DDD / CQRS
Previously... in Software Development
Read / Write
- Forced into 3rd Normal Form
- Fast Writes
- Complex and slow reads
The Light...
Command Query Responsibility Segregation
- Close friend of Command Query Separation
- For systems, not just methods
Example: Profiles
DTO Flow
- User loads a profile from
- User updates the URL field
- User tells server to Update the DTO
- Server performs validation
- Server saves the DTO
- Browser reloads the profile
CQRS Flow
- User loads a profile via query system
- User updates the URL
- UpdatePhotoUrl command sent to command system
- Handler loads the profile
- Handler sets the photo URL
- Handler saves the profile
- Browser reloads the profile via query system
CQRS: Complete
But let's look at some code anyway
The Simple Profile
Old
DTOs
DDD / CQRS
Light query objects
Commands
Transfer
Old
Get
Update
DDD / CQRS
Get (?)
SetPhotoURL
Verbs
Old
As expected
DDD / CQRS
Double
Code Size
Old
Command handler?
DTO?
DDD / CQRS
Domain Object
Business Rules
Old
Fast Reads
Fast Writes
DDD / CQRS
Fast Reads
Fast Writes
Speed
Winner
DTOs
Complex Profiles
Friends complicate everything
Old
Get
Update
DDD / CQRS
Get
SetPhotoUrl
Befriend
Verbs
Old
Getting bigger...
DDD / CQRS
Smaller growth
Code Size
Old
Still unclear...
DDD / CQRS
Domain Object
Business Rules
Old
Slow validation
Fast Writes
Slow Reads
DDD / CQRS
Fast Validation
Fast Writes
? Reads (hint: Fast)
Speed
Old
Difficult
DDD / CQRS
It Just Works
Scale Out
About them reads...
Eventual Consistency
Tailored for Speed
But it's behind...
ORM?
Reconstruction
Testing!
Commands
Events
Exceptions
Given a profile
-
When its URL is set to a shock site
- An exception is thrown
-
When its URL is set to something normal
- PhotoURLChanged event is published
-
When a friend is added
- FriendAdded event is published
-
When the same friend added again
- Exception
-
When a different friend added
- Event published
Internal State
Internal State
There are NO getters!
Behaviour Driven
The Read Model
Concurrency
Partitioning
Next Week(s)
Event Sourcing - Attempt 2
Aggregates
Bounded COntext
?
DDD / CQRS
By xwipeoutx
