CQRS way

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
• 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 Concept

Command Query Responsibility Segregation

Concept

Aggregate

• Reply 
• Snapshot
  

Horizontal scale

Advantages

• Simplicity due to real DDD
• Fast read (Thin read layer or SQLite, BerkeleyDB)
  
• Flexible read optimization (MS SQL->Redis,ElasticSearch)
  
• Time-travel (Reproduce bug)
• Easy horizontal scaling
• Read side scaling without limitation
  
• Write side is scaled  by sharding  ToMD5(id) % 10 + 1 // 1..10
• Across data centers, node on demand
  
• No data loss due to storing all events
• Audit, big data ready (forecast, fraud detection, debug)
  
• Sophisticated reporting
  
• Users who created >10 companies per month during 3 month at 2012; Activity stream
  
• Integration for free due to event sourcing

Disadvantages

• 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
  
• Long projection rebuild (in-mem, background, rebuild changed)
  
• Read side lag (due to eventual consistency)
  
• Painful aggregate refactoring
  
  

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