Typical architecture

• ORM (NH, EF)
• MS SQL
• DDD (Entity, Repository, Service)
• WebApi CRUD (REST) services

What the problem?

Problems

• You can’t do DDD
  
• Anemic domain model
• No aggregates
• CRUD
• As result Business logic is blur (server, client, dev mind)
  
• Lazy loading
  
• Query optimization is punishment
• One repository method for different reason
• Leads to entity pollution
  
• Data loss due to storing last state
  
• Doesn’t scale
• Very expensive horizontal scale, and limited vertical
  
• Transaction and locks for readers
  

Why do we use

• Experience, it works at least
  
• Devs understand it
• Mature
  
• You can google any problem
  
• Tooling
• Microsoft, Oracle propagation
• They benefit from this architecture!
• There isn't any other option
• or is there?

Free your mind

• Save state vs history
  
• We are lucky, CQRS started to live in .NET
• Greg Young and Jonathan Oliver are fathers
• CQRS completely different approach
• With pros and cons, all about trade off
  
• No silver bullet
• Time for app to lose weight
  

How?

CQRS

• Command Query Responsibility Segregation concept
  
• Pros and cons
• Production proven implementation
• NEventStore (MS SQL, Mongo)
  
• CommonDomain

Concept

Command Query Responsibility Segregation

Concept

Aggregate

• Reply 
• Snapshot
  

Horizontal scale

Advantages #1

• Simplicity due to real DDD
• Aggregate is isolated (logic inside one class)
  
• Has solid public interface for write only (no pollution)
• Simple unit tests
• Fast read
• Thin read layer on top of data store
  
• Read database should be at Web server (MS SQL + Shared Memory protocol = ~50% increase)
  
• Flexible read optimization
  
• For highload projections MS SQL can be replaced with Redis, Mongo without any problem.
• Time-travel
  
• Reproduce bug
  

Advantages #2

• Easy horizontal scaling
• Read side performance linear increases by adding new nodes
• Geo-scaling, due to 100-200ms delay
• On-demand horizontal scaling can be applied
  
• No data loss due to storing all events
• Audit for free
• Analytics on the board
• Sophisticated reporting
  
• Users who created >10 companies per month during 3 month at 2012
• Activity stream
  
• Integration for free due to event sourcing

Disadvantages #1

• Hard to understand at the beginning (eventual vs transactional consistency)
  
• Feature implementation takes more time
• Extra code should be written (command, event, projection)
• Fit for system with <30% writes
• Hot fixes slower, no possibility to run SQL to fix
  
• Write side doesn't scale, performance fixed
• Can be increased by snapshots
  

Disadvantages #2

• Long projection rebuild
• Increases all the time, can be optimized by dev
• Read side is later than write side
• Can be resolved by ManualResetEvent or SignalR
  
• Painful aggregate refactoring
• Event can not be changed (Converting events, compensation events)

CQRS code examples:

• Controller write
• Command
  
• Command handler
• Aggregate
• Event
  
• Projection
• Controller read
• Write database
  

Controller write

Command

Command handler

Aggregate

Event

Projection

Controller read

Write database