Idiomatic Domain Driven Design: implementing CQRS
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Applying DDD and CQRS can not only make the resulting design of our system simpler and more effective but, freeing us from the burdenof the one model fits all approach, also allows architects to adopt different strategies when it comes to business logic modeling. Though lot has been written about DDD and CQRS, missing working code publicly available seems to be the elephant in the room: in this talk, well find out how to implement the Command side of the Force by means of a proper Domain Model and getting to the point of switching from an entity based modeling to an event based one.
![Command Query Responsibility Segregation
A single model cannot be appropriate for
reporting, searching and transactional
behaviours
[Greg Young]](https://image.slidesharecdn.com/cqrs-141103163700-conversion-gate01/85/Idiomatic-Domain-Driven-Design-implementing-CQRS-13-320.jpg)
![CQRS feat. Transaction Script
Transaction Script
[P of EAA, 110]
Organizes business logic by procedures where each
procedure handles a single request from the
presentation.
Commands are 束modeled損 as functions, each one
representing one action that the system is capable of](https://image.slidesharecdn.com/cqrs-141103163700-conversion-gate01/85/Idiomatic-Domain-Driven-Design-implementing-CQRS-23-320.jpg)
![Next stop: Events. See you tomorrow
It really became clear to me in the last couple of years
that we need a new building block and that is the
Domain Event.
[Eric Evans]
An event is something that has happened in the past.
[Greg Young]
A domain event captures the memory of something
interesting which affects the domain
[Martin Fowler]](https://image.slidesharecdn.com/cqrs-141103163700-conversion-gate01/85/Idiomatic-Domain-Driven-Design-implementing-CQRS-25-320.jpg)
![Bibliography
[DDD] Domain Driven Design, Eric Evans , Addison-Wesley
[NAAE] Microsoft .NET: Architecting Applications for the
Enterprise (2属 ed.), Andrea Saltarello & Dino Esposito,
Microsoft Press
[MERP] Merp, https://naa4e.codeplex.com/](https://image.slidesharecdn.com/cqrs-141103163700-conversion-gate01/85/Idiomatic-Domain-Driven-Design-implementing-CQRS-26-320.jpg)
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Idiomatic Domain Driven Design: implementing CQRS
- 2. Idiomatic Domain Driven Design: implementing CQRS Andrea Saltarello CEO @ managed/designs andysal@gmail.com http://twitter.com/andysal74 http://slideshare.net/andysal
- 3. Who I am 1. CEO @ Managed Designs, striving to discover the perfect 束sustainable software development process損 Customer has to be satisfied and pay Supplier has to have a reasonable markup Team members should be satisfied with their work 2. Founder and leader of UGIdotNET (first Italian .NET User Group): I need to share experiences peer to peer 3. (Co)Author (together with Dino) of .NET: Architecting Applications for the Enterprise, by Microsoft Press 4. Basically, a software developer
- 4. Whats DDD? Domain Driven Design: Is not a methodology Is not an architecture Is not a silver bullet Is an approach aimed to tackle the complexity of software development; its main assets are: Bounded Context Ubiquitous Language Aggregates (Events)
- 5. Ubiquitous Language Its the language spoken by all the people that work on a project; the UL spans from the documentation to the source code. Basically, technicians should adopt the language spoken by the business in order to reduce friction
- 6. Bounded Context A Bounded Context is a bounduary whithin which a specific ubiquitous language applies A system is a composition (Map) of (bounded) contexts (e.g.: web store, accountability, delivery&shipment ), speaking one to each other by means of some kind of API. Due to the decoupling of contexts, we can use an ad hoc architecture for every context
- 7. Aggregates Aggregates are atomic clusters of objects that exist and interact inside a specific BC: every Aggregate acts as the (both) data and behaviour composition of an important concept which exists within the BC itself. Basically, a DDD oriented system does coordinate the work of aggregates The whole set of aggregates existing within a specific BC is the BCs Domain Model. Should a specific behaviour exceed an Aggregates bonduary, it will be implemented as a (Domain) Service Model + Services == (Domain) Layer
- 8. Andrea vs. DDD, A.D. 2004 Got it: a system is a composition Map of Bounded Contexts, each one sporting a Domain Layer created after an Ubiquitous Language. Awesome! Super Cool! Now How the hell can we translate this in code?
- 9. 2004: a Domain (Model) Odissey
- 10. DEMO Domain Model (Voucher, NSK)
- 11. Is Domain Model broken? Domain Model is (almost) ok, but to follow the Blue Book style is pretty hard and requires a lot of tradeoffs: is there a different way to have the same approach applied?
- 12. DDD has twodistinct parts. You always need one and can sometimes happily ignore the other. Analytical Strategic Valuable to everybody and every project Blue books Domain Model is just one supporting architecture, though the one being originally recommended
- 13. Command Query Responsibility Segregation A single model cannot be appropriate for reporting, searching and transactional behaviours [Greg Young]
- 14. CQRS at a glance Query Command
- 15. CQRS in a nutshell 束Doing CQRS損 means separating the stack that reads data (query) from the one that can alter the systems state (command), each one having its own ad hoc 束model損 All the actions that the system is capable of are expressed as Commands. To sum it up: 1. A user An actor ask the system to execute a command 2. Command execution alters the systems state 3. The read model is used in order to query for (updated) data
- 16. Whats a 束MODEL損, anyway? Were not trying to have a model which faithfully represents the world, but one that works within a bounded context .Net DeveloperDays 2014
- 17. Mercator projection (1569) .Net DeveloperDays 2014 Thats the world map were used to and Its wrong, but it actually works (from a certain point of view)
- 18. Galls-Peters projection (1885 1973) .Net DeveloperDays 2014
- 19. The Query side of the Force The 束Read Model損 can be served in different flavours, such as: Plain SQL (Micro) O/RM Services (xml, json, DTOs, ) LET (Layered Expression Trees)
- 20. Layered Expression Trees (LET idiom) As a business unit manager, I want to collect credits due to unpaid outgoing invoices #ubiquitouslanguage #nuffsaid Database.OutgoingInvoices. .PerBusinessUnit(businessUnitId) .ExpiredOnly() .Select(i => new {InvoiceNumber = i.Number, CustomerId = i.Customer.Id}) .AsParallel() .ForAll(i => bus.Send(new CollectDebtCommand(i.InvoiceNumber, i.CustomerId)));
- 22. The Command side of the Force Sorry, non silver bullet available for implementing commands, too. Common choices: Transaction Script Event Sourcing (we will talk about this tomorrow)
- 23. CQRS feat. Transaction Script Transaction Script [P of EAA, 110] Organizes business logic by procedures where each procedure handles a single request from the presentation. Commands are 束modeled損 as functions, each one representing one action that the system is capable of
- 24. DEMO Transaction Script feat. CQRS
- 25. Next stop: Events. See you tomorrow It really became clear to me in the last couple of years that we need a new building block and that is the Domain Event. [Eric Evans] An event is something that has happened in the past. [Greg Young] A domain event captures the memory of something interesting which affects the domain [Martin Fowler]
- 26. Bibliography [DDD] Domain Driven Design, Eric Evans , Addison-Wesley [NAAE] Microsoft .NET: Architecting Applications for the Enterprise (2属 ed.), Andrea Saltarello & Dino Esposito, Microsoft Press [MERP] Merp, https://naa4e.codeplex.com/