ImplementingEvent Sourcing in .NET

Andrea SaltarelloCEO @ managed/designs

andysal@gmail.comhttp://twitter.com/andysal74http://slideshare.net/andysal

Who I am

1. CEO @ Managed Designs, striving to discover the perfect«sustainable software development process»

• Customer has to be satisfied and pay

• Supplier has to have a reasonable markup

• Team members should be satisfied with their work

2. Founder and leader of UGIdotNET (first Italian .NET User Group): I need to share experiences in a peer to peer way

3. (Co)Author (together with Dino) of .NET: ArchitectingApplications for the Enterprise, by Microsoft Press

4. Basically, a software developer

CQRS recap

Query Command

DEMO

CQRS recap

It really became clear to me in the last couple of years that we need a new building block and that is the Domain Event.

[Eric Evans]

An event is something that has happened in the past.

[Greg Young]

A domain event … captures the memory of something interesting which affects the domain

[Martin Fowler]

Why events?

«Undo principle»: Instead of focusing on a system’s last known state, we could note down every occurring event: this way, we would be able build the state the system was in at any point in time

To cut a long story short: we end up recording an event stream

JobOrderCreated InvoiceIssuedJobOrderExtended JobOrderClosed

Event Stream vs. «My application»

Still, my users are more interested in knowing a job order’s balance or whether an invoice hasbeen paid.

Ways to achieve that:

• Rebuilding the state traversing the wholeevent stream (slow, but snapshots can help)

• CQRS to the rescue: let’s have a database dedicated to store the last known state and have it read by the read model

CQRS/ES in a nutshell

1. Application sends a command to the system

2. Command execution alters the system’s state and raises events to state success/failure

3. Events are notified to interested subscribers, suchas:

• Workflow managers (a.k.a. «Sagas») which can execute more commands

• Denormalizers, which will update the readmodel

Note: command/event dispatch will usually be managed by a Mediator («bus»)

Read

Model

Application

Layer

Domain

layer

Event

store

QUERY

COMMAND

Bu

s

EVENT Handler

Handler

Handler

Snapshot

DB

CQRS/ES at a glance

DEMO

Merp

On a technology side…

Looking for a bus and an event store: build or buy?

• Event Store:

–SQLWhatever

–MongoDB

–RavenDB

• Bus:

–NEventStore

–NServiceBus

My advice? Buy#donoteverthinkaboutmakingyourowntoolkit

The “Buy” Side of the Force

SQLWhatever MongoDB RavenDB

• Everyone’s skill• Every tool you can

need does exist• Both On-Premise

& cloud

• Free• Runs on both

Linux and Windows

• Both On-Premise & cloud

• Schemaless

• LINQ support• TX+DTC support• Schemaless• Both On-Premise

& cloud

MSMQ NEventStore NServiceBus

• ROTFL • Free• FOSS• DDD/CQRS/ES full

stack

• Advances configoptions

• Advancednotificationsfeatures

DEMO

NServiceBus

Bibliography

[DDD] Domain Driven Design, Eric Evans , Addison-Wesley

What I've learned about DDD since the book, Eric Evans, InfoQ

[NAAE] Microsoft .NET: Architecting Applications for the Enterprise (2° ed.), Andrea Saltarello & Dino Esposito, Microsoft Press

[MERP] Merp, https://naa4e.codeplex.com/