Cqrs event sourcing slide landis+gyr

CQRS and Event Sourcing With Axon Framework

© Landis+Gyr | September 25, 2020 | CONFIDENTIAL
Content
 Classical old school N-Tier Architecture and its Characteristics
 Drawbacks of Classical old school N-Tier Architecture
 DDD and Aggregates
 Event Sourcing
 CQRS
 Event Sourcing and CQRS
 Axon Framework
 Resources and QnA
2

Classical old school N-Tier Architecture
Let’s Review the
classical old school
N-tier Architecture
Client
Network
Soap/REST End Point
Business Service
Dao
Network
RDBMS
View
Model
DTO
Business
Model
ER-
Model

Characteristics
We read and write data
through the same layers
1
WRITE
READ

Characteristics
5
2
We use the same Data
model for read and
write access

Characteristics
We use coarse grained
Deployment units that
Combine read and write
3

Characteristics
7
We change datasets directly
4

Using N-Tier Architecture is not an issue
8
Many Applications will
run smooth and fine
with this kind of approach

Drawbacks of Classical old school N-Tier Architecture
 The data model is a compromise.
 You can’t scale read and write separately.
 Horizontal scale has limit.
 Normalized SQL Queries(Joins and sub queries).
 No Data history, no snapshots, no replay.
 Tendency to a monolithic application.
9

DDD
Domain-Driven Design (DDD) describes an approach to building
software that puts a lot of emphasis on the design of a model,
leveraging the use of ubiquitous language . The domain model is the
heart of software and should correctly capture and deal with the
essential complexity of the domain.
Ubiquitous Language is the term uses in Domain Driven Design for the
practice of building up a common, rigorous language between
developers and users(client).
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Aggregates
An Aggregate is a cluster of domain objects that can be treated as a
unit. It consists of a root entity and possibly one or more other
associated entities and value objects. For example, the domain model
for the online store contains aggregates such as Order and Customer.
An Order aggregate consists of an Order entity (the root), one or more
OrderLineItem value objects along with other value objects such as a
delivery Address and PaymentInformation. A Customer aggregate
consists of the Customer root entity along with other value objects
such a DeliveryInfo and PaymentInformation.
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Event Sourcing
Event Sourcing is an architectural pattern in which the state of the
application is being determined by a sequence of events.
Event is something that has happened to a domain object. An
event usually represents a state change. Consider, for example, an
Order aggregate in the online store. Its state changing events
include Order Created, Order Cancelled, Order Shipped. Events can
represent attempts to violate a business rule such as a Customer’s
credit limit.
12

Event Handling
13

Events Streams
The sequence of events in the queue is called event stream.
14

Key Points
 Event Sourcing is a technique for reliably updating state and
publishing events that overcomes limitations of other solutions.
 The design concepts for an event-driven architecture, using event
sourcing, align well with microservices architecture patterns.
 Snapshots can improve performance when querying aggregates by
combining all events up to a certain point in time.
 Event sourcing can create challenges for queries, but these are
overcome by following CQRS guidelines and materialized views.
 Event sourcing and CQRS do not require any specific tools or
software, and many frameworks exist which can fill in some of the
low-level functionality.
15

Events
 An event is something that happened in the past.(ex:- Order
Related Events)
 An Event is always immutable.
 A Create/Update/delete are just an event.
 There is no deletion of events.
 The Events use event bus which is usually implemented by a
message broker.
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Examples of Events
17
ShipmentDeliveredEvent
CustomerVerifiedEvent
CartCheckedOutEvent

Advantages of Event Sourcing
 Complete rebuild is possible.
 Temporal Queries.
 Event Replay.
 Works well with Domain Driven Design.
 Useful in Analytics based on Historical data.
18

CQRS
Command Query Responsibility Separation
CQRS, as the name suggests, splits the application into two parts. The
first part is the command-side, which handles commands (e.g. HTTP
POSTs, PUTs, and DELETEs) to create, update and delete aggregates.
These aggregates are, of course, implemented using event sourcing.
The second part of the application is the query side, which handles
queries (e.g. HTTP GETs) by querying one or more materialized views
of the aggregates. The query side keeps the views synchronized with
the aggregates by subscribing to events published by the command
side.
19

Old vs CQRS
Old architecture Idea behind CQRS
20

CQRS Code Example
21

Event Sourcing and CQRS
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Advantages
 Individual scalability and deployment options.
 Technological freedom of choice for command, query and event
handler code.
 Excellent Fit for Bounded Context (Domain Driven Design).
 Enhanced User Experience.
 The Event Store has a very high business value.
23

Axon Framework
Axon provides the Axon Framework and the Axon Server to help build
applications centered on three core concepts - CQRS (Command Query
Responsibility Segregation) / Event Sourcing and DDD (Domain Driven
Design).While many types of applications can be built using Axon, it
has proven to be very popular for microservices architectures. Axon
provides an innovative and powerful way of sensibly evolving to event-
driven microservices within a microservices architecture.
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Architecture Overview
25

Why Axon?
 Reduce Boilerplate Code.
 APIs as design Guidance.
 Infrastructure as Configuration.
 Use cases for Microservices.
 Integration(works with explicit messaging).
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Resources
CQRS by Martin Fowler
https://martinfowler.com/bliki/CQRS.html
Event Sourcing by Martin Fowler
https://martinfowler.com/eaaDev/EventSourcing.html
Axon Framework
https://axoniq.io/
https://www.infoq.com/articles/microservices-aggregates-events-
cqrs-part-1-richardson/
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