This document introduces the Command Query Responsibility Segregation (CQRS) pattern. CQRS is based on the principle of separating commands and queries. It splits the conceptual model into separate models for commands and queries. This allows applying different optimization strategies for reads versus writes. For example, queries can use denormalized data while commands update a normalized database. The read model can be populated asynchronously from the write model and stored in different data stores optimized for reads. CQRS helps handle complex domains and improves scalability.

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Introduction to CQRS
Oleksandr Loktyev

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Command-query separation (CQS)
Introduction to CQRS
A method should either change the state of an object or return a result, but not both.
1. Queries. Return results.
2. Commands. Change state.
Pros
• Method intents are well understood
• No side effects

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Command query responsibility segregation (CQRS)
Introduction to CQRS
• A pattern based on CQS principle.
• Conceptual model is split into separate models depending whether it is a
Command or Query.

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CustomerService
Customer GetCustomer(CustomerId)
CustomerSet GetCustomersWithName(Name)
void CreateCustomer(Customer)
void EditCustomerDetails(CustomerDetails)
Applying CQRS on this would result in two services:
CustomerServiceCommands
void CreateCustomer(Customer)
void EditCustomerDetails(CustomerDetails)
CustomerServiceQueries
Customer GetCustomer(CustomerId)
CustomerSet GetCustomersWithName(Name)

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And that’s it!
There is nothing more to it than that…

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Typical web architecture
Introduction to CQRS

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Domain objects are natural for CrUD operations
Introduction to CQRS
public void Undismiss(DismissedObjectDTO dto)
{
var dismissedObject = _uow.DismissedObjectRepository.Get (dto.ObjectId, dto.ObjectTypeId);
dismissedObject.EndDate = DateTime.Now;
_uow.DismissedObjectRepository.Save(dismissedObject);
if (dismissedObject.ObjectType.ID == (int) Enums.CoachingObjectType.Observation)
{
var reviewDetail = _uow.EventReviewDetailDAO.Get(dto.ObjectId);
erd.ObservationScore = 100;
_uow.EventRepository.Save(erd);
}
_uow.CommitChanges();
}

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But can be extremely inefficient for reads
Introduction to CQRS
Lazy loading:
public Company GetCameraCompany(Camera camera)
{
return camera.Vehicle.Driver.Company;
}

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One more example
Introduction to CQRS
Incorrect fetching strategy.
private static void SomeOperation(List<Customer> customers)
{
foreach (var customer in customers)
{
foreach (var order in customer.Orders)
{
// do something
}
}
}

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Something that is more natural for Read operation
Introduction to CQRS
Plain data instead of Domain Model.
private static void SomeOperation(List<Customer> customers)
{
var customerOrderDTO = _uow.customerRepository.GetCustomerOrders(customers);
foreach (var order in customerOrderDTO .Orders)
{
// do something
}
}

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Queries
Introduction to CQRS
• What’s the purpose of queries? To show data. Not objects.
• So why should the data from the database come across 5 layers through 3 model
transformations? It’s a bit overkill to display data.
• Why not just this: The UI read data from the database and displays it?

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Consequence 1
Commands and queries use the same data but they should not necessarily
use the same data model

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CQRS way
Introduction to CQRS
It is natural to use a different model to update information than the model you use to
read information.

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Simple query for grid
Introduction to CQRS
SELECT
c.Name, o.Name, ot.Type
FROM Customer c
INNER JOIN CustomerOrder co ON co.CustomerId = c.CustomerId
INNER JOIN Order o ON co.OrderId = c.OrderId
INNER JOIN OrderType ot ON o.OrderId = ot.OrderId
WHERE
o.CustomerId = 20
AND ot.Type = ‘Active’

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What if the query is slow?
Introduction to CQRS
1. Execution plan and query optimization.
2. Add indexes.
3. Move to stored procedure.
4. Completely remove ORM.
The fact is that some queries simply cannot be quick.

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Let’s add denormalization
Introduction to CQRS
SELECT
CustomerName, OrderName, OrderType
FROM CustomerOrderView co
WHERE
CustomerId = 20
AND OrderType = ‘Active’

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Consequence 2
Database can contain denormalized data for reads

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CQRS way
Introduction to CQRS
Queries tend to use denormalized data rather then classical database approach.

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Let’s go further and separate storages
Introduction to CQRS

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Read storage can be actually anything you want
Introduction to CQRS

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How to populate read storage? Synchronous way.
Introduction to CQRS

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How to populate read storage? Async way.
Introduction to CQRS

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CQRS is …
Introduction to CQRS
• CQRS is a small tactical pattern
• CQRS is learnable in 5 minutes
• CQRS can open many doors

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Benefits
• Handles domain complexity
• Applies different optimization strategies for reads and writes
• Scalability
When to use
• Complex domains
• High performance applications
• Only on specific portions of a system (Bounded Context in DDD)

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Questions?