Practical introduction to DDD, CQRS and Event Sourcing

Dennis Doomen | @ddoomen | Aviva Solutions | The Continuous Improver

Mortgage Application
Proofing Documents
Loan Officer

…all proofing documents must have been signed before the mortgage
application can be approved

Use a foreign key
constraint to
prevent removing
signatures
Transaction
boundary to
prevent tampering
with proof
concurrently.

No way to remove
a signature.

Aggregate
boundary
Protects the
signatures
Uses the Ubiquitous
Language
Bounded context

…reading should be fast and collaboration is low

HTTP API
Domain
OR/M
RDBMS
Web App
Tables
(Materialized)
Views

HTTP API
Domain
NOSQL
Web App
Documents Indexes
Elastic Search

Command Handlers
Commands
Domain
App
Query HTTP API Command HTTP API
NOSQL
DocumentsIndexes
Elastic Search
Data Access
Data Access
Use Ubiquitous
Language
Captures business
processes
Indexes are
typically eventually
consistent.

…changes have to be audited

Command Handlers
Commands
Domain
App
Query HTTP API Command HTTP API
NOSQL
DocumentsIndexes
Elastic Search
Data Access
Data Access
Audit Trail

…many folks work on the same documents concurrently

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
of testing
Great unit
of testing
Auditability
comes for
free
Can look at
the state in
the past
Can be scaled
independently
Autonomous
projections
Aligns well
with Event
Storming
Forces you to
understand your
domain
thorogoughly
No relational
table
structure
anymore
Can replay old
data against
new rules
More difficult
to envisage
domain
relationships

Dennis Doomen | @ddoomen | Aviva Solutions | The Continuous ImproverDennis Doomen | @ddoomen | Aviva Solutions | 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

…list of proofing documents with state, mortgage number and last
loan officer that signed it.

Event Store
App
Query Store
Data Access
Mortgage Proof
Projector Subscription
Query HTTP API
Mortgage Proof
Projection
Loan Officer
Projector
Loan Officer
Projection
Joins the tables
while querying
Load
Officers
Proofing
Documents

Event Store
App
Query Store
Data Access
Mortgage Proof
Projector Events
Query HTTP API
Mortgage Proof Projection
Processes events
from the mortgage
and loan officer
aggregates
Proofing Document
Projections
Contains state,
mortgage number
and loan officer info
Autonomous
Can be
asynchronous
Can be stale. Now
what?
Can be anything.

…a new version requires a schema change?

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)

Event Store
Projector
Application
Version 2
Owns schema
Version 1
X
Involves down-time until
projections are rebuild

…the terminology changes?

HTTP Command API
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

…there’s a bug in a projection?

…rebuilding a projection is slow?

Event Store
Projector Projector
RDBMS
Subscribe
Subscribe
Document
DB
Projector
RDBMS
Subscribe
Raw SQLNHibernate RavenDB
Autonomous &
independent
Storage technique
optimized for
projection
Use aggressive
caching during
rebuilds.
Caching strategy
optimized for
projector
Stream-by-stream
projections during
rebuilds.

…we have 4 years of data and rebuilding is still too slow?

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.

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

…need to capture the original name of a loan officer while signing?

Event Store
Projector
Subscription
Projection
Subscribes to events
from loan officers
and documents
Loan
Officer
Lookup

Event Store
Projector
Project loan officer events
Subscription
Subscription
Storage
(persistent)
Lookup
Event
Modifier
Modifies events with
lookup data
Receives
modified events

…then ping me at @ddoomen
…or email me at dennis.doomen@avivasolutions.nl

Practical introduction to DDD, CQRS and Event Sourcing

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