Command Query Separation And How It Helps In Your Daily Job
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The CQRS architecture has been very hot in the past two years. But by now, I think that the majority of the profession agrees that going all the way is only worthwhile for systems with some very specific requirements. Yet, I believe that the separation of reads and writes in an enterprise system has some interesting advantages. In the past year I have applied this principle to an ASP.NET WebForms project that had nothing to do with web services at all, but gave me a lot of benefits. Even my own initiative, the Silverlight Cookbook, is now based on that. I'd like to get a chance to explain you why I use this technique, why it has become part of my default reference architecture (regardless of the technology), and I'll share some of the advantages and disadvantages of that choice.
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Command Query Separation And How It Helps In Your Daily Job
- 1. Command Query Separation And what it brings to your day job Dennis Doomen
- 2. About Me • Principal Consultant • Speaker • 16 years IT experience • Public initiatives • C++ origins but since – Silverlight Cookbook 2001 addicated to C# – C# Coding Guidelines • Specialties – Fluent Assertions – Architecture • Internet – Scrum/XP – www.dennisdoomen.net – ALM – DZone MVB – @ddoomen
- 3. WHAT IS CQS AND WHY SHOULD YOU USE IT?
- 4. “…asking a question should not change the answer…” PROF. BERTRAND MEYER
- 5. CQS IN PRACTICE myObject .UseRepository("Stuff") .WithTransactionSupport() .Retries(3) .LogWarnings() .LogErrors(); var newObject = myObject .UseRepository("Stuff") .WithTransactionSupport() .Retries(3) .LogWarnings() .LogErrors();
- 6. AND ANOTHER EXAMPLE void Push(T t); T Pop(); bool IsEmpty { get; } public interface IStack<T> : IEnumerable<T> { IStack<T> Push(T value); IStack<T> Pop(); T Peek(); bool IsEmpty { get; } }
- 7. SO WHAT’S CQRS THEN?
- 8. “CQRS is a simple pattern that strictly segregates the responsibility of handling command input into an autonomous system from the responsibility of handling side-effect-free query/read access on the same system.” CLEMENS VASTERS
- 9. CQRS = CQS on Architecture Level Front-End Sustains the user No O/R intent conversions Queries Business Actions Projections Commands Query Service Command Service Command Handlers Data Access Layer Domain Model Optimized Repositories for querying Optimized for consistency Changes Relational Query Store Database Query Store Synchronous or asynchronous
- 10. Scaling Opportunities Front-End Querying Querying Command Side Querying Querying Query Store Query Store Relational Query Store Database Query Store
- 11. Myths of CQRS • It is an architecture (style) • It should be used by default • It requires Event Sourcing • It requires eventual consistency • It requires a bus/queues/asynchrony • Commands are fire-and-forget • Solves all concurrency problems • It is easy
- 12. HOW TO BUILD IT?
- 13. Approach 1. Decide if you need it 2. Reconsider if you need it 3. Design your domain using aggregates 4. Add commands 5. Add queries
- 15. Effective Aggregate Design Model true Design invariants in Small consistency Aggregates boundaries Reference Update single other aggregate per aggregates by transaction identity
- 16. How it works Execute query Front-End Send command App Query Processor Command Service Find query handler Find command handler Registry Creates Creates Query Command Find by ID and/or version Handler Handler Invoke method Unit-of- LINQ, HQL, SQL Work Domain Send domain event Aggregate Event Loads Simple Data Root Handler Access Layer Store denormalized data Store normalized data Read DB Write DB
- 17. WebAPI, What to use? WCF, POCO Front-End WCF, App POCO Autofac, Query Unity, Processor StructureMap Command Service Registry Query Command Handler Handler NHibernate Unit-of- EF, Dapper Work Domain Aggregate Event Simple Data Root Handler Access Layer Udi Dahan’s Domain Event Read DB Write DB SQL, Oracle, SQL, Oracle, RavenDB, RavenDB, NoSQL NoSQL
- 19. Reading Material • Effective Aggregate Design http://dddcommunity.org/library/vernon_2011 • Meanwhile…on the command side of my architecture http://www.cuttingedge.it/blogs/steven/pivot/entry.php?id=91 • Meanwhile…on the query side of my architecture http://www.cuttingedge.it/blogs/steven/pivot/entry.php?id=92 • Busting some CQRS myths http://lostechies.com/jimmybogard/2012/08/22/busting-some-cqrs- myths/ • Free MSDN eBook: Exploring CQRS and Event Sourcing http://www.microsoft.com/en-us/download/details.aspx?id=34774
Editor's Notes
- DisadvantagesNo concurrency; conflicting service requests are simply rejectedStaleness ignored;Scalability limited; No domain verbs; domain model does contain concepts from the domain model, just not the business actionsGranularity DTOs; What operations to expose in your WCF service? What relations to include and when? What domain entities to flatten.Unneccesary DTO conversions; Conversions from/to domain model entities, traversing of relations, eager fatchingConflicting demands; query demands denormalized schema, commands require normalized integer schema
- DisadvantagesNo concurrency; conflicting service requests are simply rejectedStaleness ignored;Scalability limited; No domain verbs; domain model does contain concepts from the domain model, just not the business actionsGranularity DTOs; What operations to expose in your WCF service? What relations to include and when? What domain entities to flatten.Unneccesary DTO conversions; Conversions from/to domain model entities, traversing of relations, eager fatchingConflicting demands; query demands denormalized schema, commands require normalized integer schema