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CQRS by Mind Map: CQRS
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A command is a simple object with a name of an operation and the data required to perform that operation.

Example: RelocateCustomerCommand

Is immutable (when using Messaging Pattern)

Is a method call (When not using the Messaging Pattern)

Execution may fail (with an exception)

QUESTION: Should value objects be used in commands?

Command Handler

Handles one or more commands

May refuse to execute the command by throwing an exception


Called Saga by Greg Young

Term "Saga" introduced by Hector Garcia-Molina and Kenneth Salem in 1987

Listens to events

Sends Commands & Events

Business Process Model and Notation (BPMN)

Saga vs. State Machine vs. Workflow (Kelly Sommers)


Is immutable

Named with a verb in the past tense

Example: CustomerRelocatedEvent

Never fails (as it already happened)

Related concepts

Messaging Pattern

Event Sourcing

storing current state as a series of events

Rebuild state by replaying series of events

Snapshots for better performance

Design by contract (DBC)

Eventual Consistency

CAP theorem

Task Based UI

Behavior-driven development (BDD)

"Given-When-Then" style tests

Natural fit for CQRS/ Event Sourcing

Introducing BDD (Dan North)

Fast Command Bus

The LMAX Architecture (Martin Fowler)


Processing 1M commands per second with Axonframework

Based on "Command Query Separation" (CQS) idea from Betrand Meyer


Listens for events

Creates and manages views

Projects incoming events to a view


A representation of the system for a special purpose

Could also be another model like an UML diagram, a Java class, Excel file, XML file or a chart


Is based on a view


Similar to a function (See DDD Map)