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ImplementingEvent-DrivenMicroservices

NikhilBarthwalwww.nikhilbarthwal.com

2018

Agenda(1/2)

• MotivationstomovefromMonolithictoMicroservices• UnderstandingtheDistributedDatainMicroservices• UsingDomainDrivenDesignforpartitioningdata• ProblemswithDistributedData• EventDrivenMicroservices

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Agenda(2/2)

• CommunicationPatternsforMicroservices• EventSourcing• CommandQueryResponsibilitySegregation• IntroducingSagas• Resourcesforfurtherreading

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Monolithic:Theoldschoolapproach…

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Database

WebService

EasytoDevelop,TestandDeploy…

Butdoesn’tscale!

• Largecodebase→Slowerdevelopment• Moretimetotest&deploy• Toomuchteaminterdependency

Microservices:TheBravenewworld...

• Smallercodebasetounderstand&change• Autonomousteams• Fastertesting&deploy

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AssemblyofLooselycouplesservices!ImageSource:https://www.slideshare.net/JontheBeach/distributed-sagas-a-protocol-for-coordinating-microservices

LooseCouplingimpliesDistributedData

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DatabaseperService!

FasterInnovation!

Looselycoupledservices

Reducesteaminterdependencies

ImageSource:http://microservices.io/patterns/microservices.html

DomainDrivenDesign(DDD)

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An approach to software development for complex needs byconnecting the implementation to an evolving model.

• Entities• ValueObjects• BoundedContext• Aggregates InfrastructureLayer

DomainLayer

ApplicationLayer

UserInterface

DDD:Aggregates

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• Clusterofdomainobjectsthatcanbetreatedasasingleunit

• Oneofitscomponentisroot• Alloutsidereferencewouldonlygototheaggregateroot

DomainModel=

CollectionofAggregates

DataPartitionStrategies:UseDDD

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Aggregatesprovide

boundariesfor

partition

ProblemswithDistributedData

• Howdowekeepdataconsistent?• Howdowequeryscattereddata?

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CannotuseACIDTransactions

BEGIN TRANSACTION

SELECT ADDRESSFROM CUSTOMERSWHERE CUSTOMER_ID=XXX

SELECT PRICEFROM PRODUCTSWHERE CUSTOMER_ID=YYY

INSERT INTO ORDERS...

COMMIT TRANSACTION

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PrivatetoCustomerservice

PrivatetoProductservice

Cannotuse2-PhaseCommit

• Coordinatorisasinglepointoffailure• Verychatty,withO(4n)messages,O(n2)withretries• Lockswoulddecreasethroughputs• Impactsavailability(CAPtheoremlimitations)

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Guaranteesconsistency,but…

ConsistencyEventualConsistency

Availability

Partition(Network)

EventualConsistency

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UseEventDrivenMicroservice Architecture!

EventDrivenArchitecture:Introduction

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• Eventoccurswhenachangehappensinsystem• Alllistenersgetnotifiedoftheevent,maytakeaction• Highlydistributed/looselycoupledarchitecture• Oftenusedforasynchronousflowsofinformation

Communication:DirectMessaging

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Benefits:

• Easierimplementation

• WorkswellwithServerless

Drawbacks:

• Increasedcoupling

• Difficulttoscale

OrderService

ProductService

CustomerService

FulfillmentService

PaymentService

ShippingService

Communication:ViaMessagebroker

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Benefits:

• Morescalable

• IncreasedDecoupling

• BetterErrorcontrol

Drawbacks:

• RequiresBrokerasHighlyAvailabile component

OrderServiceProductService

CustomerService

FulfillmentService

PaymentService

ShippingService

MessageBroker

CommunicationPatterns

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Request/Response(synchronous)

Event-Driven

(asynchronous)Tier-3:LittletoNoimpactforCustomer

Tier-2:ImpactingCustomersindirectly

Tier-1:DirectlyCustomerfacing

EventSourcing:Benefits&Drawbacks

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Benefits:• 100%accurateauditlogging• Easytemporalqueries• Applicationscanprocesssameeventsbutcreateviews

Drawbacks:• AddsComplexity• NoStrictConsistency• Longerbootuptimes(Snapshotscanhelp)

EventSourcing:Introduction

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• Modelingstatechangesassequenceofevents• Storingtheeventthatcouldtriggerthestatechange• Enablesrollingbacktoparticulartimeinhistory

Examples:

EventSourcing:Multipleviews

20ImageSource:https://www.confluent.io/wp-content/uploads/2016/09/Event-sourced-based-architecture.jpeg

Addingapplicationsthatprocessevent…

butcreateadifferentview!

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CQRSPattern

CommandQueryResponsibilitySegregation

MustforEventSourcing!

Read

Write

ImageSource:https://martinfowler.com/bliki/CQRS.html

CQRS:Benefits&Drawbacks

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Benefits:• NeededforEventSourcing• Improvedseparationofconcerns• Supportsscalablemultipledenormalizedviews

Drawbacks:• Increasedcomplexity• Potentialcodeduplication• ReplicationlagasNoStrictConsistency

Sagas:Introduction

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• Basedona1987paper• Initiallyforasingledatabaserunningononenode

• Nowadaptedfordistributedsystemswithasynchronyandpartialfailure

Transaction&RollbackTransaction

• EveryTransactionhasaRollbacktransaction• Thislogicmustbeincludedintheservice

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T1 T2 T3

R1 R2 R3

Transactions→

RollbackTransactions→

Sagas:Example

Thisactioninitiatesthesaga

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CheckCreditService WithdrawalService DepositService

DepositCheck

Longrunningtransactions…usecompensatingactionstohandlefailures!

DoesSenderhasenoughmoneyinhis

account?

Withdrawthemoney(unlessitisonhold)?

Depositthemoney(unlessaccountisdeactivated)?

DistributedSaga:Example

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Requests:

• Idempotent

• CanAbort

CompensatingRequests:

• Idempotent

• Commutative

• CanNotAbort

Source:https://speakerdeck.com/caitiem20/distributed-sagas-a-protocol-for-coordinating-microservices

Resources

• UsingDDDtodevelopMicroservices:https://www.infoq.com/news/2016/02/ddd-microservices

• OverviewofSagaPattern:http://microservices.io/patterns/data/saga.html

• DistributedSagas:https://www.slideshare.net/JontheBeach/distributed-sagas-a-protocol-for-coordinating-microservices

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Questions?

NikhilBarthwal

@nikhilbarthwalnikhilbarthwal@yahoo.comwww.nikhilbarthwal.com