Events on the outside, on the inside and at the core (jfokus jfokus2016)
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This is the talk I gave at JFokus 2016 on event-driven microservices. This presentation looks at the importance of events and the role that they play in applications. We describe how events are a key application integration mechanism and how they are used by applications to communicate with the outside world. You will learn how the microservices inside a system can use events to maintain data consistency. We discuss how easy it is to implement both of these mechanisms by developing your core business logic using an event-centric approach known as event sourcing.
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Events on the outside, on the inside and at the core (jfokus jfokus2016)
- 1. @crichardson Events on the outside, on the inside and at the core Chris Richardson Founder of Eventuate.io Founder of the original CloudFoundry.com Author of POJOs in Action @crichardson chris@chrisrichardson.net http://microservices.io http://eventuate.io http://plainoldobjects.com
- 2. @crichardson Presentation goal Events play a key role in applications Event sourcing enables the event-driven enterprise
- 4. @crichardson About Chris Consultant and trainer focusing on microservices http://www.chrisrichardson.net/
- 5. @crichardson About Chris Founder of a startup that is creating a platform that makes it easy for application developers to write microservices http://eventuate.io
- 7. @crichardson Agenda Events on the outside Events on the inside Events at the core with event sourcing Designing event-centric domain model
- 8. @crichardson Events on the outside Application A Application B Application C Event X Event Y Event Z
- 12. @crichardson An event is produced when… Creation or update of a business object Attempt to violate a business rule
- 13. @crichardson Who consumes an event? Order Application Human Notification Service Dashboard/ Monitoring Shipping Application OrderCreated Email SMS …
- 14. @crichardson How to deliver events?
- 15. @crichardson Polling for events HTTP Periodically poll for events Atom Publishing Protocol (AtomPub) Based on HTTP Head is constantly changing Tail is immutable and can be efficiently cached High-latency, inefficient
- 16. @crichardson Using WebSockets Browser Web Socket STOMP Client application Service Web Socket STOMP Application SUBSCRIBE MESSAGE MESSAGE Low latency, more efficient, but what about past events?
- 17. @crichardson Webhooks = user-defined HTTP callback Client Service register(events, callbackUrl) POST callbackUrl POST callbackUrl … https://en.wikipedia.org/wiki/Webhook Low latency, more efficient, but what about past events?
- 18. @crichardson Github webhooks https://developer.github.com/webhooks/ Installed on an organization or repository e.g. POST /repos/:owner/:repo/hooks Available events: push - push to a repository fork - repository is forked pull_request - assigned, unassigned, … push - push to a repository …
- 19. @crichardson Twilio - Telephony and SMS as a service Twilio Your Application TwiML doc HTTP GET/ POST REST API Manage resources Send SMS Initiate voice calls Webhooks handle incoming SMS and voice calls Voice SMS Phone number SMS URL +VOICE URL
- 20. @crichardson Integration hubs - Zapier, IFTTT Application abstraction: Triggers - events published by application: polling or Webhooks Action - operation supported by application, e.g. REST API end points
- 23. @crichardson messaging system Messaging-based IPC Sender RecipientChannel message
- 25. @crichardson The event-driven enterprise App A App BApp X App Y Firewall Message Broker webhooks WebSockets AtomPub App Z App C Msg Broker Client Msg Broker Client Msg Broker Client App D Msg Broker Client Inside the firewallOutside the firewall
- 26. @crichardson Agenda Events on the outside Events on the inside Events at the core with event sourcing Designing event-centric domain model
- 27. @crichardson Events on the inside Application Service A Service B Service C Event X Event YEvent Z Event X Event Y
- 28. @crichardson Traditional monolithic architecture Simple to develop test deploy scale Shopping Cart Customers Orders Mobile client REST API Browser Web UI Message Broker Adapter MySQL Adapter RDBMS … Message Broker
- 29. @crichardson But that leads* to monolithic hell For large and/or complex applications…
- 30. @crichardson Today: use a microservice, polyglot architecture Orders Customers … Shopping UI Mobile Client Browser API Gateway Order management REST API Customer Management REST API …. REST API Order Database (MongoDB) MongoDB Adapter Customer Database (Sharded MySQL) MySQL Adapter
- 31. @crichardson But now we have distributed data management problems
- 32. @crichardson Example: placing an order Order Service Customer Service Order Database Customer Database Order #1 Customer #1 No 2PC No ACID NoSQL SQL
- 33. @crichardson Customer management How to maintain invariants? Order management Order Service placeOrder() Customer Service updateCreditLimit() Customer creditLimit ... has ordersbelongs toOrder total Invariant: sum(open order.total) <= customer.creditLimit ?
- 34. @crichardson Use an event-driven architecture Services publish events when something important happens, e.g. state changes Services subscribe to events and update their state Maintain eventual consistency across multiple aggregates (in multiple datastores) Synchronize replicated data
- 35. @crichardson Event-driven application architecture Application Service A Service B Message Broker Service C Msg Broker Client Msg Broker Client Msg Broker Client Service D Msg Broker Client
- 36. @crichardson Order Management Order id : 4567 total: 343 state = CREATED Customer Management Customer creditLimit : 12000 creditReservations: {} Customer creditLimit : 12000 creditReservations: { 4567 -> 343} Order id : 4567 total: 343 state = OPEN Eventually consistent credit checking Message Bus createOrder() Publishes: Subscribes to: Subscribes to: publishes: OrderCreatedEvent CreditReservedEvent OrderCreatedEvent CreditReservedEvent
- 37. @crichardson Now there are two problems to solve….
- 38. @crichardson Problem #1: How to design eventually consistent business logic? More on that later….
- 39. @crichardson Problem #2: How to atomically update database and publish an event Order Service Order Database Message Broker insert Order publish OrderCreatedEvent dual write problem ?
- 40. @crichardson Failure = inconsistent system Order Service Order Database Message Broker insert Order publish OrderCreatedEvent X
- 42. @crichardson Agenda Events on the outside Events on the inside Events at the core with event sourcing Designing event-centric domain model
- 43. @crichardson Just publish events Application Database Message Broker update publish X
- 44. @crichardson Event sourcing For each aggregate (business entity): Identify (state-changing) domain events Define Event classes For example, ShoppingCart: ItemAddedEvent, ItemRemovedEvent, OrderPlacedEvent Order: OrderCreated, OrderCancelled, OrderApproved, OrderRejected, OrderShipped
- 45. @crichardson Persists events NOT current state Order status …. 101 ACCEPTED Order table X
- 46. @crichardson Persists events NOT current state Event table Entity type Event id Entity id Event data Order 902101 …OrderApproved Order 903101 …OrderShipped Event type Order 901101 …OrderCreated
- 47. @crichardson Replay events to recreate state Order state OrderCreated(…) OrderAccepted(…) OrderShipped(…) Events Periodically snapshot to avoid loading all events
- 48. @crichardson The present is a fold over history currentState = foldl(applyEvent, initialState, events)
- 49. @crichardson Events at the core Service Aggregate A Aggregate B Aggregate C Event X Event YEvent Z Event Z Event Y
- 50. @crichardson Domain logic = event-driven aggregates Customer AggregateOrder Aggregate Order Created Event Check Credit Command Credit Reserved Event Approve Order Command Create Order Command External request Emits events Event Command
- 51. @crichardson Request handling in an event sourced application HTTP Handler Event Store pastEvents = findEvents(entityId) Order new() applyEvents(pastEvents) newEvents = processCmd(SomeCmd) saveEvents(newEvents) (optimistic locking) Order Service apply(newEvents)
- 52. @crichardson Event Store publishes events consumed by other services Event Store Event Subscriber subscribe(EventTypes) publish(event) publish(event) Customer update() Customer Service
- 53. @crichardson Event Store publishes events consumed by other services Event Store Event Subscriber subscribe(EventTypes) publish(event) publish(event) CQRS View update() Service Xyz send notifications …
- 54. Event store = database + message broker Hybrid database and message broker Implementations: Home-grown/DIY geteventstore.com by Greg Young http://eventuate.io (mine) Event Store Save aggregate events Get aggregate events Subscribe to events
- 55. @crichardson Benefits of event sourcing Solves data consistency issues in a Microservice/NoSQL based architecture Reliable event publishing: publishes events needed by predictive analytics etc, user notifications,… Eliminates O/R mapping problem (mostly) Reifies state changes: Built in, reliable audit log, temporal queries Preserved history More easily implement future requirements
- 56. @crichardson Drawbacks of event sourcing… Requires application rewrite Weird and unfamiliar style of programming Events = a historical record of your bad design decisions Must detect and ignore duplicate events Idempotent event handlers Track most recent event and ignore older ones …
- 57. @crichardson … Drawbacks of event sourcing Querying the event store can be challenging Some queries might be complex/inefficient, e.g. accounts with a balance > X Event store might only support lookup of events by entity id Must use Command Query Responsibility Segregation (CQRS) to handle queries application must handle eventually consistent data
- 58. @crichardson Agenda Events on the outside Events on the inside Events at the core with event sourcing Designing event-centric domain model
- 59. @crichardson Use the familiar building blocks of DDD Entity Value object Services Repositories Aggregates ⟸ essential
- 60. About Aggregates Graph consisting of a root entity and one or more other entities and value objects Each core business entity = Aggregate: e.g. customer, Account, Order, Product, …. Reference other aggregate roots via primary key Often contains partial copy of other aggregates’ data Order OrderLine Item quantity productId productName productPrice customerId Address street city …
- 61. @crichardson Domain model = collection of loosely connected aggregates Order OrderLine Item quantity … Address street city … Customer Product name price
- 62. @crichardson Product service Customer serviceOrder service Easily partition into microservices Order OrderLine Item quantity … Address street city … Customer Product name price
- 63. @crichardson Transaction = processing one command by one aggregate No opportunity to update multiple aggregates within a transaction event driven eventual consistency between aggregates If an update must be atomic (i.e. no compensating transaction) then it must be handled by a single aggregate Therefore, aggregate granularity is important
- 64. @crichardson Product service Customer serviceOrder service Transaction scope = service Order OrderLine Item quantity … Address street city … Customer Product name price
- 66. Designing domain events Record state changes for an aggregate Part of the public API of the domain model ProductAddedToCart id : TimeUUID senderId: UUID productId productName productPrice … Required by aggregate Enrichment: Useful for consumers Event metadata
- 68. @crichardson Designing commands Created by a service from incoming request Processed by an aggregate Immutable Contains value objects for Validating request Creating event Auditing user activity
- 70. @crichardson Various programming models “Traditional Java” mutable object-oriented domain objects https://github.com/cer/event-sourcing-examples/tree/master/ java-spring Functional Scala with immutable domain objects https://github.com/cer/event-sourcing-using-scala-typeclasses Hybrid OO/Functional Scala with immutable domain objects https://github.com/cer/event-sourcing-examples/tree/master/ scala-spring
- 71. @crichardson OO = State + Behavior creditLimit creditReservations : Map<OrderId, Money> Customer List<Event> process(CreateCustomerCommand cmd) { … } List<Event> process(ReserveCreditCommand cmd) { … } … void apply(CustomerCreatedEvent anEvent) { … } void apply(CreditServedEvent anEvent) { … } … State Behavior
- 72. @crichardson Familiar concepts restructured class Customer { public void reserveCredit( orderId : String, amount : Money) { // verify // update state this.xyz = … } public List<Event> process( ReserveCreditCommand cmd) { // verify … return … new CreditReservedCredit(); } public void apply( CreditReservedCredit event) { // update state this.xyz = event.xyz }
- 74. @crichardson Architecture Create/update boards and tasks Change notifications Materialized views Event Store
- 76. @crichardson Summary Events are a central to modern applications Events integrate applications Events maintain data consistency in a microservices architecture Build events into the core of your application using event sourcing