address (street,city,state,zipcode,country,phone)>

<!ELEMENT item (product_id,product_name,quantity,price)>

<!ELEMENT credit_card (#PCDATA)>

<!ELEMENT given (#PCDATA)>

<!ELEMENT family (#PCDATA)>

[…ELEMENTS MISSING…..]

<!ELEMENT product_id (#PCDATA)>

<!ELEMENT product_name (#PCDATA)>

<!ELEMENT quantity (#PCDATA)>

<!ELEMENT price (#PCDATA)>

<!ELEMENT total (#PCDATA)>

Sample (Incomplete) XML Document<?xml version="1.0"?>

<!DOCTYPE orders SYSTEM "orders.dtd">

<orders>

<order>

<header>

<billing_info>

<name>

<given>John</given>

<family>Doe</family>

</name>

<address>

<street>555 Main Street</street>

<city>Mill Valley</city>

<state>California</state>

<zipcode>94520</zipcode>

<country>USA</country>

<phone>707 555-1000</phone>

</address>

Extensible Style Language (XSL)

• XML ignores presentation of data

• XSL stylesheets define presentation of XML doc in some format

XMLDoc

XSL

Compiler HTML, PDF,etc...

in browser or WWW server

XML/XSL Impact

• Major vendor support/momentum

• Presentation• Decoupling content/presentation is good!

• Presentation is the browser’s job

• Data interchange• business-to-business e-commerce

• DOM API - XML parsers produce a DOM representation of an XML document

• Microsoft’s SOAP

Summary

• Distributed object system technology is a key part of building enterprise distributed systems

• There’s a lot of :• design issues

• technology issues

• plain hard issues (ie testing)

• This course is just the ‘entrée’...