Domain Driven Design by Eric Evans

Domain-Driven Design by Eric Evans

This is a basic summary of the main points of this book. It provides some good tips on the software development process and some good pattern applications as well.

Premise of book
1. For most software projects, the primary focus should be on the domain and domain logic
2. Complex domain designs should be based on a model.


Part I - Putting the Domain Model to Work

Chapter 1 - Crunching Knowledge

Ingredients of Effective Modeling
1. Binding the model and the implementation - crude model type created and changed iteratively
2. Cultivating a language based on the model -explanations required between the software team and business
3. Developing a knowledge-rich model - captures knowledge of various kinds
4. Distilling the model - testing concepts and removing those which don't fit anymore
5. Brainstorming and experimenting

Chapter 2 - Communication and the Use of Language

When building your model use language that both sides(Development & Business) understand.
This is called "Ubiquitous language"
This helps make the model more useful for the conversation between development and business.

Chapter 3 - Binding Model and Implementation

A Domain Driven design calls for a model that doesn't just aid early analysis but is the very foundation for the design.
Tightly relating the code to an underlying model gives the code meaning and makes the model relevant.


Chapter 4 - Isolating the Domain

Give a Shipping Lane sample application with 4 tiers (User Interface, Application, Domain, and Infrastructure)
Layers can communicate down but not up.

Chapter 5 - A Model Expressed in Software

Introduce different types of objects
Entity Objects - objects defined by their identity(ie. key), can change, change state
Value Objects - objects that represent things that do not change, immutable (ie. an address)

3 attributes of a good service
1. operation relates to a domain concept that is not a natural part of an entity or value object
2. interface is defined in terms of other elements of the domain model
3. operation is stateless

Modules(aka Packages, namespaces)
grouping classes that are cohesive

Chapter 6 - The Life Cycle of a Domain Object
Talking about Aggregate Objects
Factories - Helping simpify the knowledge required to create complex objects
- each factory operation should be atomic
- the factory will be coupled to its arguments

Repositories
How to provide access to globally accessible objects
Repositories advantages include
- present clients with a simple model for obtaining persistent objects and managing their life cycle
- decouple application and domain design from persistence technolgoy, database,....
- communicate design decisions about object access
- allow easy substitution of a dummy implementation, for use in testing

Factory handles the beginning of an object's life; a Repository helps manage the middle and the end.

Chapter 7 - Using the Language: An Extended Example

An example of refinements in developing a Model-Driven Design via a Cargo Shipping System
Isolating the Domain, Distinguishing Entities and Value Objects, Aggregate Boundaries

Part III Refactoring Toward Deeper Insight

Chapter 8 - Breakthrough
Each refinement of code and model gives devlopers a clearer view.
This clarity creates the potential for a breakthrough of insights
Work on refactoring may not show much progress initially but often eventually leads to a breakthrough

Chapter 9 - Making Implicit Concepts Explicit
Listen to the language the domain experts use. These are hints of a concept that might benefit the model.
Scrutinize Awkwardness in language describing the model.
There may be a missing concept
Descibed patterns such as Specification - testing objects for certain criteria

Chapter 10 - Supple Design
Intention-Revealing Interfaces - better expression
Side-Effect-Free Functions - make safe and simple
Assertions - make side effects explicit
Standalone classes - Lower coupling, elimnate all other concepts from the picture
Closure of Operations - where it fits, define an operation whose return typ is the same as the type of its arguments
- this provides a high-level interface without introductin any dependency on other concepts
Conceptual Contours - decompose design elements into cohesive units, taking into consideration your intuation


Chapter 11- Applying Analysis Patterns
Analysis patterns are groups of concepts that represent a common construction in business modelling.
It may be releveant to only one domain or it may span many domains. [Fower 1997, p.8]
-they help feed into the dynamo of knowledge crunching and refactoring toward deeper insight and stimulates development
-when you use a term from a well-known analysis pattern, take care to keep the basic concept it designates intact

Chapter 12 - Relating Design Patterns to the Model
Strategy - decoupling an algorithm via an interface which allows other algorithms to be interchanged
Composite - treating individual objects and compositions of objects uniformly

Chapter 13 - Refactoring Toward Deeper Insight
Focus on:
1. Live in the domain
2. Keep looking at things a different way
3. Maintain an unbroken dialog with domain experts

Part IV - Strategic Design

Chapter 14 - Maintaining Model Integrity
Bounded Context - explicitly define the context within wich a model applies
Continuous Integration - institute a process of merging all code and other implementation artifacts frequently
Context Map - identify each model in play on the project and define its BOUNDED CONTEXT
Shared Kernel - designate some subset of the domain model that the 2 teams agree to share
Customer/Supplier Development Teams - establish a clear customer/supplier relationship between two teams
Conformist - if there is an upstream/downstream relationship, the downstream should conform to the upstream group
Anticorruption Layer - create an isolating layer to provide clients with functionality in terms of their own domain model.
Separate Ways - declare a bounded context to have no connection to others at all(avoid cost of integration)
Open Host Service - define a protocol that gives access to your subsystem as a set of Services
Published Language - use a well-documented shared language that can express the necessary domain infomration as a common medium of communication


Chapter 15 - Distillation
Core Domain - Boil the model down. Put most valuable and specialized concepts into the Core Domain, apply top talent to the Core Domain
Generic Subdomains - identify cohesive subdomains that are not motivation for your project, leave no trace of project specifics there
Domain Vision Statement - write short description of the Core domain ("value proposition"), statement to guide
Highlighted Core - very brief document (3-7 pgs) that describes the core domain
Cohesive Mechanisms - partition a conceptually cohesive mechanism into a separate lightweight framework(use intention revealing interface)
Segregated Core - refactore the model to separate the core concepts from supporting players and strengthen the cohesion of the CORE while reducing its coupling to other code
Abstract Core - Identify the most fundamental concepts in the model and factor them into distinct classes, abstract classes, or interfaces, place in own module


Chapter 16 - Large Scale Structure
Devising a pattern of rules or roles and relationshiops that will span the entire system and that allows some understanding of each part's place in the whole
Evolving Order - the conceptual large-scale structure evovolving with the application, possibly changing to a completely different type of structure along the way, avoid straight-jacketing
System Metaphor - when a concrete analogy to the system emerges that captures the imagination of team members and seems to lead thinking in a useful direction, adopt it as a large-scale structure
Responsibility Layers - look at conceptual dependencies in your model and the varying rates and sources of change of different parts of your domain, if you identify a natural strata in the domain, cast them as broad abstract responsibilities
Knowledge Level - a group of objects that describes how another group of objects should behave, crate a distinct set of objects that can be used to describe and constrain the structure and behavior of the basic model
Pluggable Component Framework - distill an abstract core of interfaces and interactions and create a framework that allows diverse implementations of those interfaces to be freely substituted

Chapter 17 - Bringing the Strategy Together
Six essentials for Strategic Design Decision Making
1. Decisions must reach the entire team
2. The decision process must absorbe feedback
3. The plan must allow for evolution
4. Architecture teams must not siphon off all the best and brightest
5. Strategic design requires minimalism and humility
6. Objects are specialists; developers are generalists

Dynamic Checkerboard Code

An interesting problem is to draw a dynamic checkerboard or chessboard with the constraints that the height and width of the board are variable along with the fact that the dimension of the individual squares themselves have variable defining their own height and width. The board must be drawn with a simple text output.

The following java solution is proposed

/*
 * Create a function that can draw a checkerboard with boardwidth by boardlength where each square is squarewidth by squarelength.
 * It is to be drawn with System.out.prints only.
 */
public class DrawCheckerBoard {

 public static void drawCheckerBoard(int checkerBoardWidth, int checkerBoardHeight, int squareWidth, int squareHeight)
 {
  for (int i = 0 ; i < checkerBoardHeight ; i++)
  {
   for (int j = 0 ; j < squareHeight ; j++)
   {
    for(int k = 0 ; k < checkerBoardWidth ; k++)
    {
       //determine character to draw for square
       //based on the which checkerboard spot (i and k indices)
       String character = "X";
       if ((i+k)%2==0)
       {
      character = "O";
       }
       for (int l = 0 ; l < squareWidth ; l++)
       {
      System.out.print(character);
       }
    }//end of line
    System.out.println("");
   }
  }  

 }  

 public static void main(String[] args)
 {
  
  System.out.println("4 by 4 board with each square being 2 by 2");
  drawCheckerBoard(4,4,2,2);

  System.out.println("8 by 8 board with each square being 4 by 4");
  drawCheckerBoard(8,8,4,4);

 }
 
}

COBIT Powerpoint Presentation

I researched to find out what COBIT was by reading IT Governance Based on Cobit 4.1: A Management Guide (Paperback)
by Koen Brand and Harry Boonen.

COBIT is an IT Framework for Governance. It has similarities to ITIL and some overlap. One person told me that COBIT is what you build and ITIL is how you do it.

Anyways, I made a brief powerpoint presentation on COBIT which may be viewed from Here

Obviously very high level, but with pictures and some text, it will help.

Determining Leap Year in Oracle and Days in February

Recently, I was playing around with a leap year issue and the number of days in February. I would like to share the following snippets of commands to help some people save some time.

--2 ways to determine if it is a LEAP YEAR in ORACLE

SELECT 2008 , Decode( mod(2008, 4), 0, decode( mod(2008, 400), 0, 'Yes', decode( mod(2008, 100), 0, 'No', 'Yes') ), 'No' ) as LEAP_YEAR
FROM DUAL;

or

SELECT DECODE(29, (SELECT to_date('01032008', 'DDMMYYYY') - to_date('01022008', 'DDMMYYYY')FROM DUAL),'LEAPYEAR','NOTLEAPYEAR')
FROM DUAL;

--2 ways to determine the NUMBER OF DAYS IN FEBRUARY in ORACLE

SELECT (EXTRACT (DAY FROM LAST_DAY(ADD_MONTHS(TRUNC(TO_DATE('2008','YYYY'),'YYYY'),1))))
FROM DUAL;

or

SELECT to_date('01032008', 'DDMMYYYY') - to_date('01022008', 'DDMMYYYY')
FROM DUAL;



Of course the input to these queries requires the year that you are inquiring about.

Powerpoint ITIL Presentation

I have recently done some research into ITIL including the reading of Foundations of IT Service Management Based on ITIL V3 by Jan Van Bon, Arjen de Jong, and Axel Kolthof.

ITIL is a very broad level topic about IT services and how to approach them.

I did make a brief high level powerpoint presentation on an Introduction to ITIL.

This presentation may be downloaded from Here

It is a basic high level presentation, it is difficult to present the topic without getting lost in the depth and breadth. This presentation just presents a little background and a little overview of the lifecycle phases in Version 3.

Service Oriented Architecture with Java

I recently read “Service Oriented Architecture with Java” by Binildas CA, Malhar Barai, and Vicenzo Caselli. This book, available at http://www.packtpub.com, provides a look at some of the tools in the java world that can be applied to support a Service Oriented Architecture.

There are 6 Chapters in all:

Chapter 1: The Mantra of SOA
This chapter reviews basic tiered architecture, EA and the basic points of benefit of SOA including better integration, business agility, asset re-use, increase ROI

Chapter 2: Web Services and SOA
Practically all current SOA implementations now are built upon web services. XML over the Http protocol is covered. Representational State Transfer(REST) is covered. Main java implementations of web services are introduced including JAX-WS 2, Axis2, Spring-WS, and XFire/CXF 2.0.

Chapter 3 : Web Service Implementations
Code is presented for getting a web service up and running in JAX-WS2, Axis2, Spring-WS, and XFire/CXF 2.0 The coded examples are very easy to follow and can get a developer up and running quickly.

Chapter 4: Data and Services – All Roads Lead to Enterprise Service Bus
This chapter reviews JDO(Java Data Objects) as an alternative to JDBC along with sample code and examples. Service Data Objects(SDO) are covered as a way to abstract data within and SOA. Apache Tuscany DSO is covered with an example. Service Component Architecture(SCA) is described along with a Tuscany SCA java example
Benefits of MOM and ESB are also covered. OpenESB is covered as an open source option for implementing an ESB.

Chapter 5 – Traditional Integration Technology
2 Case Studies are presented showing the advantages of an SOA based architecture over that of EAI.

Chapter 6 – Goals We Can Achieve with SOA
Loose Coupling, Reusability, Seamless Integration, Return on Investment(ROI)

All in all this is a pretty good book. It’s focus is definitely to provide information on a SOA implementation in a java oriented environment. This book covers the basics of the open source options to getting java based web services and infrastructure. I would strongly recommend this book to those trying to do open source SOA implementations in java.

Run .NET Code from a shared drive/directory

Security is an issue whenever running code from an untrusted source like a shared or network drive. It would be nice to be able to put an application on a share drive to run whether it be in C# or VB .NET.

Normally, when you try to run .NET code on a shared network drive you will get an exception like System.Security.Permissions.SecurityPermission .


There is a way to relax the security via the caspol.exe command(For .Net Framework 2.0 "C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\CasPol.exe -s off").

An application may be launched with one simple batch file as follows:

AllInOne.BAT:
@ECHO OFF

REM Relax security constraints for .NET
START C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\CasPol.exe -s off

REM Pause until first command has executed
Set Count=1
:LOOP
Set /A Count=%Count%+1
If %Count% lss 100 GoTo :LOOP

REM Launch .NET application
"""SomeExecutable.exe"""

REM Remove relaxation of security constraints
TASKKILL /im CasPol.exe

@ECHO ON
*****************************

A pause is required to allow caspol to start up before launching the .NET application.

But of course this implementation does meet the requirements of launching the .NET application from the shared location but there are those DOS windows which are open and probably not that desirable.

These can be avoided with 2 BAT files and 2 VBS files as follows. This is just one way of doing it but there are probably better ways out there.

launch.bat (Main file)
****************
@ECHO OFF
REM Main Launch for .NET application on share drive
REM VBS script used so there is not a DOS console opened up

START launchtasks.vbs

@ECHO ON
****************

launchtasks.vbs
****************
On Error Resume Next

CmdLine = """tasks.bat"""
Params = ""
CmdLine = CmdLine & " " & Params

'Launch without window
wscript.echo fShellRun(CmdLine)

Function fShellRun(sCommandStringToExecute)
' This function will accept a string as a DOS command to execute.
' It will then execute the command in a shell, and capture the output into a file.
' That file is then read in and its contents are returned as the value the function returns.
Dim oShellObject, oFileSystemObject, sShellRndTmpFile
Dim oShellOutputFileToRead, iErr
Set oShellObject = CreateObject("Wscript.Shell")
Set oFileSystemObject = CreateObject("Scripting.FileSystemObject")
sShellRndTmpFile = oShellObject.ExpandEnvironmentStrings("%temp%") & oFileSystemObject.GetTempName
On Error Resume Next
oShellObject.Run sCommandStringToExecute & " > " & sShellRndTmpFile, 0, True
iErr = Err.Number
On Error GoTo 0
If iErr <> 0 Then
fShellRun = ""
Exit Function
End If
fShellRun = oFileSystemObject.OpenTextFile(sShellRndTmpFile,1).ReadAll
oFileSystemObject.DeleteFile sShellRndTmpFile, True
End Function
****************

tasks.bat
****************
@ECHO OFF

REM Relax security constraints for .NET
REM Run this in VBS to avoid a DOS WINDOW
START caspol.vbs

REM Pause until first command has executed
Set Count=1
:LOOP
Set /A Count=%Count%+1
If %Count% lss 70 GoTo :LOOP

REM Launch .NET application
"""SomeExecutable.exe"""

REM Remove relaxation of security constraints (The >NUL is to avoid showing the info on the processes terminated)
TASKKILL /im CasPol.exe >NUL

@ECHO OFF
****************


caspol.vbs
****************
On Error Resume Next

'The command below will also depend on the .NET version of the application you are running

CmdLine = """C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\CasPol.exe"""
Params = "-s off"
CmdLine = CmdLine & " " & Params

'New way without window
wscript.echo fShellRun(CmdLine)

Function fShellRun(sCommandStringToExecute)
' This function will accept a string as a DOS command to execute.
' It will then execute the command in a shell, and capture the output into a file.
' That file is then read in and its contents are returned as the value the function returns.
Dim oShellObject, oFileSystemObject, sShellRndTmpFile
Dim oShellOutputFileToRead, iErr
Set oShellObject = CreateObject("Wscript.Shell")
Set oFileSystemObject = CreateObject("Scripting.FileSystemObject")
sShellRndTmpFile = oShellObject.ExpandEnvironmentStrings("%temp%") & oFileSystemObject.GetTempName
On Error Resume Next
oShellObject.Run sCommandStringToExecute & " > " & sShellRndTmpFile, 0, True
iErr = Err.Number
On Error GoTo 0
If iErr <> 0 Then
fShellRun = ""
Exit Function
End If
fShellRun = oFileSystemObject.OpenTextFile(sShellRndTmpFile,1).ReadAll
oFileSystemObject.DeleteFile sShellRndTmpFile, True
End Function
****************