#JaxLondon: Building microservices with Scala, functional domain models and Spring Boot

@crichardson

Building microservices with Scala, functional domain models and Spring Boot

Chris Richardson

Author of POJOs in ActionFounder of the original CloudFoundry.com

@[email protected]://plainoldobjects.com

@crichardson

Presentation goal

Share my experiences with building an application using Scala, functional domain

models, microservices, event sourcing, CQRS, and Spring Boot

@crichardson

About Chris

@crichardson

About Chris

Founder of a buzzword compliant (stealthy, social, mobile, big data, machine learning, ...) startup

Consultant helping organizations improve how they architect and deploy applications using cloud, micro services, polyglot applications, NoSQL, ...

@crichardson

Agenda

Why build event-driven microservices?

Overview of event sourcing

Designing microservices with event sourcing

Implementing queries in an event sourced application

Building and deploying microservices

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Let’s imagine that you are building a banking app...

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Domain model

Account

balance

open(initialBalance)debit(amount)credit(amount)

MoneyTransfer

fromAccountIdtoAccountIdamount

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Tomcat

Traditional application architecture

Browser/Client

WAR/EAR

RDBMS

Customers

Accounts

Transfers

Banking Banking UI

developtest

deploy

Simple to

Load balancer

scale

Spring MVC

SpringHibernate

...

HTMLREST/JSON

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Problem #1: monolithic architecture

Intimidates developers

Obstacle to frequent deployments

Overloads your IDE and container

Obstacle to scaling development

Requires long-term commitment to a technology stack

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Solution #1: use a microservice architecture

Banking UI

Account Management Service

Transaction Management Service

Account Database

Transaction Database

Standaloneservices

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Problem #2: relational databases

Scalability

Distribution

Schema updates

O/R impedance mismatch

Handling semi-structured data

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Solution #2: use NoSQL databases

Avoids the limitations of RDBMS

For example,

text search ⇒ Solr/Cloud Search

social (graph) data ⇒ Neo4J

highly distributed/available database ⇒ Cassandra

...

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Different modules use different databases

IEEE Software Sept/October 2010 - Debasish Ghosh / Twitter @debasishg

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But now we have problems with data consistency!

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Problem #3: Microservices = distributed data management

Each microservice has it’s own database

Some data is replicated and must be kept in sync

Business transactions must update data owned by multiple services

Tricky to implement reliably without 2PC

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Problem #4: NoSQL = ACID-free, denormalized databases

Limited transactions, i.e. no ACID transactions

Tricky to implement business transactions that update multiple rows, e.g. http://bit.ly/mongo2pc

Limited querying capabilities

Requires denormalized/materialized views that must be synchronized

Multiple datastores (e.g. DynamoDB + Cloud Search ) that need to be kept in sync

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Solution to #3/#4: Event-based architecture to the rescue

Microservices publish events when state changes

Microservices subscribe to events

Synchronize replicated data

Maintains eventual consistency across multiple aggregates (in multiple datastores)

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Eventually consistent money transfer

Message Bus

MoneyTransferService AccountService

transferMoney()

Publishes:Subscribes to:

Subscribes to:

publishes:MoneyTransferCreatedEvent

AccountDebitedEvent

DebitRecordedEvent

AccountCreditedEventMoneyTransferCreatedEvent

DebitRecordedEvent

AccountDebitedEventAccountCreditedEvent

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To maintain consistency a service must

atomically publish an event whenever

a domain object changes

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How to generate events reliably?

Database triggers, Hibernate event listener, ...

Reliable BUT

Not with NoSQL

Disconnected from the business level event

Limited applicability

Ad hoc event publishing code mixed into business logic

Messy code, poor separation of concerns

Unreliable, e.g. too easy to forget to publish an event

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How to atomically update datastore and publish event(s)

Use 2PC

Database and message broker must support 2PC

2PC is best avoided

Use datastore as a message queue

1. Update database: new entity state & event to publish

2. Publish event & mark event as published

Eventually consistent mechanism

See BASE: An Acid Alternative, http://bit.ly/ebaybase

• Difficult to implement when using a NoSQL database :-(

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Agenda






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Event sourcingFor each aggregate:

Identify (state-changing) domain events

Define Event classes

For example,

Account: AccountOpenedEvent, AccountDebitedEvent, AccountCreditedEvent

ShoppingCart: ItemAddedEvent, ItemRemovedEvent, OrderPlacedEvent

@crichardson

Persists events NOT current state

Account

balance

open(initial)debit(amount)credit(amount)

AccountOpened

Event table

AccountCredited

AccountDebited

101 450

Account tableX101

101

101

901

902

903

500

250

300

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Replay events to recreate state

Account

balance

AccountOpenedEvent(balance)AccountDebitedEvent(amount)AccountCreditedEvent(amount)

Events

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Aggregate traits

Map Command to Events

Apply event returning updated Aggregate

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Account - command processing

Prevent overdraft

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Account - applying eventsImmutable

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Request handling in an event-sourced application

HTTPHandler

EventStore

pastEvents = findEvents(entityId)

Account

new()

applyEvents(pastEvents)

newEvents = processCmd(SomeCmd)

saveEvents(newEvents)

Microservice A

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Event Store publishes events - consumed by other services

EventStore

EventSubscriber

subscribe(EventTypes)

publish(event)

publish(event)

Aggregate

NoSQLmaterialized

view

update()

update()

Microservice B

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Persisting events

Ideally use a cross platform format

Use weak serialization:

enables event evolution, eg. add memo field to transfer

missing field ⇒ provide default value

unknown field ⇒ ignore

JSON is a good choice

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Optimizing using snapshots

Most aggregates have relatively few events

BUT consider a 10-year old Account ⇒ many transactions

Therefore, use snapshots:

Periodically save snapshot of aggregate state

Typically serialize memento of the aggregate

Load latest snapshot + subsequent events

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Event Store APItrait EventStore {

def save[T <: Aggregate[T]](entity: T, events: Seq[Event], assignedId : Option[EntityId] = None): Future[EntityWithIdAndVersion[T]]

def update[T <: Aggregate[T]](entityIdAndVersion : EntityIdAndVersion, entity: T, events: Seq[Event]): Future[EntityWithIdAndVersion[T]]

def find[T <: Aggregate[T] : ClassTag](entityId: EntityId) : Future[EntityWithIdAndVersion[T]]

def findOptional[T <: Aggregate[T] : ClassTag](entityId: EntityId) Future[Option[EntityWithIdAndVersion[T]]]

def subscribe(subscriptionId: SubscriptionId): Future[AcknowledgableEventStream]}

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Business benefits of event sourcing

Built-in, reliable audit log

Enables temporal queries

Publishes events needed by big data/predictive analytics etc.

Preserved history ⇒ More easily implement future requirements

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Technical benefits of event sourcing

Solves data consistency issues in a Microservice/NoSQL-based architecture:

Atomically save and publish events

Event subscribers update other aggregates ensuring eventual consistency

Event subscribers update materialized views in SQL and NoSQL databases (more on that later)

Eliminates O/R mapping problem

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Drawbacks of event sourcing

Weird and unfamiliar

Events = a historical record of your bad design decisions

Handling duplicate events can be tricky

Application must handle eventually consistent data

Event store only directly supports PK-based lookup (more on that later)

@crichardson

Example of an eventual consistency problem

Scenario:

1.Create the user

2.Create shopping cart

3.Update the user with the shopping cart’s id

The user temporarily does not have a shopping cart id!

Client might need to retry their request at a later point

Server should return status code 418??

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Handling duplicate messages

Idempotent operations

e.g. add item to shopping cart

Duplicate detection:

e.g. track most recently seen event and discard earlier ones

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Agenda






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The anatomy of a microservice

Event Store

HTTP Request

HTTP Adapter

Aggregate

Event Adapter

Cmd

Cmd

EventsEvents

Xyz Adapter

Xyz Request

microservice

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Asynchronous Spring MVC controller

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MoneyTransferService

DSL concisely specifies:1.Creates MoneyTransfer aggregate2.Processes command3.Applies events4.Persists events

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MoneyTransfer Aggregate

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Handling events published by Accounts

1.Load MoneyTransfer aggregate2.Processes command3.Applies events4.Persists events

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Agenda






@crichardson

Let’s imagine that you want to display an account and it’s recent transactions...

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Displaying balance + recent credits and debits

We need to do a “join: between the Account and the corresponding MoneyTransfers

(Assuming Debit/Credit events don’t include other account, ...)

BUTEvent Store = primary key lookup of individual aggregates, ...

⇒Use Command Query Responsibility Separation

Define separate “materialized” query-side views that implement those queries

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Query-side microservices

Event Store

Updater - microservice

View UpdaterService

EventsReader - microservice

HTTP GET Request

View Query Service

ViewStore

e.g. MongoDB

Neo4JCloudSearch

update query

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Persisting account balance and recent transactions in MongoDB

{ id: "298993498", balance: 100000, transfers : [

{"transferId" : "4552840948484", "fromAccountId" : 298993498, "toAccountId" : 3483948934, "amount" : 5000}, ...

], changes: [ {"changeId" : "93843948934", "transferId" : "4552840948484", "transactionType" : "AccountDebited", "amount" : 5000}, ... ] }

Denormalized = efficient lookup

Transfers that update the account

The sequence of debits and credits

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Updating MongoDB using Spring Data

class AccountInfoUpdateService (mongoTemplate : MongoTemplate, ...) extends CompoundEventHandler {

@EventHandler def recordDebit(de: DispatchedEvent[AccountDebitedEvent]) = { ... val ci = AccountChangeInfo(...)

mongoTemplate.updateMulti( new Query(where("id").is(de.entityId.id).and("version").lt(changeId)), new Update(). dec("balance", amount). push("changes", ci). set("version", changeId), classOf[AccountInfo]) }

@EventHandler def recordTransfer(de: DispatchedEvent[MoneyTransferCreatedEvent]) = ...

}

insert/In-place update

duplicate event detection

updates to and from accounts

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Retrieving account info from MongoDB using Spring Data

class AccountInfoQueryService(accountInfoRepository : AccountInfoRepository) {

def findByAccountId(accountId : EntityId) : AccountInfo = accountInfoRepository.findOne(accountId.id)

}

case class AccountInfo(id : String, balance : Long, transactions : List[AccountTransactionInfo], changes : List[ChangeInfo], version : String)

case class AccountTransactionInfo(changeId : String, transactionId : String,

transactionType : String, amount : Long, balanceDelta : Long)

trait AccountInfoRepository extends MongoRepository[AccountInfo, String]

Implementation generated by Spring Data

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Other kinds of viewsAWS Cloud Search

Text search as-a-Service

View updater batches aggregates to index

View query does text search

AWS DynamoDB

NoSQL as-a-Service

On-demand scalable - specify desired read/write capacity

Document and key-value data models

Useful for denormalized, UI oriented views

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Dealing with eventually consistent views

Scenario:

Client creates/updates aggregate

Client requests view of aggregate

But the view might not yet have been updated

Solution:

Create/Update response contains aggregate version

Query request contains desired version

Alternatively:

“Fake it” in the UI until the view is updated

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Agenda






@crichardson

Building microservices with Spring Boot

Makes it easy to create stand-alone, production ready Spring Applications

Automatically configures Spring using Convention over Configuration

Externalizes configuration

Generates standalone executable JARs with embedded web server

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Spring Boot simplifies configuration

Spring Container

Application components

Fully configured application

ConfigurationMetadata

•Typesafe JavaConfig•Annotations•Legacy XML

Default Configuration

Metadata

Spring BootYou write less

of this

Inferred from CLASSPATH

@crichardson

Tiny Spring configuration for Account microservice

@Configuration@EnableAutoConfiguration@Import(classOf[JdbcEventStoreConfiguration]))@ComponentScanclass AccountConfiguration {

@Bean def accountService(eventStore : EventStore) = new AccountService(eventStore)

@Bean def accountEventHandlers(eventStore : EventStore) = EventHandlerRegistrar.makeFromCompoundEventHandler( eventStore, "accountEventHandlers", new TransferWorkflowAccountHandlers(eventStore)) @Bean @Primary def scalaObjectMapper() = ScalaObjectMapper

}

Service

Event handlers

Scan for controllers

Customize JSON serialization

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The Main program

object BankingMain extends App {

SpringApplication.run(classOf[AccountConfiguration], args :_ *)

}

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Building with Gradle

buildscript { repositories { mavenCentral() } dependencies { classpath("org.springframework.boot:spring-boot-gradle-plugin:1.0.0.RELEASE") }}

apply plugin: 'scala'apply plugin: 'spring-boot'

...

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Running the microservice

$ java -jar build/libs/banking-main-1.0-SNAPSHOT.jar --server.port=8081...11:38:04.633 INFO n.c.e.e.bank.web.main.BankingMain$ - Started BankingMain. in 8.811 seconds (JVM running for 9.884)

$ curl localhost:8081/health{"status":"UP", "mongo":{"status":"UP","version":"2.4.10"}, "db":{"status":"UP","database":"H2","hello":1}}

Built in health checks

Command line arg processing

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NodeJS-based API gateway

Motivations

Small footprint

Efficient, event-driven I/O

Availability of modules: express, request, googleapis, ...

Routes requests to the appropriate backend service based on URL prefix

Handles Google/OAuth-based authentication

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Jenkins-based deployment pipeline

Build & Testmicro-service

Build & TestDockerimage

Deploy Docker image

to registry

One pipeline per micro-service

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Production environment

EC2 instance running Docker

Deployment tool:

1.Compares running containers with what’s been built by Jenkins

2.Pulls latest images from Docker registry

3.Stops old versions

4.Launches new versions

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Summary

Event Sourcing solves key data consistency issues with:

Microservices

Partitioned/NoSQL databases

Spring and Scala play nicely together

Spring Boot makes it very easily to build production ready microservices

NodeJS is useful for building network-centric services

@crichardson

Questions?

@crichardson [email protected]

http://plainoldobjects.com http://microservices.io