Aligning Bounded Contexts with Subdomains in Legacy Code

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Aligning Bounded Contexts with Subdomains in Legacy Code NDC Oslo 2021 Mufrid Krilic Domain-Driven Design Coach www.cowork.no

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Training and Coaching • Agile Leadership • “Tight-Loose-Tight” • Domain-Driven Design with NDC Workshops • bit.ly/domainstorytelling-online • Blog • https://mufridk.medium.com

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Book recommendation #1

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Strategic Domain-Driven Design The Principles

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Strategic DDD • Focusing on Core domain • Problem and solution space • Subdomains and Bounded Contexts • Linguistic boundaries • Ubiquitous Language • The two pillars of DDD

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Core Domain • The thing that distinguishes you from the competitors • “Not every part of the system will be well-designed” • Generic subdomain • Supporting subdomain

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Problem Space and Solution Space Problem space • Domain analysis to discover inherent Subdomains Solution space • Model your application accordingly in multiple Bounded Contexts

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Linguistic Boundaries Ubiquitous Language • The same language everywhere • Conversations • Documentation • Code

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The Two Pillars of DDD 1. Ubiquitous Language 2. Bounded Context Bounded contexts in your application are defined by linguistic boundaries

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Book recommendation #2

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Boundaries Why?

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Making Sense of the Problem Space • Aligning with the business • Reducing the translation effort • Ubiquitous Language

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Reducing the Cognitive Load • Legacy Code • Being able to navigate • …. and understand • Hidden boundaries in legacy code • Look for subdomains

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Book recommendation #3

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Boundaries Where?

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Observable boundaries • Technical • Network • Containers • Organizational • Teams • Departments • Easily observable boundaries

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Not so easily observable? • The “4+1” View Model of Software Architecture • Philippe Kruchten • https://bit.ly/4plus1Paper

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https://bit.ly/4plus1Paper

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Dependencies What are they?

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Logical View Dependencies The reason for increased coupling • The amount of data that is passed across the boundaries

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Cost of Dependencies in a Distributed System • Distributed logical dependencies • Across software and process views • Across the organization • Microservices

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The Story Begins…. Story about a team dealing with legacy system

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Business Case • Physicians prescribe medications • Integration with systems for prescription delivery in pharmacies • Focus on putting patient needs and safety on the top

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Use Case Prescribing pre-approved medication

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Drawn with egon.io

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Use Case Prescribing non-approved medication

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Drawn with egon.io

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Book recommendation #4

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Looking for Subdomains .... in Legacy Environments

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Subdomains in Problem Space • Pivotal Event • NonApprovedMedicationPrescribed • Time-related activity • While waiting for approval the physician stops the prescription process

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Drawn with egon.io

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Traces of Bounded Contexts in Development View • Are there any established boundaries in the Development View? • 4+1 Model • .NET projects • Namespaces

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Dependencies in Logical View • Analysis of • using statements • .NET project references • NuGet package references • Http calls to remote services • Event-based communication with remote services

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Bounded Contexts …. and Context Mapping

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DDD Context Mapping Downstream: depending on upstream Upstream: depended on downstream Conformist: Upstream model referenced directly in Downstream context

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Bounded Contexts in Legacy Code • Respect existing dependencies and relationships • Explicit decision on the amount of work needed to break things up

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Decision Time

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Application Bounded Context • Establish observable boundary • Visual Studio Solution • Prescription Bounded Context remains as is • 1 week time-box • Summarize lessons learned regardless of the outcome

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Step One Establish Visual Studio Solution

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Move existing .NET projects • Between solutions within the same repository • git mv • Exclude/Include Project Code compiles and unit tests run

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Integration tests? • Valuable enough feedback with unit tests only? • Integration/E2E tests • Harder to maintain • Can provide runtime feedback

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Work on E2E tests? • Pros • Early feedback on bounded context/subdomain alignment • Cons • Considerable work needed just to get the tests running • Could actually slow down the feedback loop Focus on unit tests only at this stage

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Step Two Cleaning up obsolete references

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Made obsolete in step one • .NET project references • using statements Uncovers hidden dependencies

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Classes and interfaces - Defined in Application BC - Referenced in Prescription BC

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Stable Abstraction Principle Packages that are maximally stable should be maximally abstract. • Extract common type definitions to a separate module

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DDD Context Mapping Shared Kernel: Two contexts share a subset of the domain model

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Step 3 Make E2E tests to run

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The end of the story…. • Some E2E tests in Prescription context required DLLs from Application context • Why?

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Valid business reason for Prescription BC to refer to Application BC - Beware of Circular Dependencies!

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Business requirement unlocked • As a physician while prescribing a non-approved medicament • I need to see prior applications for the same medicament • To be able to prescribe non-approved medicaments on a life-long basis if required by patient condition Requires dependency from Prescription BC to Application BC

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Can we avoid circular dependencies? • 4+1 model • Deployment dependencies ok • Circular logical dependencies • All the integration tests in one module • Referencing different bounded contexts • All DLLs would be available

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View Model Composition • Composing data from different Bounded Contexts on front-end • Suggested reading • Mauro Servienti • https://milestone.topics.it/ • The physician needs only to see the prior applications • Data shown for decision-support

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Takeaways for the Team ▪ Regular Architectural Reviews – continuous learning about subdomain boundaries ▪ Stable Abstraction Principle – Define desired relationships between the modules ▪ Discover Logical Dependencies in Code Review – 4+1 Model ▪ “You can always make things more cohesive” – Kent Beck Ella Fitzegerald – «They Cant’t Take That Away From Me»

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Book recommendation #5

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Photos • By !Original: MddVector: Wikimpan - Based on File:4+1 Architectural View Model.jpg by User:Mdd, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=50144028 • https://unsplash.com/@renemolenkamp • https://unsplash.com/@jancanty • https://unsplash.com/@afafa • https://unsplash.com/@walkator • https://unsplash.com/@woutvanacker • https://unsplash.com/@polarmermaid • https://unsplash.com/@cdd20 • https://unsplash.com/@roaming_angel • https://unsplash.com/@micheile • https://unsplash.com/@patrickperkins • https://unsplash.com/@melany_tuinfosalud • https://unsplash.com/@timmossholder