Event Sourcing is becoming more mainstream these days, and a lot of conferences have demonstrated the pros and cons of this architecture style from multiple angles. I've done a few of these and publishes a lot of articles on best practices and solutions to common problems. But what nobody did was to show you how to build an event sourced systems in the .NET ecosystem. There are a lot of open-source projects that you can use, so you'll need to find a right mix and your own code to that. So let me show you how I would implement this in .NET with almost 10 years of Event Sourcing experience behind my belt.

1. With examples from .NET
Design Patterns for implementing Event
Sourcing
Dennis Doomen
@ddoomen | The Continuous Improver

2. About Me
Hands-on architect in the .NET space with 26 years of experience on
an everlasting quest for knowledge to build the right software the right
way at the right time
@ddoomen | The Continuous Improver

4. Event Sourcing? What and why
Dennis Doomen | @ddoomen | The Continuous Improver

5. Command Handlers
Commands
Domain Model
Event Store
Events
App
Query Store
Data Access
Projectors
Events
Query HTTP API
Projections
Events
Command HTTP API
Great unit
for testing
Great unit
for testing
Auditability
comes for free
Can look at the
state in the
past
Can be scaled
independently
Aligns well with
Event Storming
Forces you to
understand your
domain thoroughly
No relational table
structure needed
anymore.
Can replay old data
against new rules
More difficult to
envisage domain
relationships
Great for
replication
Why? And why not?
Dennis Doomen | @ddoomen | The Continuous Improver
Replay
production
scenarios
Need to deal with
versioning issues
Bugs are sometimes
non-trivial
Rebuilding can take
time
Can be a source of
bugs for the
inexperienced
May have to deal
with eventual
consistency
May require end-
to-end tests to
cover everything
Lot of devs perceive it
as complex.
Supports fine-
grained
merging

6. Dennis Doomen | @ddoomen | The Continuous Improver
Application
Domain
NoSQL / RDBMS
OR/M / DAL
Web UI, HTTP API, etc
Lucene Index
Document Projector
Web UI, HTTP API, etc
Web UI, HTTP API, etc
Domain
Commands
Events
Event Store
Projections
Projectors
Uses Event
Sourcing
Uses traditional
CRUD architecture
Indexing-based
architecture
Subcribe
to
webhooks
Coarse-
grained
HTTP
requests.
Bus
Subscribe
Publish coarse-
grained event
When? And when not?

7. How?
Dennis Doomen | @ddoomen | The Continuous Improver

8. Application
Command
Service
Correct Customer
Email Handler
Customer
Unit of
Work
Projector
Data Access
Layer
Read Database
Write
Database
CorrectCustomerEmailCommand
HTTP API / In-process invocation
Get<Customer>(identity)
Correct Email
Event Store
Load(events)
Apply
Get changes
CustomerEmailCorrectedEvent
Submit changes
History
Aggregates.NET
SimpleDomain
Nstore
Eventuous
MartenDB (PostgreSQL)
Eventuous
Nstore
NEventStore (*)
SqlStreamStore
Equinox
Brighter
MediatR (*)
Eventuous
Propulsion
LiquidProjections (*)
Event Sourcing in .NET
Dennis Doomen | @ddoomen | The Continuous Improver
Any RDBMS
Any NoSQL
EventStoreDB

9. Aggregate (Custom)
Dennis Doomen | @ddoomen | The Continuous Improver

10. Event Source base (Custom)
Dennis Doomen | @ddoomen | The Continuous Improver

11. Aggregate State (Custom)
Dennis Doomen | @ddoomen | The Continuous Improver

12. Projector (Custom)

13. Projector (Custom)
See also https://github.com/liquidprojections/LiquidProjections/blob/master/Samples/ExampleHost/CountsProjector.cs#L17

14. Aggregate (Eventuous)
See also https://github.com/liquidprojections/LiquidProjections/blob/master/Samples/ExampleHost/CountsProjector.cs#L17

15. Event Source (NEventStore)
See also https://github.com/liquidprojections/LiquidProjections/blob/master/Samples/ExampleHost/CountsProjector.cs#L17

16. Best practices
Dennis Doomen | @ddoomen | The Continuous Improver

17. Event Store
Projector Projector
RDBMS
Subscribe
Subscribe
Document DB
Projector
RDBMS
Subscribe
Raw SQL
NHibernate RavenDB
Lookup
Autonomous &
independent
Storage technique
optimized for projection
No joining of projections
Avoid reusing the
projections for multiple
purposes
Use aggressive caching
during rebuilds.
Caching strategy
optimized for projector
Owned by projector
Dennis Doomen | @ddoomen | The Continuous Improver
Stream-by-stream
projections during
rebuilds
About well-factored
projections

18. Customer #123
Correct Customer Shipping
Address Command
Customer Shipping
Address Corrected Event
Event Store
Treat as a
message, not a
type
Order #456
Correct Shipping Address
Command Handler
Application
Customer Shipping Address
Corrected Handler
Order Redirected Event.
Identity != natural
key
Identify
partition key
Primitive types only
Avoid terms like
Create, Update,
Delete, Change.
Avoid property
change events
Don’t use as inter-
domain contracts
Don’t expose
outside the domain
Dennis Doomen | @ddoomen | The Continuous Improver
Inside the domain

19. Customer #123
Correct Customer Shipping
Address Command
Customer Shipping
Address Corrected Event
Event Store
Order #456
Correct Shipping Address
Command Handler
Application
Order Redirected Event
Dennis Doomen | @ddoomen | The Continuous Improver
Transaction Boundaries

20. Command Service
Some Command Handler
Customer #123
Event Store
Customer Created Event
Get<Customer>(“123”)
Customer Created Event
Converter
Customer Enrolled Event
May split or merge
events
Can also run as part
of migration
May change the
identity
Unaffected Events
Dennis Doomen | @ddoomen | The Continuous Improver
Versioning

21. Dealing with schema changes
Dennis Doomen | @ddoomen | The Continuous Improver

22. • Fix a code bug in the projection
• Restructure the projection for performance reasons
• New features require changes to projections.
Dennis Doomen | @ddoomen | The Continuous Improver
Reasons for reprojections

23. Event Store
Projector
Application
Version 2
Version 1
X Involves down-time until
the projection has been
rebuild.
Reads data from old
database
Dennis Doomen | @ddoomen | The Continuous Improver
In-place upgrades

24. Event Store
Projector
Application Application
Network Load Balancer
Event Store
Version 1 Version 2
events
Projection
Projector
Projection
bring off-line
Returns HTTP 503 (Service
Unavailable)
Returns HTTP 503 (Service
Unavailable)
Dennis Doomen | @ddoomen | The Continuous Improver
Requires full
rebuild of the
projections
Practically no
downtime.
Out-of-place / Blue-Green
upgrades

25. Q&A
…ping me at @ddoomen
…email me at dennis.doomen@avivasolutions.nl

26. Resources
Event Sourcing: The Good, The Bad and The Ugly
Liquid Projections
Eventuous