In this talk, we need to know what microservices is, and are you really need to do it? You may never need microservices unless crunched business core value.

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Kim Kao, Solutions Architect
Mar 13, 2019
Implementing Microservices by DDD
DDD – A collaboration way to go

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A typical day for a customer new to AWS...
Manager -
“We are going to run workload(s) on AWS.
We have new sub-systems/module to develop with legacy services.
Container is good, Lambda is awesome. It’s great to have whole
cloud native advantage if you guys migrate all service into
microservice, serverless...”
Developer - “Not a problem. I’ll make it …”

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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 splitting 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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What are Microservices?

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= Microservices ?

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100/200/300/1000?
LOC

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What Are Microservices?
“A software architecture style in which complex
applications are composed of small, independent
processes communicating with each other using language-
agnostic APIs. These services are small, highly decoupled
and focus on doing a small task, facilitating a modular
approach to system-building.” - Wikipedia
https://en.wikipedia.org/wiki/Microservices

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Way to divide services from Monolith
• By noun(s)
• By Your Experience(s)
• By Team’s decision
• Up to Leader/Manager/CxO ...
• No right or power to influence

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Case study - Gilt
Problem behind Microservices

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Gilt
The limited-selling website for limited time purchases, the flash purchase start time is except for 9
:00 pm on Wednesday and Sunday, and the rest are sold at 12 noon in the US East, lasting 36-48
hours and then off the shelf. Popular brands usually go out quickly.

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Monolith
Load
Balancer
Account Service
Cart Service
Shipping Service
StoreFront UI
Browser
Database
Data Access
Service

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Account
Database
Inventory
Database
Shipping
Database
Migrate to Microservices
Load
Balancer
StoreFront
UI
Browser
Account
Service
Cart Service
Shipping
Service
Load
Balancer
Load
Balancer
Load
Balancer

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Challenge is Coming
• Authentication & Authorization
• Session mechanism is broken
• Centralized & Robust alternative one is required
• Service Discovery
• Write the traffic routing code(s) in logic?
• Re-try, Failover, Caching, ...
• Persistence
• Isolated Persistence for each Service

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A Classic solution on AWS
Load
Balancer
StoreFront
UI
Browser
Account
Database
Account
Service
Cart Service Inventory
Database
Shipping
Service
API
Gateway
Load
Balancer
Load
Balancer
Load
Balancer
Shipping
Database

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Authentication & Authorization
• Token based solution fit in
• Cognito
• 3rd party Authentication Federation
• Amazon, Google, Apple, Facebook...
• Self developed/hosted centralize A&A
services

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Service Discovery
• Client Side-Service Discovery
• Application Load Balancer-based Service
Discovery
• DNS-Based Service Discovery
• Service Discovery using ECS Event
Stream
• Configuration Management
• New * App Service Mesh (preview)
• Cloud Map

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Microservices
For Persistence ?
Stateful Persistent always be the top issue !!!

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• Accept all API calls to one RDS Cluster
• RDS Instance Sizing predict by services scaling ability
• DBA(s) : How to manage Connection Pool ?
• Read replica can’t help on Write intention
• RDS I/O be the bottleneck when highly Scale up
• All Services impacted while any failure occurred
• Option : Upgrade Read Replica to Master
Launching 1 RDS cluster

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Launching N * RDS for Independency?

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• DynamoDB
• Perfect deal with high transaction (write)
• Serve each table for only one transaction/service intention
• De-Normalize schema is required
• Prevent revision issue
• Hard to do complexity Join Query
• Cost High
• Low Performance
• Coding for iteration & aggregation
• Apply all RDS tables into DynamoDB?
Using NoSQL Solution?

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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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Transaction Dependencies between Microservices
• Upstream-Downstream co-relationship
• One Transaction invoke local API and Remote API
Book Rental
Service
Book Flight
Service
Trip Service
Book Hotel
Service
Exception / Error ?
Exception / Error ?
Exception / Error ?
!"
#
!$
#
!#
#
+
+
Transaction Compensate

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Saga : Alternative for 2 Phase -commit

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Context
• You have applied the Database per Service pattern.
Each service has its own database.
• Some business transactions, however, span multiple
service so you need a mechanism to ensure data
consistency across services.

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Problem
How to maintain data consistency across services?

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Forces
• 2 Phase-Commit(PC) is a well-known solution for years
• The 2PC coordinator also represents a Single Point of
Failure, which is unacceptable for critical systems

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Solutions
• Each local transaction updates self
and publishes a message/event to
trigger the next local transaction in
the saga.
• If a local transaction fails because
it violates a business rule then the
saga executes a series of
compensating transactions that
undo the changes that were made
by the preceding local
transactions.
https://microservices.io/patterns/data/saga.html

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Two ways of Saga Pattern
• Choreography - each local transaction publishes
domain events that trigger local transactions in other
services
• Orchestration - an orchestrator (object) tells the
participants what local transactions to execute
https://microservices.io/patterns/data/saga.html

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Choreography-based Saga
https://microservices.io/patterns/data/saga.html
2
1
3.b
3.a
4.a
4.b
• Logic in code
• Compensate chain
• Multiple states present in transaction

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Orchestration -based Saga
https://microservices.io/patterns/data/saga.html
• Coordinator stand alone
• Each service provide
a pair function for Success or Fail
• Abstract compensate logic
out of Codes1
1.1 2 3
45

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Resulting Context
• This pattern has the following benefits:
• It enables an application to maintain data consistency across multiple services without
using distributed transactions
• This solution has the following drawbacks:
• The programming model is more complex. For example, a developer must design
compensating transactions that explicitly undo changes made earlier in a saga.
• There are also the following issues to address:
• In order to be reliable, a service must atomically update its database and publish a
message/event. It cannot use the traditional mechanism of a distributed transaction
that spans the database and the message broker. Instead, it must use one of the
patterns listed below.

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Implementing Saga on AWS

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AWS Step Functions

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• Each Service provide cancel operation
• State change trigger cancel operation
• Simplify Complexity rules
• Just a JSON file

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Code snippet
https://github.com/humank/lambda-saga-pattern

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How to adopt Microservices?

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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

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Better way to decompose Monolith
Domain
Expert
Matters

43. &

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How to break Monolith?

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Methodology -1
Use Case Modeling

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Methodology -2
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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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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Modeling - Intent matters

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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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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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Coffee shop experience
DDD by EventStorming

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Go through Event Storming approach
Don’t tell tech only
Don’t sell tech partially
Aim for Core value
Figure out trigger
and result

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Seat
occupied
Menu offered
Ordered 2 cups
of Americano
Paid
Order
received
Coffee
made up
Customers
Left
Table
Cleaned
*Key Business Events in Coffeeshop

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Event Trigger
• Client
• Server
• Counter
• Barista
ACTORS

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Most Valuable / Risky Events

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Order
Make Up
Payment
Event Bus
(pub/sub)
Put
event
Event
Event
CloudWatch Event
.
.
.
Any other messaging
technology
Coffee shop Domain implementation Core Domain
Sub Domain
(Messaging)
Support Domain
Core Domain

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

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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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Take Away
Know Why/What/How
• Do you really need Microservices?
• Leverage Business Events to aggregate
context and form up Service Boundary
• There is no C4 to solve distributed persistence
issue
• State Machine to do Transaction Compensate
(Step Functions way to go)
• DDD is good to collaborate Business and
Technology guys by speaking Ubiquitous
Language
• Crunch Problem, then design solution

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Implementing DDD on AWS
AWS : YikaiKao@amazon.com
Commounty : DDD Taiwan@FB
Telegram : YikaiKao
WeChat : YikaiKao
Twitter : @YikaiKao
GitHub Repos