Developing complex distributed systems is hard. There are many challenges. How do we identify and tackle them? How do others do it?

1. Applying ADSD and NServiceBus
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2. Introduction
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3. Who we are?
● For around 20 years, Strasz Assessment Systems has provided solutions for the Assessments
industry, so typically, our clients offer education, certification or licensure programs.
● The system we will be talking about today is the one we have built for one of our largest clients, the
AICPA.
● The AICPA produces, delivers and scores the Uniform CPA exam.
● Our end to end solution for them includes content authoring of test questions, automated assembly
into an exam, delivery in a test center, scoring, reporting and invoicing
● Some of our other clients include AAMC who offer the MCAT exam, Inteleos who offer sonography
and physician certifications, and NCCPA who certify physician assistants.
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4. Technology Stack
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5. Technology Stack
● We are currently in the middle of a cloud migration effort. So our latest technology stack is
NServiceBus, .Net core, Azure Functions, Azure Cosmos Db, Azure Service Bus, Azure Cognitive
Search, Angular & Typescript and MonoRepo with NX.
● On the other hand, the CPA exam was computerised in the early 2000s so there are a lot of legacy
technologies in play.
● Our on-prem stack includes NServiceBus, RabbitMQ, RavenDb and SQL Server
● We also use IBM CPLEX engine which uses linear programming and user defined constraints to
automate exam creation.
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6. Need for SOA
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7. CPA Exam Administration
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8. AICPA Systems and Data Flow
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9. Example: Test Question (Item)
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10. CMS: Need for SOA
● CPA exam computerized around 20 years ago
○ Model mirrors database structure
○ Business Rules Implicit in the system
● Example issue with CMS
○ Different aspects of an Item that CMS needs to support
■ Question
■ Scoring Rules
■ Inventory
■ Statistics
■ Reference management
■ Classification management
○ Technical coupling of separate business capabilities.
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11. CMS: Need for SOA
● Example issue with CMS
○ Item development goes through various lifecycle stages from Conception to PreTest to Operational
■ Complex and inter-dependent workflows, reviews, validation rules
○ Snapshot for rescore or re-reporting
■ Candidate may request rescore for an exam they took in the past.
○ Increased cost of change
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12. Duplication: Need for SOA
● Example issue with Adaptive Routing
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13. Duplication: Need for SOA
● Example issue with Adaptive Routing
○ Routing rules and logic are imported into every system.
○ In SOA, a single service owns this data and behavior, not systems.
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14. CMS: Scoring Rules (Before)
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“Whatever the cause, software that lacks a concept at the foundation of its design is, at best, a mechanism that does useful things without
explaining its actions.”
― Eric Evans, Domain-Driven Design: Tackling Complexity in the Heart of Software

15. CMS: Scoring Rules Service (After)
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16. Challenges we faced
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17. Challenges
● Getting buy in from business.
○ Though we have an excellent product team, we rightly needed to show that we were not further
complicating the system.
○ External review of our codebase and SOA proposal by Paul Rayner who is one of the leading voices in
the DDD community.
● Getting developer buy-in, thanks to our practice and commitment to Continuous Learning we were
able to slowly but surely get the team on-board.
● Doing the actual work was challenging too. Unlike tactical DDD patterns, it was difficult to find a
reference architecture for ADSD
○ Trust the team’s collective background in design and we engaged Particular software (Mauro Servienti)
to consult with us I think a couple of hours every week during implementation
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18. Finding Service Boundaries
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19. Finding Service Boundaries
● Udi talks about slicing an entity based on which properties participate in a transaction and which ones
never will.
● For folks with DDD background, you can pay attention to language. For example in the context of
Questions, an item has response fields, stem, stimulus, widgets and exhibits. In the context of Scoring
rules item has Measurement Opportunities, Keys, Assertions.
● Other ways to find boundaries if you have a large relational database is to look at all the tables and see
which ones have more relationships between them. They are likely to be more cohesive.
● Remember the share nothing approach above all else. Only Ids can be shared. When correct service
boundaries are established they will seem obvious.
● We even discovered a service boundary when deep into implementation. Routing rules between
segments in an exam for example - intro, break, timeout, survey are different from routing rules
between test questions.
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20. ADSD Course: Benefits
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21. ADSD Course: Benefits
● Our architects attended the course in NYC.
● Having watched Udi’s presentations over the years the course helped connect a lot of the concepts
we had heard from him.
● We had extracted one bounded context/service from our Cms monolith, the one for Scoring rules.
● We were struggling with Composite UI questions in extracting more boundaries.
● Technical services like ItOps and Branding critical to eliminating data duplication
● Having our architects attend the course also meant that it was easier to bring the rest of the
development team along.
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22. Rolling your own messaging framework
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23. Rolling your own messaging framework
● Our history is we had MSMQ on-prem, then RabbitMq on prem (since .Net core does not support
DTC), then Azure Storage Queues (because it seemed ridiculously inexpensive) and now we are using
Azure Service Bus.
● Easier to switch transport because it sits behind an NServicebus abstraction.
● Messaging design patterns like Sagas/ProcessManager, Outboxing and even behaviors like
ReplyToOriginator are complex and not easy to roll on your own.
● Each transport offers different delivery guarantees and has different limitations, so you are likely going
to write a lot of plumbing code.
● Time spent writing plumbing code = time not spent working on business logic.
● Best to source infrastructure code from the community. You cannot keep up yourself. for ex - even
log4net does not support structured logging yet.
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24. Applying NServiceBus
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25. Sagas: Process Manager
● Compare with Point to Point interaction
● CQRS journey example (Before) -
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26. Sagas: Process Manager
● CQRS journey example (After) -
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27. Sagas: Process Manager Example
● Vendor Import
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28. Sagas: Domain Model Example
● Percent Correct Scoring Policy
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29. Service with Sagas as Aggregates
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30. Sagas: Testability
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31. Sagas: Benefits
● Model Business Process and manage Process State
● Handle out of order messages in the overall operation.
● Engage domain experts for compensating actions
● No need for data retrieval, save, concurrency checks
● Small aggregate boundaries
● Testable
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32. Composite UI
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33. Request Interceptor: Server
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34. Request Interceptor: Server
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35. Request Interceptor: Client
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36. Other Code Samples
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37. Request Interceptor (with Subscription): Server
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38. Service with Functions As ACs
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39. NServiceBus Style Handlers: Client
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