Over the years I've spoken many times about what Event Sourcing is and shared many of the good, the bad and the ugly parts of it in blog posts and various talks. However, I've never talked about how to actually build a system based on this architecture style. I keep getting the same questions over and over again. Like when to apply Event Sourcing and at what architectural level. How to deal with transactional boundaries within and outside the domain. How to build projections that are autonomous, reliable and self-supporting. How to deal with upgrades and blue-green deployments. But also on how to handle bugs, design mistakes and crashing projections. Having made a lot of these mistakes myself over these years, it's time to share my current thoughts and opinions about this. Since the .NET space has a pretty rich set of open-source projections to support this, the examples and code will be .NET. But the concepts are universal, so don't let that scare you off.
14. 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
Dennis Doomen | @ddoomen | The Continuous Improver
NES
Aggregates.NET
SimpleDomain
NStore
EventStore
NEventStore (*)
SQLStreamStore (*)
NStore
NES
Marten
Handelier
Brighter
MediatR (*)
Projac
LiquidProjections (*)
EventStore
15. 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 StoreProjections
Projectors
Uses Event
Sourcing
Uses traditional
CRUD
architecture
Indexing-based
architecture
Subcribe
to
webhooks
Coarse-
grained
HTTP
requests.
Bus
Subscribe
Publish coarse-
grained event
16. 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
17. 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
18. 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
19. 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
27. Document #1
Created Event
Event Store
Graph
Projector
Document #1
Closed Event
(other events)
Projector with
active
projections
Archiving
Projector
Start archiving
Document #1
Marked As
Archivable Event
Mark all events
as archivable
Tracks dependencies
between documents
Deletes
projections
related to
Document #1
Can skip all
archivable events
during next
rebuild.
Dennis Doomen | @ddoomen | The Continuous Improver
28. Event Store
Lucene
Projector
Document #1
Marked As
Archivable Event
Allows projectors
to clean up
Lucene Index
Take snapshot
Purge events
Tombstone
$tombstone
stream
Application
Document
Projector
Search
Tracks deleted streams
for future references
Stream Tombstoned
Event
Search
Dennis Doomen | @ddoomen | The Continuous Improver