Andrea Saltarello shows how to evolve systems architecture leveraging CQRS principles and NServiceBus Sagas.

1. Andrea Saltarello
@andysal74
CEO @ Managed Designs
http://www.manageddesigns.it
Making workflow implementation
easy with CQRS

2. Introduction to ServiceInsight for NServiceBusMaking workflow implementation easy with CQRS
A workflow consists of an orchestrated and
repeatable pattern of business activity enabled by the
systematic organization of resources into processes that
transform materials, provide services, or process
information.[1]
It can be depicted as a sequence of operations,
declared as work of a person or group,[2] an organization
of staff, or one or more simple or complex mechanisms.
[Wikipedia]

3. Introduction to ServiceInsight for NServiceBusMaking workflow implementation easy with CQRS
A simple workflow: Invoicing
Issue
Payment
Ship
NotifyExpires IsPaid Collect
No

4. Introduction to ServiceInsight for NServiceBusMaking workflow implementation easy with CQRS
A simple workflow: Invoicing
Either a user or a scheduler fires code such as:
public void NotifyCustomersAboutPertainingExpiredInvoices()
{
var expiredInvoices = from i in db.OutgoingInvoices
where SomeClausesPointingToStillUnpaidExpiredInvoices
select i;
foreach(var invoice in expiredInvoices)
{
NotifyExpiredInvoiceToCustomer(invoice);
}
}

5. Introduction to ServiceInsight for NServiceBusMaking workflow implementation easy with CQRS
Enter CQRS
Presentation layer
Application layer
Infrastructure layer
Domain Model
Domain layer
Presentation layer
Infrastructure layer
CQRS
Queries
Data
access
Commands
Application
+
Domain

6. Introduction to ServiceInsight for NServiceBusMaking workflow implementation easy with CQRS
Why CQRS?
We faced a lot of complexity in modeling
We thought it was inherent domain complexity
Long story short: a single model caring about all aspects of the domain is
hard. Enter CQS
Command/Query Separation
(cit. Bertrand Meyer – 1980s)
Query
> Returns data
> Doesn’t alter state
Command
> Alter state
> Doesn’t return data

7. Introduction to ServiceInsight for NServiceBusMaking workflow implementation easy with CQRS
CQRS in action
As a business unit manager, I want to notify
customers about expired invoices
Database
.OutgoingInvoices.
.PerBusinessUnit(businessUnitId)
.ExpiredOnly()
.Select(i => new {InvoiceNumber = i.Number, CustomerId =
i.Customer.Id})
.AsParallel()
.ForAll(i => NotifyCustomer(i.InvoiceNumber, i.CustomerId));

8. Introduction to ServiceInsight for NServiceBusMaking workflow implementation easy with CQRS
The result
Pros:
• Easy to read
• Close to Ubiquitous Language
Cons:
• Synchronous (lower scalability)
• Either a scheduling engine or human interaction is
required

9. Introduction to ServiceInsight for NServiceBusMaking workflow implementation easy with CQRS
Commands: a different strategy
1. Application sends a command to the system
2. Commands are dispatched to workflow managers
(a.k.a. Sagas) which will execute them and then state
success/failure accordingly
3. Responses are notified to interested subscribers (a.k.a.
handlers) such as Denormalizers, which will
(eventually) update the read model’s database
Note: command/responses dispatch will be managed by a
Mediator

10. Introduction to ServiceInsight for NServiceBusMaking workflow implementation easy with CQRS
CQRS in action, part 2
As a business unit manager, I want to notify
customers about expired invoices
Database
.OutgoingInvoices.
.PerBusinessUnit(businessUnitId)
.ExpiredOnly()
.Select(i => new {InvoiceNumber = i.Number, CustomerId = i.Customer.Id})
.AsParallel()
.ForAll(i => mediator.Send(new NotifyPaymentDueCommand(i.InvoiceNumber,
i.CustomerId)));

11. Introduction to ServiceInsight for NServiceBusMaking workflow implementation easy with CQRS
The result, part 2
Pros:
• Easy to read
• Close to Ubiquitous Language
• Asynchronous
Cons:
• Either a scheduling engine or human interaction is
required

12. Introduction to ServiceInsight for NServiceBusMaking workflow implementation easy with CQRS
Queues or buses? That is the question (semi cit. )
Although both categories are viable options and support
common features (e.g.: message durability), a bus is generally
preferable due to advanced capabilities (e.g.: message
scheduling)
Mediator, who art thou?

13. Introduction to ServiceInsight for NServiceBusMaking workflow implementation easy with CQRS
Sagas 1-2-3
In a messaging system, a saga orchestrate a set of
messages. The main benefit of using a saga is that it
allows us to manage the interaction in a stateful manner
(easy to think and reason about) while actually working in a
distributed and asynchronous environment.
[Ayende]
Though Saga are not proper workflow/process manager,
they are effective at pretending to be 

14. Sagas in action

15. Introduction to ServiceInsight for NServiceBusMaking workflow implementation easy with CQRS
More resources:
• “Microsoft .NET: Architecting Applications for the Enterprise”
by Andrea Saltarello and Dino Esposito, Microsoft Press (2014)
• Merp (http://naa4e.codeplex.com)
Links

16. Thank you