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

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

@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/

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

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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?

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

@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is an old method, technology, computer system, or application program/code, "of, relating to, or being a previous or outdated computer system," yet still in use.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

@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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@kenny_baas 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

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

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@kenny_baas 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

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

@kenny_baas Instead of one canonical language, create multiple bounded languages

@kenny_baas Business Architects Developers

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

@kenny_baas 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

@kenny_baas 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

@kenny_baas Business Architects Developers

@kenny_baas Photo by Evan Qu on Unsplash Guiding heuristic Start communicating business change in contextual boundaries that form the bounded context (purpose, need, responsibility)

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@kenny_baas Photo by Evan Qu on Unsplash 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

@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.

@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

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

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

@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

@kenny_baas Photo by Evan Qu on Unsplash Guiding heuristic Discover and identify bounded context with EventStorming

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@kenny_baas Photo by Evan Qu on Unsplash Design heuristics ● Split according to the language. ● Split according to the departments. ● Split according to the users.

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@kenny_baas Photo by Evan Qu on Unsplash Guiding heuristic Pick some boundaries to start with and iterate.

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@kenny_baas 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/

@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

@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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@kenny_baas 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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@kenny_baas 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); }

@kenny_baas 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) { ………………

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

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@kenny_baas Photo by Evan Qu on Unsplash Design heuristics (Competing) 1. “Integrate through an API on application layer” 2. “Integrate through an API on the database layer”

@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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@kenny_baas Photo by Evan Qu on Unsplash Guiding heuristic “Either remove a bubble context, or dedicate resources and people to move towards an autonomous bubble context”

@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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@kenny_baas Photo by Evan Qu on Unsplash Value heuristic “Use Test-Driven Development and good unit testing practices to get fast feedback, and become more confident on changes”

@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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@kenny_baas Photo by Evan Qu on Unsplash Guiding heuristic “Don’t rebuild functionality that works as intended and has no or little expected changes coming up. (Including software characteristics)”

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

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@kenny_baas Photo by Evan Qu on Unsplash Guiding heuristic “You can use Monolith and Microservices architecture together!!”

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

@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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@kenny_baas Photo by Evan Qu on Unsplash Design heuristic “Implement requirements and information based on the responsibility of your bounded context”

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

@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

@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”

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