Domain Driven Design Taiwan Community meetup series. This slides learn from ddd_eu, DDD china and sort of public videos on youtube. Focus on Why DDD matters, what is microservices, and how can you adopt microservices.

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Kim Kao ( )
Jan 26, 2019
Why DDD matters
Crunch problems, design solutions

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uncertainty.equals(dangerous) == true

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A typical day for problem solving...
Manager -
“We are going to launch new product.
We are challenged by competitors, several niche economics there."
“I heard of microservices(anything else), sounds good to quick deliver.
Make sure you are aware on this, and release on time!!!”
Developer - “...........”

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Problem Solving

6. &
& &

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Application is …
Requirements Software Modeling Collaboration

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Eric Evans
Ubiquitous Language
Bounded Context
Collaboration

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Sheet music works

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When talk about problem

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IoT
SSO
Server
less
Network
Container
Firecracker
AI/ML
DW
Front
End DB
CI/CD

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Knowledge from stakeholders
Use Case Modeling

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Through Classic to Agile
ICONIX Processing

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Object Modeling …
ER Modeling …
xxx Modeling …
A lack of Collaboration with non-tech stakeholders
Not “Ubiquitous Language”

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Way to collaborate
• Point out the events
• Who send the command
• Find the Noun(s)
• Have all team voices
Commands
Events
Aggregate

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Domain Driven Design

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Intent matters

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Domain Driven Design (DDD)
• What is DDD?
• Not only a Technology / Methodology
• Set of principles and patterns for focusing the design effort where
it matters most
• It’s all about
• Understanding the domain where the software will be applied
• Create highly expressive model of that domain
• Distil Ubiquitous language

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Domain (Strategy) Modeling – Basic Terms
• Domain
• Subject area where the software will be applied
• Example : VOD Domain, EC Domain, Banking Domain
• Subdomain
• Logically separated part of the Domain
• Example : Ingestion, Streaming, Geo Location
• Domain Model
• Software model for solution of a domain problem
• Bounded Context
• Explicit boundary where Domain Model lives

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Tell about story

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Domain is about capability
Grab key events to
Abstraction to give

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Dive into relationships and why
Strategic DDD

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Pattern in Practice
Tactical DDD

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Approach
• Collect the “Aggregates” which stand for similar concept in
the “Bounded Context”.
• Prioritize the “Domains”, which one is Core Domain, rest for
Sub Domain, even Utility Domain.
• Get Consensus, not Compromise
• Focus on Core Domain

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Tell story by coding

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Microservices?

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100
LOC

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200
LOC

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400?
1000?

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Trap in Microservices

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Crunch Bounded Context
• 1st focus on high Cohesion
• Always Low Coupling later

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“All Things Distributed”
“Design to Fail”
“Redundancy is hard”
“Server provisioning hell .....................................”

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It’s about capability
• Are you ready to deal with M:N transaction compensation ?
• Are you ready to embrace the rapidly change by contract ?

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When you should dive in Microservices
Team
Partners
Business
Operation
Coding
Value

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Migration Strategy
Assess and
prioritize, one app
at a time
Re-host (lift-and-shift)
data center or Cloud
Re-platform (lift-tinker-shift)
VMs à containers
Re-factor
monolith à microservices
Re-invent (cloud-native)
new serverless microservices
Determine
Core/Sub/utility
Domain

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How BBoM to Microservices
• Only one domain a time
• Test Driven
• Value Driven
• Change Driven
• Adapter in BBoM
• Port in Microservices

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Event handler

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Domain Events means …
Un Cer tain ty
the state of being uncertain.

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De duplicator
Event 3 Event 1 Event 2 Event 1 Event 3 Event 2 Event 1
…
Want In-Order, actually Uncertainty

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Uncomfortable
It’s okay to feel this way.
@VaughnVernon

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Dependencies

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Focus on Contract and Protocol
• Service owner is consumer
• Expose SDK means projection technical decisions to consumer
• “Operation” complexity spread to consumer
• Consumer should be able to use any language/technology
and change over time!
• AWS
• API Gateway, Lambda Execution, any composable API call

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Some exceptions
• Centralized Logging
• Monitoring
• Distributed tracing

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Principle

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Adopt Microservices?

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Business Wants
https://vaughnvernon.co/tag/event-storming/

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But You Want
https://vaughnvernon.co/tag/event-storming/

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Challenge on migration to Microserivces
• Legacy looks like Big ball of mud(BBOM)
• Heavy dependency with external system
• No idea on split BBOM
• No idea to find out system boundary
• Which service(s) worth to do
• Human resources allocation
• Team, out sourcing, ISVs solution

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Business operation without whole picture
The Blind Men and the Elephant
Is It correct to all in microservices or serverless ?

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Precondition to do microservices
Rapid Provisioning
Basic monitoring
Rapid application deployment
Martin
Fowler
Realize the business flow, way to decompos

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Better way to decompose Monolith
Domain
Expert
Matters

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Learn and respect Domain experts

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How DDD can help you
• Business strategy on resource allocation
• Best resources should be put in most key/core domain
• Buy or out-sourcing common domain and sub domain
• Service re-architecture
• Form up the system context boundary
• Knowing the upstream-downstream relationship between domains
• Meaningful to do microservice (separate computing/persist, and
API communication )
• Service Migration
• Good to re-architecture

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Migration Business Case
Know Why/What/How
• Options to Optimize
• Learn and speak same language with
Domain expert
• Capture key business events, if it matters
• Adopt appropriate AWS services to
implement Solution

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Sharpen your saw

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DDD stuff

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Event Storming

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Specification by Example

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Programming Skills level up
• Test Driven Design
• Behavior Driven Design
• Refactoring
• CI/CD

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Practice DDD
Telegram : YikaiKao
WeChat : YikaiKao
Twitter : @DddTaiwan, @YikaiKao
GitHub Repos

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Thank You!

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Let’s explore
Event Storming