Architecturing Architectures with NServiceBus

NServiceBus

Lightweight framework for designing distributed systems in .NET
An Enterprise Service Bus
Helps to make distributed systems more:
- Scalable
- Reliable
- Extensible
Services and support provided by Particular
Open Source but not free
Binaries available through Nuget

Website

OrderService

PlaceOrder(order)

Request Order

Response OrderId

Synchronous

The website is blocked while calling the Order Service

Remote Procedure Call

Website

Message Queue

Message: PlaceOrder

Asynchronous

The website is NOT blocked while the place order is processed

Message oriented architecture

Windows Service

Process the PlaceOrder message

Connect

Receive

PlaceOrder

Messages in NServiceBus

Just simple .NET classes with properties (POCO)
2 Types of messages
- Commands
  - Do Something: PlaceOrder
  - Sent from one (or more) senders, processed by one endpoint
  - Implement ICommand
- Event
  - Something has happened: OrderWasPlaced
  - Published from one sender, processed by several endpoints/subscribers
  - Implement IEvent

Queue(eueueueu)s

FIFO datastructure for storing and retrieving messages
Default queue in NServiceBus is MSMQ
Support for
- ActiveMQ
- RabbitMQ
- SQL Server
- Windows Azure Queues
- Windows Azure ServiceBus
One endpoint has one queue, send-only endpoints do not have queues

Using NServiceBus to integrate systems

CQRS

Command and Query Responsibility Segregation

CQS

"Command Query Separation" describes the principle that an object's methods should be either commands or queries. A query returns data and does not alter the state of the object; a command changes the state of an object but does not return any data. The benefit is that you have a better understanding what does, and what does not, change the state in your system.

CQRS

CQRS is simply the creation of two objects where there was previously only one. The separation occurs based upon whether the methods are a command or a query. A command is any method that mutates state and a query is any method that returns a value.

-Greg Young, CQRS, Task Based UIs, Event Sourcing agh!

CustomerService

void MakeCustomerPreferred(CustomerId)
Customer GetCustomer(CustomerId)
CustomerSet GetCustomersWithName(Name)
CustomerSet GetPreferredCustomers()
void ChangeCustomerLocale(CustomerId, NewLocale)
void CreateCustomer(Customer)
void EditCustomerDetails(CustomerDetails)

CustomerWriteService

void MakeCustomerPreferred(CustomerId)
void ChangeCustomerLocale(CustomerId, NewLocale)
void CreateCustomer(Customer)
void EditCustomerDetails(CustomerDetails)

CustomerReadService

Customer GetCustomer(CustomerId)
CustomerSet GetCustomersWithName(Name)
CustomerSet GetPreferredCustomers()

CQRS

CQRS is a simple pattern that strictly segregates the responsibility of handling command input into an autonomous system from the responsibility of handling side-effect-free query/read access on the same system. Consequently, the decoupling allows for any number of homogeneous or heterogeneous query/read modules to be paired with a command processor.

In simple terms, you don't service queries via the same module of a service that you process commands through. In REST terminology, GET requests wire up to a different thing from what PUT, POST, and DELETE requests wire up to.
—Clemens Vasters (CQRS Advisors Mail List)