The document discusses event sourcing, a method where application state changes are stored as a sequence of immutable events, enabling features like audit trails and temporal queries. It covers the integration of Reactor, a library for creating reactive applications, with Spring StateMachine to handle complex state management efficiently. Additionally, it highlights practical examples and code snippets demonstrating the implementation of event sourcing and state machines in applications.

1. #JCConf
Event Sourcing with Reactor
and Spring Statemachine
Jimmy Lu
jilu@digitalriver.com
Digital River, Inc.

2. Agenda
• Event Sourcing
• Reactor
• Spring Statemahcine
• Demo of Event Sourcing with Reactor
and Spring Statemachine
• Summary

3. Event Sourcing
• All changes to application state are
stored as a sequence of events
• Not only manage current state
• Replays sequence of events to
reconstruct application state
• Natively support audit trail
• Eventually consistent
• Commonly works with CQRS pattern

4. https://msdn.microsoft.com/en-us/library/dn589792.aspx

5. Event Store
• Append-only store (events are
immutable)
• Series immutable events
• compensating event
• Acts as the source of truth of
materialised view
• Simple schema for storing event
• Behaves like a database with message
broker characteristics

6. Event Store
• Better work with version data format
• Consider creating snapshots at specific
intervals if event stream is too large
• Consumers of the events must be
idempotent
• Event ID typically maps to individual
entities
• The order of the event is important

7. Event store
https://geteventstore.com/

8. Benefits of Event Sourcing
• A solution to ensure atomicity when
mutating entity states while publishing
events simultaneously
• 100% accurate audit logging which is not
an afterthought
• Easy temporal queries
• Single event centric model
• Conflict management
• Simplified/Better testing

9. A Taste of Event Sourcing
• // Akka Persistence
• public void onReceiveRecover(Object msg) {
• if (msg instanceof Evt) {
• state.update((Evt) msg);
• } else if (msg instanceof SnapshotOffer) {
• state = (ExampleState)((SnapshotOffer)msg).snapshot();
• } else {
• unhandled(msg);
• }
• }
http://doc.akka.io/docs/akka/snapshot/java/persistence.html#Event_sourcing

10. • // Akka Persistence
• public void onReceiveCommand(Object msg) {
• if (msg instanceof Cmd) {
• final String data = ((Cmd)msg).getData();
• final Evt evt1 = new Evt(data + "-" + getNumEvents());
• final Evt evt2 = new Evt(data + "-" + (getNumEvents() + 1));
• persistAll(asList(evt1, evt2), new Procedure<Evt>() {
• public void apply(Evt evt) throws Exception {
• state.update(evt);
• if (evt.equals(evt2)) {
• getContext().system().eventStream().publish(evt);
• }
• }
• });
• } else if (msg.equals("snap")) {
• saveSnapshot(state.copy());
• }
• ......
• }
http://doc.akka.io/docs/akka/snapshot/java/persistence.html#Event_sourcing

11. • public class Account {
• ......
•
• public List<Event> process(OpenAccountCommand cmd) {
• return EventUtil.events(new AccountOpenedEvent(cmd.getInitialBalance()));
• }
•
• public List<Event> process(CreditAccountCommand cmd) {
• return EventUtil.events(new AccountCreditedEvent(cmd.getAmount(), cmd.getTransactio
nId()));
• }
•
• public void apply(AccountOpenedEvent event) {
• balance = event.getInitialBalance();
• }
•
• public void apply(AccountDebitedEvent event) {
• balance = balance.subtract(event.getAmount());
• }
•
• public void apply(AccountCreditedEvent event) {
• balance = balance.add(event.getAmount());
• }
• }
https://github.com/cer/event-sourcing-examples

12. Reactor
• A foundational library for building reactive
fast-data applications on the JVM
• An implementation of the Reactive Streams
Specification
• Building on top of the Disruptor RingBuffer
• Functional and reactive to allow for easy
composition of operations
• 10’s of millions of operations per second
(even up to 100’s if you have enough
hardware horsepower)

13. Snippet of Reactor
• static {
• // Only done once, statically, and shared across this classloader
• Environment.initialize();
• }
• // Create a Stream subclass we can sink values into
• Broadcaster<String> b = Broadcaster.create();
• b
• // dispatch onto a Thread other than 'main'
• .dispatchOn(Environment.cachedDispatcher())
• // transform input to UC
• .map(String::toUpperCase)
• // only let certain values pass through
• .filter(s -> s.startsWith("HELLO"))
• // produce demand
• .consume(s -> System.out.println(Thread.currentThread() + ": " + s));
• // Sink values into this Broadcaster
• b.onNext("Hello World!");
• // This won't print
• b.onNext("Goodbye World!");
• // Must wait for tasks in other threads to complete
• Thread.sleep(500);
http://projectreactor.io/

14. Reactor Modules
• reactor-core
• reactor-stream
• reactor-bus
• reactor-net

15. http://projectreactor.io/docs/reference/

16. reactor-core
http://projectreactor.io/docs/reference/

17. reactor-stream
http://projectreactor.io/docs/reference/

18. reactor-bus
http://projectreactor.io/docs/reference/

19. reactor-net
http://projectreactor.io/docs/reference/

20. What/Why State Machine
• Application is and may exist in a finite
number of states and then something
happens which takes your application
from one state to the next.
• What will drive a state machine are
triggers which are either based on
events or timers.
• Behavior is always guaranteed to be
consistent
• Easily debugged due to ways how
operational rules are written in stone

21. Spring Statemachine (SSM)
• Easy to use flat one level state machine for
simple use cases.
• Hierarchical state machine structure to
ease complex state configuration.
• State machine regions to provide even
more complex state configurations.
• Usage of triggers, transitions, guards and
actions.
• Distributed state machine based on a
Zookeeper
• State machine event listeners.

22. Abstractions Provided by SSM
• States
• Hierarchical
States
• Regions
• Transitions
• Guards
• Actions
• Pseudo States
– Initial State
– Terminate State
– History State
– Choice State
– Fork State
– Join State
• Distributed States

23. Turnstile
http://docs.spring.io/spring-statemachine/docs/current/reference/htmlsingle/

24. Turnstile Code Sample
• public enum States {
• LOCKED, UNLOCKED
• }
•
• public enum Events {
• COIN, PUSH
• }
• public void configure(StateMachineStateConfigurer<States, Events> states) thro
ws Exception {
• states.withStates()
• .initial(States.LOCKED)
• .states(EnumSet.allOf(States.class));
• }
http://docs.spring.io/spring-statemachine/docs/current/reference/htmlsingle/

25. Turnstile Code Sample
• public void configure(StateMachineTransitionConfigurer<States, Events> tra
nsitions) throws Exception {
• transitions
• .withExternal()
• .source(States.LOCKED)
• .target(States.UNLOCKED)
• .event(Events.COIN)
• .and()
• .withExternal()
• .source(States.UNLOCKED)
• .target(States.LOCKED)
• .event(Events.PUSH);
• }
http://docs.spring.io/spring-statemachine/docs/current/reference/htmlsingle/

26. Washer
http://docs.spring.io/spring-statemachine/docs/current/reference/htmlsingle/

27. Washer Code Sample
• public void configure(StateMachineStateConfigurer<States, Events> states)
throws Exception {
• states
• .withStates()
• .initial(States.RUNNING)
• .state(States.POWEROFF)
• .end(States.END)
• .and()
• .withStates()
• .parent(States.RUNNING)
• .initial(States.WASHING)
• .state(States.RINSING)
• .state(States.DRYING)
• .history(States.HISTORY, History.SHALLOW);
• }
http://docs.spring.io/spring-statemachine/docs/current/reference/htmlsingle/

28. Building Event Sourced
Applications
• The states (finite, normally represented
by enumeration) of a domain object are
maintained by spring state machine
• Pub/Sub events by reactor-bus within
the application
• Event stream could be mapped to
reactor-stream with reactive streams
support
• Distributed state could be used to
enforce strong consistency of domain
objects

29. Some Thoughts About The
Idea
• Why not existing frameworks?
• Lighter-weight state machine?
• Lookup API for a state machine?
– State nextState = statemahcine.lookup(currentState, event);
• Distributed state or not?
• https://github.com/spring-
projects/spring-statemachine/issues/7
• Choice of event store? Kafka?
Cassandra?

30. Reference
• Event Sourcing by Martin Fowler
• Event Sourcing Basics
• Developing event-driven microservices
with event sourcing and CQRS
• https://github.com/cer/event-sourcing-
examples/wiki
• Akka Persistence Documentation
• Spring Statemachine Documentation
• Reactor Documentation