Domain-Driven Design heuristics for dealing with legacy software @ Techorama 2022

Domain-Driven Design heuristics
for dealing with legacy software
Kenny Baas-Schwegler
@kenny_baas
Photo by Mathew Schwartz on Unsplash

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@kenny_baas

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@kenny_baas
No I won’t rant a lot about
“The next internet explorer type microsoft legacy for generations to come”

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Legacy Software
– Kenny Baas-Schwegler
@kenny_baas
Photo by Gabriella Clare Marino on Unsplash
I <3

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@kenny_baas
“anything that provides a plausible aid or direction in the
solution of a problem but is in the final analysis unjustified,
incapable of justification, and potentially fallible.
– Billy Vaughn Koen - Nuclear Engineer
Heuristic
https://wirfs-brock.com/blog/2019/03/20/growing-your-personal-design-heuristics/

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@kenny_baas
Photo by Evan Qu on Unsplash
Design heuristic
“Favour monolithic architecture over microservices architecture”

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@kenny_baas

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@kenny_baas
Photo by Christopher Burns on Unsplash
But…But I thought we should get rid of
Legacy software, not create more?

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@kenny_baas
Photo by Manish Tulaskar on Unsplash
Monolithic architecture != BAD
Monolithic architecture != Legacy

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@kenny_baas
Legacy software
In computing, a legacy system is an old method, technology,
computer system, or application program/code, "of, relating
to, or being a previous or outdated computer system," yet still
in use.
https://en.wikipedia.org/wiki/Legacy_system

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@kenny_baas
Monolithic architecture
A single-tiered software
application in which diﬀerent
components combined into a
single program form a single
deployable unit. It can be a MVC
approach or an API with a
database.

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To communicate eﬀectively, the code must be based on the
same language used to write the requirements
- the same language that the developers speak
with each other and with domain experts
- Eric Evans

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https://www.huffingtonpost.co.uk/entry/american-tweets-about-great-british-bake-off_uk_61733eaee4b03072d6f6d012

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We all know or should know that language is ﬂuid, liquid, subject to the
whims of the people. Language evolves, as it should. Because language
changes to accommodate new users, the older users resist and complain.
http://tednellen.blogspot.com/2013/04/language-is-fluid.html

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It is not the domain experts knowledge
that goes to production, it is the
assumption of the developers
that goes to production
- Alberto Brandolini

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Instead of one canonical language,
create multiple bounded languages

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Business
Architects
Developers

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A straight line between 2 points
corresponds to a compass direction in
reality..

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A straight line between 2 points corresponds to
a compass direction in reality..
• Except for points located in Greenland
• Except for points located in Africa

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A model is a simpliﬁed representation of a thing
or phenomenon that intentionally emphasizes
certain aspects while ignoring others.
Abstraction with a speciﬁc use in mind.
— Rebecca Wirfs-Brock

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Business
Architects
Developers

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Guiding heuristic
Start communicating business change in
contextual boundaries that form the bounded context
(purpose, need, responsibility)

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@kenny_baas

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Design heuristic
When designing new bounded contexts, they should have one team who is
owning that bounded context. A team can have multiple bounded context

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@kenny_baas
Photo by Gloria Cretu on Unsplash
Recap
● A legacy system is an old application program/code still in use.
● Monolith is a deployment strategy.
● Legacy != Monolith, and most legacy software is build as monoliths
● Most monoliths components are not designed in a DDD way, creating a
disconnect with the business strategy and creating a big ball of mud.

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@kenny_baas
Photo by Felix Mooneeram on Unsplash
BigScreen
● Fictitious company
● International cinema chain with 80 cinema’s located in 12 diﬀerent countries
● Custom software system
● The current system is completely entangled, a big ball of mud (BBoM) that has
been around for many years

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Big Ball of Mud
Software system that lacks a
perceivable architecture and the
model is implicit.

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Photo by Fusion Medical Animation on Unsplash
Then came Covid-19

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Requirements
● Reopen in approx 2-3 months
● 1.5 meters apart from reservations
● Sign a health check
● Able to go back when needed

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Guiding heuristic
Discover and identify bounded context with EventStorming

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Design heuristics
● Split according to the language.
● Split according to the departments.
● Split according to the users.

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Guiding heuristic
Pick some boundaries to start with and iterate.

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Using collaborative modelling to build a
shared understanding of your domain and use it to
guide your design _is_ the philosophy behind DDD
though. The rest is principles, patterns, and practices.
— Mathias Verraes
https://verraes.net/2021/09/what-is-domain-driven-design-ddd/

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@kenny_baas
Requirements
● Reopen in approx 2-3 months
● 1.5 meters apart from reservations
● Sign a health check
● Able to go back when needed

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@kenny_baas
3 Strategies + Evolution
1. Bubble context
2. Autonomous bubble
3. Exposing Legacy as a service (LaaS)
➔ Expanding a bubble
https://www.domainlanguage.com/wp-content/uploads/2016/04/GettingStartedWithDDDWhenSurroundedByLegacySystemsV1.pdf

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Anti-corruption Layer (ACL)
Translation layers can be simple, even elegant, when bridging
well-designed bounded contexts with cooperative teams. But when
control or communication is not adequate to pull oﬀ a shared kernel,
partner or customer/supplier relationship, translation becomes more
complex. The translation layer takes on a more defensive tone.
Therefore:
As a downstream client, create an isolating layer to provide your system
with functionality of the upstream system in terms of your own domain
model. This layer talks to the other system through its existing interface,
requiring little or no modiﬁcation to the other system. Internally, the layer
translates in one or both directions as necessary between the two models.
ACL is more than a Translation Layer!
From: https://www.domainlanguage.com/ddd/reference/
https://virtualddd.com/sessions/20

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Repositories
For each type of aggregate that needs global
access, create a service that can provide the
illusion of an in-memory collection of all
objects of that aggregate’s root type.
Set up access through a well-known global
interface. Provide methods to add and remove
objects, which will encapsulate the actual
insertion or removal of data in the data store.
public interface AuditoriumSeatings
{
Task GetAuditoriumSeating (ShowId
showId);
}

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public class AuditoriumSeatingAdapter : AuditoriumSeatings
{
private readonly IProvideAuditoriumLayouts _auditoriumSeatingRepository
;
private readonly IProvideCurrentReservations _reservationsProvider
;
public AuditoriumSeatingAdapter
(IProvideAuditoriumLayouts auditoriumSeatingRepository
,
IProvideCurrentReservations reservationsProvider
)
{
_auditoriumSeatingRepository = auditoriumSeatingRepository
;
_reservationsProvider = reservationsProvider
;
}
public async Task> GetAuditoriumSeating
(ShowId showId)
{
var auditoriumDto = await _auditoriumSeatingRepository
.GetAuditoriumSeatingFor
(showId.Id);
var reservedSeatsDto = await _reservationsProvider
.GetReservedSeats
(showId.Id);
return AdaptAuditoriumSeatingDto(auditoriumDto, reservedSeatsDto
);
}
private static AuditoriumSeating AdaptAuditoriumSeatingDto(AuditoriumDto auditoriumDto,
ReservedSeatsDto reservedSeatsDto
)
{
var rows = new Dictionary();
foreach (var (rowName, seatDtos) in auditoriumDto.Rows)
{ ………………

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@kenny_baas
Ports and Adapters
The hexagonal architecture, or
ports and adapters architecture, is
an architectural pattern used in
software design. It aims at creating
loosely coupled application
components that can be easily
connected to their software
environment by means of ports
and adapters. This makes
components exchangeable at any
level and facilitates test
automation.

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Design heuristic
(repeated)
“Favour monolithic architecture over microservices architecture”

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Design heuristics
(Competing)
1. “Integrate through an API on application layer”
2. “Integrate through an API on the database layer”

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1. Bubble context

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Guiding heuristic
“Either remove a bubble context, or dedicate resources and people to move
towards an autonomous bubble context”

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@kenny_baas
Photo by Yong Chuan Tan on Unsplash
Black hole anti-pattern
When a POC is put to production and not getting the time and
people to decouple, making the bubble burst
and be absorbed back by the Big Ball of Mud

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Value heuristic
“Use Test-Driven Development and good unit testing practices
to get fast feedback, and become more conﬁdent on changes”

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1. Bubble context

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Guiding heuristic
“Don’t rebuild functionality that works as intended
and has no or little expected changes coming up.
(Including software characteristics)”

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@kenny_baas
Follow Jabe Bloom: https://twitter.com/cyetain
Follow Ben at HiredThought: https://twitter.com/HiredThought

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Guiding heuristic
“You can use Monolith and Microservices architecture together!!”

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https://github.com/ddd-crew/ddd-starter-modelling-process

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1. Bubble context

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Design heuristic
“Implement requirements and information
based on the responsibility of your bounded context”

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1. Bubble context

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Reference from Trond Hjorteland: Sociotechnical System
https://www.researchgate.net/publication/306242078_Assessing_the_impact_of_new_technology_on_complex_sociotechnical_systems

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@kenny_baas
Photo by David Ramírez on Unsplash
“Every tradeoff we make, that we might see as technical
has also user and biz effects and are hence a sociotechnical tradeoff”
– Ruth Malan / @ruthmalan

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@kenny_baas
Photo by Gabriella Clare Marino on Unsplash
“Our problems are not legacy software systems,
these can be easily changed giving enough time and people.
Our problem is we are dealing with legacy sociotechnical system,
who have legacy best practices competing with emerging practices”

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@kenny_baas
#CatTax
@kenny_baas
Baasie.com
xebia.com/blog/author/kbaas/
https://speakerdeck.com/baasie
https://virtualddd.com/

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Domain-Driven Design heuristics for dealing with legacy software @ Techorama 2022

Domain-Driven Design heuristics for dealing with legacy software @ Techorama 2022

Kenny Baas-Schwegler

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