In this talk i'll try to show you how DDD can help us solve some of the hardest challenges in architecting a micro-services eco-system: 1. Making sure that things which change for same reason are co-located inside the same microservice. 2. Elegant and efficient event driven integration of microservices

1. Microservices: Splitting the
Monolith the DDD Way
January 2, 2017
#indoerswetrust

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ABOUT ME
Erik Ashepa
● 10 years of software development and
management experience
● 5 years practicing DDD
● Currently R&D Group Manager @
Fiverr

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THIS PRESENTATION
Part 1: Bounded contexts and microservices
● Definitions
● Why split the monolith?
● How to do it?
● Examples
Part 2: Integrating microservices
● Definitions
● Why integrate?
● How to do it?
● Examples

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Domain
sphere of knowledge and activity
around which the application logic
revolves.
DEFINITIONS

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Domain-driven design (DDD)
an approach to developing software
for complex needs by deeply
connecting the implementation to an
evolving model of the core business
concepts
DEFINITIONS

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DEFINITIONS
Bounded Context
DDD deals with large models by dividing them
into different Bounded Contexts and being
explicit about their interrelationships.

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WHY?
Models should be designed to answer
specific questions and not be general
purpose do everything spaghetti
monsters.
Avoid big ball of mud

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Facilitate team autonomy
WHY?
less coordination and synchronization
between teams and groups increases team
velocity.

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Operational burden
WHY?
● No way to scale parts of the
application
● Painful continuous delivery - High
build & test times and “fear factor”
for each deploy.

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HOW?
MICROSERVIES
BOUNDED CONTEXTS
* Not a free lunch

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PAGE TITLE
HOW?
MICROSERVICES BOUNDARIES
SHOULD BE ALIGNED WITH
BOUNDED CONTEXTS.
IDEALLY A 1-1 RELATIONSHIP.

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Bounded contexts boundaries should
be driven by:
● Semantics (context and meaning in
different sub-domains)
● Business-processes steps or activities
● Preferably *not* data which can be
duplicated
PAGE TITLE
HOW?

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EXAMPLE

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EXAMPLE

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EXAMPLE
WE CAN
DO BETTER!

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EXAMPLE

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INTEGRATING MICROSERVICES
Why do we need to integrate?

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INTEGRATING MICROSERVICES
Why do we need to integrate?
● Cascading command triggers

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INTEGRATING MICROSERVICES
Why do we need to integrate?
● Cascading command triggers
● Synchronize State changes

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DEFINITIONS
Domain Events
Capture things that can trigger a
change to the state of the
application you are developing.

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Properties of effective domain events
● Capture the intent of the user
● Always in the past tense.
● Naming aligned with the business
PAGE TITLE
HOW?

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INTEGRATING MICROSERVICES
Cascade command triggers using domain events and event handlers
1. Commands create events which are handled by event handlers (i.e: PlaceOrderCommand
triggers OrderPlacedEvent and handled by OrderPlacedEventHandler)
2. Publish events so other bounded contexts will get a chance to handle them.

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EXAMPLE

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INTEGRATING MICROSERVICES
Synchronize State Changes
● Query database table directly.
● API for getting the latest version of an object. (/users/3/orders/1234)
● Use state captured on Domain Events
● Publish and Consume State Change Events (i.e: OrderCreated, OrderUpdated)

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TRANSPORTING DATA
How to publish Domain Evens and
Synchronize State between Microservices:
● Request Reply (RPC, REST)
● Publish and Subscribe (Message Broker)

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TRANSPORTING DATA
Request Reply (RPC, REST)
● Simple: Easy to set up and synchronous.
● No indirection: Services call each other explicitly.
● Temporal Coupling: Communicating services must be running to exchange information.

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TRANSPORTING DATA
Publish and Subscribe (Message Broker)
● Complex: More moving parts and asynchronous.
● Indirection: Services don’t call each other, they use an intermediary.
● No Temporal Coupling: Eventually communicating services will consume the information.

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EXAMPLES
● REST API for consuming domain events (/user/2/events)
● WebSocket API for consuming state change events (/user/2/sce/)
● REST API for querying user resource (/user/2/)
● Kafka topic for publishing and consuming domain events/state change events
● Database table for storing domain events (A.K.A event sourcing)

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SLAYING DRAGONS

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RESOURCES
Domain Driven Design Israel @
Meetups.com
Join the DDD-IL community @ Meetups for more:
● Modellathons
● Event Storming
● Advanced Topics
● Lots of arguments

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** BTW: We’re hiring **
Thank You