Essential Software Architecture

Session 1: Introduction to Software Architecture 1

Session Outline

 IT Systems Application Domain  What is Software Architecture?

 Definitions  What does an architect do?

 Architectures and technologies  Further reading 2

IT Systems Application Domain

    IT systems are everywhere    Banks Shops Internet sites Large, complex, heterogeneous, distributed applications Use commercial-of-the-shelf middleware, databases, web servers, application packages Major problems are architecture design, technology selection, application and business integration 3

What is Software Architecture?

  It’s about software design  All architecture is software design, but not all design is software architecture  Part of the design process Simply, architecture focuses on ‘issues that will be difficult/impossible to change once the system is built’  Quality attributes like security, performance   Non-functional requirements like cost, deployment hardware More on these later in this session 4

Defintions - ANSI/IEEE Std 1471-2000



“Architecture is the fundamental organization of a system, embodied in its components, their relationships to each other and the environment, and the principles governing its design and

evolution

.”

5

Definitions - SEI



“The software architecture of a program or computing system is the structure or structures of the system, which comprise software elements, the externally visible properties of those elements, and the relationships among them.

” 6

Definitions – Garlan and Shaw



“[Software architecture goes] beyond the algorithms and data structures of the computation; designing and specifying the overall system structure emerges as a new kind of problem. Structural issues include gross organization and global control structure; protocols for communication, synchronization, and data access; assignment of functionality to design elements; physical distribution; composition of design elements; scaling and performance; and selection among

design alternatives.”

7

Architecture Defines Structure

 Decomposition of system in to components/modules/subsystems  Architecture defines:  Component interfaces  What a component can do  Component communications and dependencies  How components communicate  Component responsibilities  Precisely what a component will do when you ask it 8

Structure and Dependencies

   Excessive component dependencies are bad!

Key architecture issue   Identifying components that may change Reduce direct dependencies on these components Creates more modifiable systems C1 C2 C3 C4 Third Party Component C1 C2 C3 C4 AL Third Party Component Four components are directly dependent on a third party component. If the third party component is replaced with a new component with a different interface, changes to each component are likely.

C

Diagram Key

Component Only the AL (abstraction layer) component is directly dependent on the third party component. If the third party component is replaced, changes are restricted to the AL component only Dependency 9

Architecture Specifies Component Communication

 Communication involves:   Data passing mechanisms, e.g.:    Function call Remote method invocation Asynchronous message Control flow   Flow of messages between components to achieve required functionality Sequential   Concurrent/parallel Synchronization 10

Architecture Patterns/Styles

   Patterns catalogue successfully used structures that facilitate certain kinds of component communication    client-server Message broker Pipeline Patterns have well-known characteristics appropriate for particular types of requirements Patterns are very useful things …  Reusable architectural blueprints    Help efficiently communicate a design Large systems comprise a number of individual patterns “Patterns and Styles are the same thing – the patterns people won” [anonymous SEI member] 11

Architecture addresses NFRs

   Non-functional requirements (NFRs) define ‘how’ a system works NFRs rarely captured in functional requirements  Aka. architecture requirements  Must be elicited by architect NFRs include:    Technical constraints Business constraints Quality attributes 12

Architecture is an Abstraction

   Architecture provides an abstract view of a design  Hides complexity of design  May or may not be a direct mapping between architecture elements and software elements Example – A Marketecture  informal depiction of system’s structure and interactions.  portray the design philosophies embodied in the architecture Every system should have a marketecture:  Easy to understand  Helps discussion during design, build, review, sales (!) process 13

Decomposition

  Hierarchical decomposition is a powerful abstraction mechanism  Partitions design  Allocate components to development teams Why isn’t the

Client

component decomposed?

Top Level Architecture Description Client Broker Server Messag e Handler Director y Server Security Server Request Handler Data Store Diagram Key

C Component Dependency 14

Architecture Views

  A software architecture represents a complex design artifact Many possible ‘views’ of the architecture  Cf. with buildings – floor plan, external, electrical, plumbing, air-conditioning 15

Philippe Krutchen - 4+1 View Model

    

Logical view:

describes architecturally significant elements of the architecture and the relationships between them.

Process view:

describes the concurrency and communications elements of an architecture.

Physical view:

depicts how the major processes and components are mapped on to the applications hardware.

Development view:

software components as held in e.g. a configuration management tool. captures the internal organization of the

Architecture use cases:

capture the requirements for the architecture; related to more than one particular view 16

SEI – Views and Beyond

   

Module:

structural view of the architecture, comprising the code modules such as classes, packages and subsystems

Component and Connector:

describes the behavioral aspects of the architecture. Components are objects, threads or processes, and connectors describe how components interact.

Allocation:

shows how the processes are mapped to hardware and gives a view of the source code in the configuration management systems.

This course will focus on

structural

views and

behavioral

17

What does an Architect do?

 Many responsibilities:  Liaison with stakeholders  Technology knowledge  Software engineering  Risk managements  Long list at:  http://www.sei.cmu.edu/ata/arch_duties.html 18

Architectures and Technologies

 Architects reduce risks by using proven design patterns  Must map abstract design pattern to concrete implementation  Software vendors have created (COTS) technologies that explicitly support widely used patterns  Makes implementation of patterns easier  Reduces risk if technology is well built 19

Architectures and Technologies

Abstract

Architectural Patterns/Styles Application Servers Messaging Message Brokers Object Brokers

Concrete COTS technologies

Process Orchestration

  Each technology has multiple vendors/open source versions Architects need to choose technology wisely   Proof of concept prototype Detailed feature evaluation 20

Summary

 “

The life of a software architect is a long (and sometimes painful) succession of sub-optimal decisions made partly in the dark”

 Architecture involves complex design decisions  Architect role is much more than just technical design  Made harder by early lifecycle nature of much of the design 21

Selected Further Reading

    L. Bass, P. Clements, R Kazman. Software Architecture in Practice, Second Edition. Addison Wesley, 2003.

R. Wirfs-Brock, A. McKean. Object Design: Roles, Responsibilities, and Collaborations. Addison Wesley, 2002.

M. Fowler. Patterns of Enterprise Application Architecture. Addison-Wesley, 2002.

I.Gorton, A Liu.

Performance Evaluation of Alternative Component Architectures for Enterprise JavaBean Applications,

in IEEE Internet Computing, vol.7, no. 3, pages 18-23, 2003.

22

Essential Software Architecture

Session 2: Introduction to the Case Study 23

ICDE System

 Information Capture and Dissemination Environment (ICDE) is a software system for providing intelligent assistance to  financial analysts  scientific researchers  intelligence analysts  analysts in other domains 24

ICDE Schematic

Internet Local information repositories ICDE Repository Analyst ICDE Recording Software 3 rd Party Tools 25

ICDE Use Cases

Analyst * *

ICDE

* Capture User Actions * * * 3rd Party Tools * User Assistance * * Query User Actions * * * Data Store 26

Case Study Context

   ICDE version 1.0 in production Basically a complex,

raw

information capture tool, GUI for looking at captured data 2 tier client-server, single machine deployment   Java, Perl, SQL, Programmatic access to data through very complex SQL (38 tables, 46 views) 27

ICDE version 2.0

   ICDE v2.0 scheduled for development in 12 month timeframe  Fixed schedule, budget Major changes to:  Enhance data capture tools (GUI)  Support 3 rd party tool integration, testing, data access and large production scale deployments (100’s of users) Very few concrete requirements for the 3 rd party tool support or release to full production environment 28

ICDE v2.0 Business Goals

Business Goal

Encourage third developers party tool

Supporting Technical Objective

Simple and reliable programmatic access to data store for third party tools Heterogeneous (i.e.

non-Windows) support for running third party tools platform Allow third party tools to communicate with ICDE users from a remote machine Promote the ICDE concept to users Scale the data collection and data store components to support up to 150 users at a single site Low-cost deployment for each ICDE user workstation 29

  

Architecturally Significant Requirements for ICDE v2.0

ICDE project requirements:     Heterogeneous platform support for access to ICDE data Instantaneous event notification (local/distributed) Over the Internet, secure ICDE data access Ease of programmatic data access ICDE Project team requirements:  Insulate 3 rd evolution party projects and ICDE tools from database    Reliability for multi-tool ICDE deployments Scalable infrastructure to support large, shared deployments Minimize license costs for a deployment Unknowns  Minimize dependencies, making unanticipated changes potentially easier 30

Summary

 ICDE is a reasonably complex system  Will be used to illustrate concepts during the remainder of this course 31

Essential Software Architecture

Session 3: Quality Attributes 32

What are Quality Attributes

 Often know as –ilities  Reliability  Availability  Portability  Scalability   Performance (!) Part of a system’s NFRs  “how” the system achieves its functional requirements 33

Quality Attribute Specification

  Architects are often told:  “My application must be fast/secure/scale” Far too imprecise to be any use at all  Quality attributes (QAs) must be made precise/measurable for a given system design, e.g.



“It must be possible to scale the deployment from an initial 100 geographically dispersed user desktops to 10,000 without an increase in effort/cost for installation and configuration.”

34

Quality Attribute Specification

 QA’s must be concrete  But what about testable?

 Test scalability by installing system on 10K desktops?

  Often careful analysis of a proposed solution is all that is possible “It’s all talk until the code runs” 35

Performance

 Many examples of poor performance in enterprise applications  Performance requires a:  Metric of amount of work performed in unit time  Deadline that must be met  Enterprise applications often have strict performance requirements, e.g.

 1000 transactions per second  3 second average latency for a request 36

Performance - Throughput

 Measure of the amount of work an application must perform in unit time  Transactions per second  Messages per minute  Is required throughput:  Average?

 Peak?

 Many system have low average but high peak throughput requirements 37

Throughput Example

CPU % MST (msp) 300 250 200 150 100 50 0 0 5 10 # of threads 15 20   Throughput of a message queuing system  Messages per second (msp)  Maximum sustainable throughput (MST) Note throughput changes as number of receiving threads increases 38

Performance - Response Time

 measure of the latency an application exhibits in processing a request    Usually measured in (milli)seconds Often an important metric for users Is required response time:   Guaranteed?

Average?

 E.g. 95% of responses in sub-4 seconds, and all within 10 seconds 39

Response Time

 Example shows response time distribution for a J2EE application 40

Performance - Deadlines

 ‘something must be completed before some specified time’  Payroll system must complete by 2am so that electronic transfers can be sent to bank  Weekly accounting run must complete by 6am Monday so that figures are available to management  Deadlines often associated with batch jobs in IT systems.

41

Something to watch for …

    What is a    Transaction?

Message?

Request?

All are application specific measures.

System must achieve 100 mps throughput  BAD!!

System must achieve 100 mps peak throughput for

PaymentReceived

messages  GOOD!!!

42

ICDE Performance Issues

  Response time:  Overheads of trapping user events must be imperceptible to ICDE users Solution for ICDE client:  Decouple user event capture from storage using a queue 5. Write event to ICDE database queue 1. Trap user event 2. Write event to queue 3. Return to user thread 4. Read event from queue 43

Scalability



“How well a solution to some problem will work when the size of the problem increases.”

 4 common scalability issues in IT systems:  Request load  Connections  Data size  Deployments 44

Scalability – Request Load

 How does an 100 tps application behave when simultaneous request load grows? E.g.

 From 100 to 1000 requests per second?

 Ideal solution, without additional hardware capacity:  as the load increases, throughput remains constant (i.e. 100 tps), and response time per request increases only linearly (i.e. 10 seconds). 45

Scalability – Add more hardware …

Scale-up: Single application instance is executed on a multiprocessor machine Scale-out: Application replicated on different machines

Application Application CPU Application Application Application 46

Scalability - reality

   Adding more hard ware should improve performance:  scalability must be achieved without modifications to application architecture Reality as always is different!

Applications will exhibit a decrease in throughput and a subsequent exponential increase in response time.   increased load causes increased contention for resources such as CPU, network and memory each request consumes some additional resource (buffer space, locks, and so on) in the application, and eventually these are exhausted 47

Scalability – J2EE example

2500 2000 1500 1000 500 0 0 200 400 600 800

No. of Clients

1000 1200 WAS SB JBoss SB IAS SB SS SB WLS SB BES SB I.Gorton, A Liu,

Performance Evaluation of Alternative Component Architectures for Enterprise JavaBean Applications, in IEEE Internet Computing, vol.7, no. 3, pages 18-23, 2003.

48

Scalability - connections

  What happens if number of simultaneous connections to an application increases  If each connection consumes a resource?

 Exceed maximum number of connections?

ISP example:     Each user connection spawned a new process Virtual memory on each server exceeded at 2000 users Needed to support 100Ks of users Tech crash ….

49

Scalability – Data Size

  How does an application behave as the data it processes increases in size?  Chat application sees average message size double?

  Database table size grows from 1 million to 20 million rows?

Image analysis algorithm processes images of 100MB instead of 1MB? Can application/algorithms scale to handle increased data requirements?

50

Scalability - Deployment

 How does effort to install/deploy an application increase as installation base grows?

 Install new users?

 Install new servers?

 Solutions typically revolve around automatic download/installation  E.g. downloading applications from the Internet 51

Scalability thoughts and ICDE

 Scalability often overlooked.

    Major cause of application failure Hard to predict Hard to test/validate Reliance on proven designs and technologies is essential  For ICDE - application should be capable of handling a peak load of 150 concurrent requests from ICDE clients.

 Relatively easy to simulate user load to validate this 52

Modifiability

 Modifications to a software system during its lifetime are a fact of life.  Modifiable systems are easier to change/evolve  Modifiability should be assessed in context of how a system is likely to change  No need to facilitate changes that are highly unlikely to occur  Over-engineering!

53

Modifiability

 Modifiability measures how easy it

may

be to change an application to cater for new (non-) functional requirements. 

‘may’

– nearly always impossible to be certain  Must estimate cost/effort  Modifiability measures are only relevant in the context of a given architectural solution.  Components  Relationships  Responsibilities 54

Modifiability Scenarios

 Provide access to the application through firewalls in addition to existing “behind the firewall” access.

  Incorporate new features for self-service check-out kiosks.

The COTS speech recognition software vendor goes out of business and we need to replace this component.

 The application needs to be ported from Linux to the Microsoft Windows platform.

55

Modifiability Analysis

 Impact is rarely easy to quantify  The best possible is a:  Convincing impact analysis of changes needed  A demonstration of how the solution can accommodate the modification without change.  Minimizing dependencies increases modifiability  Changes isolated to single components likely to be less expensive than those that cause ripple effects across the architecture. 56

Modifiability for ICDE

 The range of events trapped and stored by the ICDE client to be expanded.  Third party tools to communicate new message types.  Change database technology used  Change server technology used 57

Security

 Difficult, specialized quality attribute:  Lots of technology available   Requires deep knowledge of approaches and solutions Security is a multi faceted quality … 58

Security

    

Authentication:

Applications can verify the identity of their users and other applications with which they communicate.

Authorization:

Authenticated users and applications have defined access rights to the resources of the system.

Encryption:

encrypted. The messages sent to/from the application are

Integrity:

This ensures the contents of a message are not altered in transit.

Non-repudiation:

The sender of a message has proof of delivery and the receiver is assured of the sender’s identity. This means neither can subsequently refute their participation in the message exchange.

59

Security Approaches

 SSL  PKI  Web Services security  JAAS  Operating system security  Database security  Etc etc 60

ICDE Security Requirements

 Authentication of ICDE users and third party ICDE tools to ICDE server  Encryption of data to ICDE server from 3 rd party tools/users executing remotely over an insecure network 61

Availability

 Key requirement for most IT applications  Measured by the proportion of the required time it is useable. E.g.

 100% available during business hours  No more than 2 hours scheduled downtime per week   24x7x52 (100% availability) Related to an application’s reliability  Unreliable applications suffer poor availability 62

Availability

  Period of loss of availability determined by:    Time to detect failure Time to correct failure Time to restart application Strategies for high availability:  Eliminate single points of failure   Replication and failover Automatic detection and restart  Recoverability (e.g. a database)  the capability to reestablish performance levels and recover affected data after an application or system failure 63

Availability for ICDE

 Achieve 100% availability during business hours  Plenty of scope for downtime for system upgrade, backup and maintenance.  Include mechanisms for component replication and failover 64

Integration

 ease with which an application can be incorporated into a broader application context  Use component in ways that the designer did not originally anticipate  Typically achieved by:  Programmatic APIs  Data integration 65

Integration Strategies

Interoperability through an API facade

API Application Data Third Party Application

Interoperability achieved by direct data access

   Data – expose application data for access by other components API – offers services to read/write application data through an abstracted interface Each has strengths and weaknesses … 66

ICDE Integration Needs

 Revolve around the need to support third party analysis tools.  Well-defined and understood mechanism for third party tools to access data in the ICDE data store. 67

Misc. Quality Attributes

 Portability  Can an application be easily executed on a different software/hardware platform to the one it has been developed for?  Testability  How easy or difficult is an application to test?  Supportability  How easy an application is to support once it is deployed?

68

Design Trade-offs

  QAs are rarely orthogonal  They interact, affect each other    highly secure system may be difficult to integrate highly available application may trade-off lower performance for greater availability high performance application may be tied to a given platform, and hence not be easily portable Architects must create solutions that makes sensible design compromises   not possible to fully satisfy all competing requirements Must satisfy all stakeholder needs  This is the difficult bit!

69

Summary

 QAs are part of an application’s non functional requirements  Many QAs  Architect must decide which are important for a given application  Understand implications for application  Understand competing requirements and trade offs 70

Selected Further Reading

   L. Chung, B. Nixon, E. Yu, J. Mylopoulos, (Editors). Non-Functional Requirements in Software Engineering Series: The Kluwer International Series in Software Engineering. Vol. 5, Kluwer Academic Publishers. 1999. J. Ramachandran. Designing Security Architecture Solutions. Wiley & Sons, 2002.

I.Gorton, L. Zhu. ACM Press

Tool Support for Just-in-Time Architecture Reconstruction and Evaluation: An Experience Report

. International Conference on Software Engineering (ICSE) 2005, St Loius, USA, 71

Essential Software Architecture

Session 4: A Guide to Middleware Architectures and Technologies 72

Introduction

    Middleware is the plumbing or wiring of IT applications Provides applications with fundamental services for distributed computing Insulates applications from underlying platform (OS, DBMS, etc) APIs Lots of middleware exists    Different purposes Different vendors Different standards and proprietary technologies 73

Middleware Classification

Business Process Orchestrators Message Brokers Application Servers Transport

BizTalk, TIBCO StaffWare, ActiveBPEL BizTalk, WebSphere Message Broker, SonicMQ J2EE, CCM, .NET

Message-Oriented Middleware, Distributed Objects Systems

74

Outline

 CORBA  Message-oriented middleware  J2EE  Message brokers  Business process orchestrators 75

CORBA

   Venerable distributed object technology Still widely used in telecomms, defense Many different implementations

Client

Object Reference

request

client ORB Server

Servant

reply

server ORB Network

76

CORBA Code Example

module ServerExample { interface MyObject { string isAlive(); }; };

Server CORBA IDL

class MyServant extends _MyObjectImplBase { public String isAlive() { return "\ nLooks like it…\n"; } } ORB orb = ORB.init(args, null); MyServant objRef = new MyServant(); orb.connect(objRef); ORB orb = ORB.init(args, null); // Lookup is a wrapper that actually access the CORBA Naming // Service directory – details omitted for simplicity MyServant servantRef = lookup(“Myservant”)String reply = servantRef.isAlive();

Client

77

CORBA – Some Thoughts

   Many associated services, eg    Naming Notification Transactions Synchronous technology, client-server relatively tightly coupled Remote calls can/will fail  State management in server objects creates ‘interesting’ recovery issues 78

Messaging - MOM

 Basic Message Oriented Middleware (MOM) provides features like:  Asynchronous communications between processes, applications and systems  Send-and-forget  Delivering messages despite failures  Transactional Messaging  Deliver all messages in a transaction, or none  Persistence  Messages can be logged at the server and hence survive server failure 79

Basic Messaging

   Send (queue, message)  Put message onto queue Receive (queue, message)  Get message from queue No dependency on state of receiving application on message send send queue receive 80

Persistence

send queue receive    Receipt of message at queue implies message is written to disk log Removal of message from queue deletes message from disk log Trade-off performance versus reliability 81

MOM Server

Senders Sending Applications

Message Handler Thread Pool

MOM Server Receivers Sending Applications

Peer-to-peer MOM technologies are the alternative design 82

MOM Transactions

Begin transaction ... update database record put message on queue ... commit transaction Begin transaction ... get message from queue update database record ... commit transaction 83

MOM Transactions

 Sender and receiver do *not* share a transaction   Rollback on receiver does not affect the sender (already committed) ‘Synchronous’ operations are not atomic  Request/response is 3 transactions not 1  Put to request queue   Get from request queue, put to response queue Get from response queue Request queue send receive receive Response queue send 84

Scaling MOM

MOM Server ApplicationQ Senders MOM Server ApplicationQ Receivers

85

Messaging – Some thoughts

 Highly attractive asynchronous technology  Supports loosely-coupled, dynamic applications  Scales well, high throughput possible  Many implementations, various qualities of service  caveat emptor 86

Publish-Subscribe Messaging

   Extension of MOM to provide 1-to-N, N-to-1, and N-to N communications Messages are ‘published’ to logical

subjects

or

topics

Subscribers receive all messages from subjects they subscribe to Sub Pub

Create/ Publish

Subject

Register/ Subscribe

Sub Sub 87

Publish-Subscribe with Multicast

Subscriber rvd Subscriber rvd rvrd Publisher rvd rvrd

Based on TIBCO Rendezvous

rvd Subscriber rvd Subscriber rvd Subscriber

88

Performance

Milliseconds 700 600 500 400 300 200 100 0 10 20 30 40 50 No. Of Subscribers MC1 MC2 QB

89

Subject/Topic Naming

Sydney DevGroup SupportGroup Information Information work gossip Sydney Sydney/DevGroup Sydney/DevGroup/Information Sydney/DevGroup/Information/work Sydney/DevGroup/Information/gossip Sydney/SupportGroup Sydney/SupportGroup/Information Sydney/SupportGroup/Information/work Sydney/SupportGroup/Information/gossip work gossip Sydney/*/Information Sydney/DevGroup/*/* Sydney/DevGroup/** 90

Publish-Subscribe – Some Thoughts

 Highly decoupled messaging style   Publishers don’t know about subscribers Subscribers don’t know who is publishing   Publishers and Subscribers can dynamically appear and disappear Issues –  Reliability  Transactions  Security  Performance 91

J2EE Overview

Client tier Web Tier Business Component tier EIS Tier Browser-based client applications (HTML, applets, DHTML/scripting) HTTP Web server Servlets, JSPs RMI Application components EJBs JCA Java client applications Java RMI Container Services Components eg. JTS, JMS ERPs, CRMs, Mainframe TP systems JDBC RDBMS CAS COM Bridge, RMI over IIOP Windows/COM client applications 92

J2EE Application Server

  In J2EE, the application server container provides the execution environment for the J2EE-specific components  EJBs   Message-driven beans Connectors Container provides additional services for hosted components      Transactions Security Directory Threading Connection pooling 93

EJB Container

EJB Container EJB Pool Application Server

Transaction Service Lifecycle Management Persistence Connection Pool Thread Pool Directory Service Security Service 94

Beans and State

EJB Container

Stateless bean pool state state state state

EJB Clients

Stateful beans state state state 95

Deployment Descriptors

EntityStock.BrokerHome db.entitystock.BrokerHome db.entitystock.Broker db.entitystock.BrokerBean

Stateless Container

EntityStock.BrokerHome Remote * Required

96

J2EE – Some Thoughts

         Standards-based, multiple vendors, portable Good open source technology available Quality of implementations varies considerably Java only, wide platform support Performance is good, but varies between suppliers Scalable, fail over support through clustering Good integration with web technologies Supports various design patterns, flexible but more complex (e.g. stateful beans/scalability, entity beans) Standards evolving, need to monitor 97

Message Brokers - Motivation

Key:

Message = Web Component In-format In-format Legacy System #1 In-format Legacy System #2 In-format Legacy System #3 In-format Legacy System #4 In-format Queue Read Message Transform Legacyformat API call 98

What if …

 the common

In-format

changes?

message format  any legacy system API changes?

 any of the transformations needs modifying? 99

Alternative Solution

Key:

Message = L1-format Legacy System #1 L2-format Legacy System #2 Web Component In-format Message Broker L3-format Legacy System #3 L4-format Legacy System #4  Transformations in broker  Simplified endpoints  Decouples Web and legacy components 100

Message Brokers

 Developed specifically for Enterprise Application Integration (EAI)  Add new layers of functionality to MOM  Message transformation  Rules engine  Intelligent routing  Adapters  Typically (but not necessarily) built on top of a MOM layer 101

Message Broker Features

   Message transformation – transform between different source/target formats    Graphical message format definition and mapping tools High performance transformation engines Message format repositories Intelligent routing  Route messages based on message content Rules Engine   Scripting language, built-in functions Application programming environment 102

Message Brokers

Input Messages Hub and Spoke Architecture Transformation Routing Rules Processing Output Messages 103

Example - WMQI

104

BizTalk Mapping Tool

105

Adapters

      An adapter is a component that resides between the message broker and the source/target systems Simplify complexity of end system interface through an abstraction layer Thin adapters - simple wrappers Thick adapters   Programmable Abstract representation of services and meta-data Centralized adapters co-located with broker Distributed adapters execute in own process and may be located with source/target system 106

Message Brokers – Some Thoughts

    Embeds transformations/routing in broker  Can get complex Possible scaling issues  Need to replicate brokers Failure handling  Lightweight, rarely designed to recover from failure Often proprietary technology  Good open source, standards-based like Mule now available 107

Business Process Orchestration

    Commonly known as workflow Aim is to automate business processes which need to access data and business logic across disparate back-end applications Builds on EAI to ensure business processes are executed in the defined order using the required data Builds on middleware providing:  Process execution engine   Visual process definition tools Process monitoring tools 108

Typical Scenario

 Business process automation Siebel Accounts Payable Customer Purchasing Sales desk Accounts Receivable Oracle SAP Customer Receiving Credit Validation Shipping 109

Example - BizTalk

110

BPO Architecture

Adapter Message Broker Adapter Adapter 111

BPEL

 Web Services standard for describing workflows  Many design and execution tools  Eg ActiveBPEL  Version 2.0 is a significant improvement 112

Integration Issues – Point-to-Point

  Point-to-Point evolution Spaghetti architecture, hard to modify potentially (N 2 -N) interfaces

1 business process = 4 interfaces 5 business processes = 20 interfaces

113

Broker Spaghetti

 No free lunch …  Just relocates the spaghetti

message broker

114

Enterprise Data Model

    Source sends message to target with common message format as payload.

Target receives message and transforms common format into its own local data representation.

2xN transformations, no broker needed Getting agreement is the tough bit …

Enterprise Data Model

115

 

Summary

Middleware:  makes building complex, distributed, concurrent applications simpler.

 institutionalizes proven design practices by supporting them in off-the-shelf middleware technologies.

Architect’s job is to ‘mix n’match’ technologies to create appropriate solutions    Analyze trade-offs Open-minded (no hammer/nail thinking) No good/evil, its just technology 116

Essential Software Architecture

Session 5: A Software Architecture Process 117

A Software Architecture Process

 Architects must be versatile:    

Work with the requirements team:

The architect plays an important role in requirements gathering by understanding the overall systems needs and ensuring that the appropriate quality attributes are explicit and understood.

Work with various application stakeholders:

Architects play a pivotal liaison role by making sure all the application’s stakeholder needs are understood and incorporated into the design.

Lead the technical design team:

Defining the application architecture is a design activity.

Work with the project management:

budgets, schedules Planning, estimates, 118

An Architecture Process

 Highly iterative  Can scale to small/large projects Determine Architectural Requirements Architecture Design Validation 119

Determine Architectural Requirements

  Sometime called:   architecturally significant requirements architecture use cases essentially the quality and non-functional requirements for a system. Functional Requirements Determine Architecture Requirements Stakeholder Requirements Architecture Requirements 120

Examples

    A typical architecture requirement : 

“Communications between components must be guaranteed to succeed with no message loss”

Some architecture requirements are constraints:  “

The system must use the existing IIS-based web server and use Active Server Page to process web requests

” Constraints impose restrictions on the architecture and are (almost always) non-negotiable. They limit the range of design choices an architect can make. 121

Quality Attribute Requirements

Quality Attribute

Performance Security Resource Management Usability Availability Reliability Scalability Modifiability

Architecture Requirement

Application performance must provide sub-four second response times for 90% of requests.

All communications must be authenticated and encrypted using certificates.

The server component must run on a low end office-based server with 512MB memory.

The user interface component must run in an Internet browser to support remote users.

The system must run 24x7x365, with overall availability of 0.99.

No message loss is allowed, and all message delivery outcomes must be known with 30 seconds The application must be able to handle a peak load of 500 concurrent users during the enrollment period.

The architecture must support a phased migration from the current Forth Generation Language (4GL) version to a .NET systems technology solution.

122

Constraints

Constraint

Business Development

Architecture Requirement

The technology must run as a plug-in for MS BizTalk, as we want to sell this to Microsoft.

The system must be written in Java so that we can use existing development staff.

Schedule Business The first version of this product must be delivered within six months.

We want to work closely with and get more development funding from

MegaHugeTech Corp

, so we need to use their technology in our application.

123

Priorities

   All requirements are not equal   

High:

the application must support this requirement.

Medium:

stage this requirement will need to be supported at some

Low:

this is part of the requirements wish list.  Tricky in face of conflicts, eg: Reusability of components in the solution versus rapid time-to-market. Making components generalized and reusable always takes more time and effort.

 Minimal expenditure on COTS products versus reduced development effort/cost. COTS products mean you have to develop less code, but they cost money.

It’s design – not meant to be easy!

124

Architecture Design

 Design steps are iterative  Risk identification is a crucial output of the design Architecture Views Architecture Requirements Choose Architecture Framework Allocate Components Architecture Document 125

Choosing the Architecture Framework

 Choose a architecture pattern/patterns that suit requirements  No magic formula  Analyze requirements and quality attributed supported by each pattern  Complex architectures require creative blending of multiple patterns.

126

N-Tier Client Server Pattern

  

Separation of concerns:

Presentation, business and data handling logic are clearly partitioned in different tiers.

Synchronous communications:

Communications between tiers is synchronous request-reply. Each tier waits for a response from the other tier before proceeding.

Flexible deployment:

There are no restrictions on how a multi-tier application is deployed. All tiers could run on the same machine, or each tier may be deployed on its own machine. Client Tier Web Server Tier Business Logic Tier Data Management Tier Web Client Web Client Web Server Application Server Databases Web Client 127

N-Tier Client Server – Quality Attribute Analysis

Quality Attribute

Availability

Issues

Failure handling Modifiability Performance Scalability Servers in each tier can be replicated, so that if one fails, others remain available. Overall the application will provide a lower quality of service until the failed server is restored.

If a client is communicating with a server that fails, most web and application servers implement transparent failover. This means a client request is, without its knowledge, redirected to a live replica server that can satisfy the request.

Separation of concerns enhances modifiability, as the presentation, business and data management logic are all clearly encapsulated. Each can have its internal logic modified in many cases without changes rippling into other tiers.

This architecture has proven high performance. Key issues to consider are the amount of concurrent threads supported in each server, the speed of connections between tiers and the amount of data that is transferred. As always with distributed systems, it makes sense to minimize the calls needed between tiers to fulfill each request.

As servers in each tier can be replicated, and multiple server instances run on the same or different servers, the architecture scales out and up well. In practice, the data management tier often becomes a bottleneck on the capacity of a system.

128

Messaging Pattern

 

Asynchronous communications:

Clients send requests to the queue, where the message is stored until an application removes it.

Configurable QoS:

The queue can be configured for high-speed, non-reliable or slower, reliable delivery. Queue operations can be coordinated with database transactions.

Loose coupling:

direct binding between clients and servers. There is no

Client Client Client Que ue Server Server Server

129

Messaging – Quality Attribute Analysis

Quality Attribute

Availability Failure handling Modifiability Performance Scalability

Issues

Physical queues with the same logical name can be replicated across different messaging server instances. When one fails, clients can send messages to replica queues.

If a client is communicating with a queue that fails, it can find a replica queue and post the message there. Messaging is inherently loosely coupled, and this promotes high modifiability as clients and servers are not directly bound through an interface. Changes to the format of messages sent by clients may cause changes to the server implementations. Self-describing, discoverable message formats can help reduce this dependency on message formats.

Message queuing technology can deliver thousands of messages per second. Non reliable messaging is faster than reliable, with the difference dependent of the quality of the messaging technology used.

Queues can be hosted on the communicating endpoints, or be replicated across clusters of messaging servers hosted on a single or multiple server machines. This makes messaging a highly scalable solution.

130

Publish-Subscribe Pattern

  

Many-to-Many messaging:

Published messages are sent to all subscribers who are registered with the topic.

Configurable QoS:

In addition to non-reliable and reliable messaging, the underlying communication mechanism may be point-to point or broadcast/multicast.

Loose Coupling:

and subscribers. As with messaging, there is no direct binding between publishers

Publisher Topic Subscriber Subscriber Subscriber

131

Publish-Subscribe – Quality Attribute Analysis

Quality Attribute Issues

Availability Failure handling Topics with the same logical name can be replicated across different server instances managed as a cluster. When one fails, publishers send messages to replica queues.

If a publisher is communicating with a topic hosted by a server that fails, it can find a live replica server and send the message there. Modifiability Publish-subscribe is inherently loosely coupled, and this promotes high modifiability. New publishers and subscribers can be added to the system without change to the architecture or configuration. Changes to the format of messages published may cause changes to the subscriber implementations.

Performance Scalability Publish-subscribe can deliver thousands of messages per second, with non reliable messaging faster than reliable. If a publish-subscribe broker supports multicast/broadcast, it will deliver multiple messages in a more uniform time to each subscriber.

Topics can be replicated across clusters of servers hosted on a single or multiple server machines. Clusters of server can scale to provide very high message volume throughput. Also, multicast/broadcast solutions scale better than their point-to-point counterparts.

132

Broker Pattern

  

Hub-and-spoke architecture:

The broker acts as a messaging hub, and senders and receivers connect as spokes.

Performs message routing:

The broker embeds processing logic to deliver a message received on an input port to an output port.

Performs message transformation:

The broker logic transforms the source message type received on the input port to the destination message type required on the output port.

Sender-1 Sender-2

inPort1 inPort2

Broker

OutPort1

Receiver-1

OutPort2

Receiver-2

133

Broker Pattern - Quality Attribute Analysis

Availability

Quality Attribute

Failure handling Modifiability Performance Scalability

Issues

To build high availability architectures, brokers must be replicated. This is typically supported using similar mechanisms to messaging and publish-subscribe server clustering.

As brokers have typed input ports, they validate and discard any messages that are sent in the wrong format. With replicated brokers, senders can fail over to a live broker should one of the replicas fail.

Brokers separate the transformation and message routing logic from the senders and receivers. This enhances modifiability, as changes to transformation and routing logic can be made without affecting senders or receivers.

Brokers can potentially become a bottleneck, especially if they must service high message volumes and execute complex transformation logic. Their throughput is typically lower than simple messaging with reliable delivery.

Clustering broker instances makes it possible to construct systems scale to handle high request loads.

134

Process Coordinator Pattern

  

Process encapsulation:

The process coordinator encapsulates the sequence of steps needed to fulfill the business process. The sequence can be arbitrarily complex.

Loose coupling:

The server components are unaware of their role in the overall business process, and of the order of the steps in the process.

Flexible communications:

Communications between the coordinator and servers can be synchronous or asynchronous. Start process request step1

Server-1 Process Coordinator

step2 step3

Server-2

Process results

Server-3

step4

Server-4

135

Process Coordinator – Quality Attribute Analysis

Quality Attribute Issues

Availability The coordinator is a single point of failure. Hence it needs to be replicated to create a high availability solution.

Failure handling Modifiability Failure handling is complex, as it can occur at any stage in the business process coordination. Failure of a later step in the process may require earlier steps to be undone using compensating transactions. Handling failures needs careful design to ensure the data maintained by the servers remains consistent.

Process modifiability is enhanced because the process definition is encapsulated in the coordinator process. Servers can change their implementation without affecting the coordinator or other servers, as long as their external service definition doesn’t change.

Performance Scalability To achieve high performance, the coordinator must be able to handle multiple concurrent requests and manage the state of each as they progress through the process. Also, the performance of any process will be limited by the slowest step, namely the slowest server in the process.

The coordinator can be replicated to scale the application both up and out. 136

Allocate Components

 Need to:      Identify the major application components, and how they plug into the framework.

Identify the interface or services that each component supports.

Identify the responsibilities of the component, stating what it can be relied upon to do when it receives a request.

Identify dependencies between components.

Identify partitions in the architecture that are candidates for distribution over servers in a network  And independent development 137

Some Design Guidelines

     Minimize dependencies between components. Strive for a loosely coupled solution in which changes to one component do not ripple through the architecture, propagating across many components.  Remember, every time you change something, you have to re test it.

Design components that encapsulate a highly “cohesive” set of responsibilities. Cohesion is a measure of how well the parts of a component fit together. Isolate dependencies on middleware and any COTS infrastructure technologies. Use decomposition to structure components hierarchically. Minimize calls between components, as these can prove costly if the components are distributed. 138

A Simple Design Example

read Write Order

OrderInput

New Orders Check Order Write Order Get Order

SendEmail Validate Store

Error Log

Customer System Order System Email Server Figure Key

Existing Component New Component Dependency Database Persistent Queue 139

Example Design

   Based on messaging Application components are:    

OrderInput:

encapsulating the order processing logic, and writing to the queue.

responsible for accessing the new orders database,

Validate:

encapsulates the responsibility of interacting with the customer system to carry out validation, and writing to the error logs if an order is invalid.

Store:

responsibility of interacting with the order system to store the order data.

SendEmail:

removes a message from the queue, formats an email message and sends it via an email server. It encapsulates all knowledge of the email format and email server access.

Clear responsibilities and dependencies 140

Architecture Validation

     Aim of the validation phase is to increase confidence of the design team that the architecture is fit for purpose.  The validation has to be achieved within the project constraints of time and budget The trick is to be as rigorous and efficient as possible.

 Validating an architecture design poses tough challenges.

‘coz it’s a design that can’t be executed or tested  consists of new and COTS components that have to be integrated Two main techniques: 1.

2.

manual testing of the architecture using test scenarios. construction of a prototype that creates a simple archetype of the desired application aim of both is to identify potential flaws in the design so that they can be improved before implementation commences.  Cheaper to fix before built 141

Scenarios

    Part of SEI’s ATAM work Involves defining:   some kind of stimulus that will have an impact on the architecture. working out how the architecture responds to this stimulus. If the response is desirable, then a scenario is deemed to be satisfied by the architecture. If the response is undesirable, or hard to quantify, then a flaw or at least an area of risk in the architecture may have been uncovered.

142

Scenario Examples

Stimulus Quality Attribute

Availability Modifiability Security Modifiability Scalability

Response

The network connection to the message consumers fails. A new set of data analysis components must be made available in the application.

No requests are received on a user session for ten minutes. The supplier of the transformation engine goes out of business.

The concurrent user request load doubles during the 3 week enrollment period.

Messages are stored on the MOM server until the connection is restored. Messages will only be lost if the server fails before the connection comes back up.

The application needs to be rebuilt with the new libraries, and the all configuration files must be updated on every desktop to make the new components visible in the GUI toolbox.

The system treats this session as potentially insecure and invalidates the security credentials associated with the session. The user must logon again to connect to the application.

A new transformation engine must be purchased. The abstract service layer that wraps the transformation engine component must be re implemented to support the new engine. Client components are unaffected as they only use the abstract service layer.

The application server is scaled out on a two machine cluster to handle the increased request load. 143

Scenarios for Order Processing Example

Quality Attribute

Modifiability Availability

Stimulus Response

The

Customer System

application is updated to an Oracle database.

packaged The

Validate

component must be rewritten to interface to the Oracle system.

The email server fails.

Messages build up in the email server restarts. Messages are then sent by the not affected.

OrderQ SendEmail

until the component to remove the backlog. Order processing is

Reliability The Customer or Order systems are unavailable.

Needs fixing ….

If either fails, order processing halts and alerts are sent to system administrators so that the problem can be fixed.

144

Prototyping

    Scenarios can’t address everything: 

“On Friday afternoon, orders must be processed before close of-business to ensure delivery by Monday. Five thousand orders arrive through various channels (Web/Call centre/business partners) five minutes before close-of business.”

Only one way – build something!

Proof-of-concept prototype:

designed be built in a way that can satisfy the requirements?

Can the architecture as

Proof-of-technology prototype :

expected?

Does the technology (middleware, integrated applications, libraries, etc) selected to implement the application behave as 145

Prototyping Strategy

   Build minimal system required to validate architecture, eg: An existing application shows that the queue and email systems are capable of supporting five thousand messages in five minutes So:  Write a test program that calls the

Customer System

validation APIs five thousand times, and time how long this takes.

 Write a test program that calls the

Order System

store APIs five thousand times, and time how long this takes.

146

Prototyping Thoughts

   Prototypes should be used judiciously to help reduce the risks inherent in a design. Only way to address:     Performance Scalability Ease of integration Capabilities of off-the-shelf components Need to be carefully scoped and managed.  Ideally take a day or two, a week or two at most.   Usually thrown-away so keep them cheap Don’t let them acquire a life of their own 147

Summary

 3 step, iterative architecture design process  Can be customized to small/meduim/large projects  Agnostic to overall process framework (ie RUP, agile, waterfall, etc) 148

Essential Software Architecture

Session 6: Documenting a Software Architecture 149

Architecture Documentation

 Architecture documentation is a thorny issue  Commonly there is no documentation covering the architecture.  If it is, it’s out-of-date, inappropriate and basically not very useful.  Also projects that have masses of architecture related information  Sometimes invaluable, but often it’s out-of-date, inappropriate and not very useful! 150

Documenting an Architecture is good!

 Others can understand/evaluate the design.  We can understand the design after a period of time.

 Others in the project team and development organization can learn from the architecture.

 We can do analysis on the design, perhaps to assess its likely performance, or to generate standard metrics.

151

But it’s difficult …

    No universally accepted architecture documentation standard. An architecture can be complex, and documenting it in a comprehensible manner is time consuming and non-trivial. An architecture has many possible views. Documenting all the potentially useful ones is time consuming and expensive.

An architecture design often evolves Keeping the architecture documents current is often forgotten, especially with time and schedule pressures in a project.

152

Think carefully about what to document

    Project complexity  A small project may only need a ‘marketecture’ Project longevity   One-off stop gap software?

Strategic, long-term, will evolve?

Needs of stakeholders    Small team, a whiteboard might be ok Large, dislocated team needs more Integrators? Testers? Programmers?

Need to spend documentation dollars/euros wisely on high value products 153

UML 2.0

 UML is a powerful way to document an architecture  Provides a relatively formal, unambiguous description  New features in UML 2.0 appropriate for architectures   Good tools available, some free Can be used to depict various structural/behavioral architecture views 154

Component Diagram

id Component View OrderProcessing

«table»

NewOrders

read 1 writeQ 1

MailQueue

validate 1 1

CustomerSystem

1 writeQ 1

OrderQueue

readQ 1

SendEmail

1 send 1

MailServer OrderSystem

155

Class Diagram

cd OrderProcessing OrderReader

1 1 1 1

Validate

1

QueueWriter

1

Store

156

Sequence Diagram

sd Interactions OrderReader

NewOrders readOrderData success:= validateOrder

Validate

success:= newOrder success:= storeOrder CustomerSystem

Store

success:= writeQueue success:= acknowledgeOrderSuccess OrderQueue

QueueWriter

success:= writeQueue MailQueue 157

Deployment Diagram

dd Deployment View OrderServer OrdersDB

«table»

:NewOrders

JDBC «executable»

: OrderProcessing

SOAP

CustomerSystem CRM

«executable»

: CustomerSystem MOMServer :MailQueue ERP OrderSystem

«executable»

: OrderSystem :OrderQueue MailServer :MailServer :SendEmail

158

Component Interfaces

id Component View OrderProcessing

«table»

NewOrders

JDBC QueueWrite

MailQueue

QueueWrite QueueRead

SendEmail

CustomerServices

OrderQueue

QueueRead SMTP

MailServer CustomerSystem OrderSystem

159

Component Decomposition

id Component View

«table»

NewOrders

JDBC

OrderProcessing

QueueWrite getOrders writeConfirmation validateOrder writeOrder

MailQueue

QueueRead QueueWrite

SendEmail

CustomerServices

OrderQueue

QueueRead SMTP

MailServer

validate

CustomerSystem OrderSystem

ProvidedInterface1

cd Component View OrderProcessing

«delegate»

or: OrderReader qw: QueueWriter

«delegate» getOrders writeConfirmation

val: Validate st: Store

«delegate» validateOrder «delegate» writeOrder 160

Document Template

 Documentation is easier if there’s a template to use  Reduces start-up time for projects by providing ready-made document structures  familiarity gained with the document structure aids in the efficient capture of project design details.  help with the training of new staff 161

Template Headings

3 3.1

3.2

3.3

3.4 3.5

4 4.1 4.2

Architecture Documentation Template Project Name: XXX 1 Project Context 2 2.1

Architecture Requirements Overview of Key Objectives 2.2

2.3

2.4

2.5

2.6

Architecture Use Cases Stakeholder Architectural Requirements Constraints Non-functional Requirements Risks Solution Relevant Architectural Patterns Architecture Overview Structural Views Behavioral Views Implementation Issues Architecture Analysis Scenario analysis Risks 162

Summary

  Some documentation is nearly always a good idea Trick is to produce ‘just enough’ and no more  requires upfront planning and thinking  Commitment to keeps docs current   UML 2.0 makes architecture documentation easier Some good UML 2.0 tools, try ‘em out.

163

Essential Software Architecture

Session 7: ICDE Case Study Design 164