Introduction to CQRS (Command Query Responsibility Segregation)

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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?