Connecting the dots - building and structuring a functional application in Scala

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CONNECTING THE DOTS BUILDING AND STRUCTURING A FUNCTIONAL APPLICATION IN SCALA JAKUB KOZŁOWSKI, DISNEY STREAMING YOW! LAMBDA JAM 2021 Photo by Kumiko SHIMIZU on Unsplash

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PROBLEM STATEMENT

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PROBLEM STATEMENT We want to build an application

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PROBLEM STATEMENT We want to build an application There are some sources of data (databases, APIs, event streams)

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PROBLEM STATEMENT We want to build an application There are some sources of data (databases, APIs, event streams) We need to serve HTTP traffic

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PROBLEM STATEMENT We want to build an application There are some sources of data (databases, APIs, event streams) We need to serve HTTP traffic Some things need to run in the background additionally

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PROBLEM STATEMENT We want to build an application There are some sources of data (databases, APIs, event streams) We need to serve HTTP traffic Some things need to run in the background additionally We want to do it with FP

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DEPENDENCY GRAPH TYPICAL APPLICATION

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DEPENDENCY GRAPH def database: Database def businessLogic(db: Database): BusinessLogic def server(logic: BusinessLogic): Server def backgroundProcesses(logic: BusinessLogic): Processes

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DEPENDENCY GRAPH def database: Database def businessLogic(db: Database): BusinessLogic def server(logic: BusinessLogic): Server def backgroundProcesses(logic: BusinessLogic): Processes def build: (Server, Processes) = { val logic = businessLogic(database) (server(logic), backgroundProcesses(logic)) }

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DEPENDENCY GRAPH This could be us, but the real world exists... def database: Database def businessLogic(db: Database): BusinessLogic def server(logic: BusinessLogic): Server def backgroundProcesses(logic: BusinessLogic): Processes def build: (Server, Processes) = { val logic = businessLogic(database) (server(logic), backgroundProcesses(logic)) }

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RESOURCES.

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RESOURCES. But we can just kill our application when it quits, right?

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CONNECTION POOLS?

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CONNECTION POOLS? But we can use try-finally, right?

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def getConnection(db: Database): Connection def returnConnection(conn: Connection): Unit def doWork(conn: Connection): Result TRY-FINALLY

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def getConnection(db: Database): Connection def returnConnection(conn: Connection): Unit def doWork(conn: Connection): Result TRY-FINALLY val result = { val c = getConnection(db) try doWork(c) finally returnConnection(c) }

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def getConnection(db: Database): IO[Connection] def returnConnection(conn: Connection): IO[Unit] def doWork(conn: Connection): IO[Result] TRY-FINALLY IN FP val result: IO[Result] = getConnection(db).bracket { c !=> doWork(c) }(returnConnection)

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MORE RESOURCES? Can't keep nesting bracket forever

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MORE RESOURCES? Can't keep nesting bracket forever ABSTRACTION? How to hide details?

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RESOURCE DATA STRUCTURE

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RUNNING A RESOURCE?

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RUNNING A RESOURCE?

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COMPOSITION?

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WHY AM I TALKING ABOUT THIS? def database: Database def businessLogic(db: Database): BusinessLogic def server(logic: BusinessLogic): Server def backgroundProcesses(logic: BusinessLogic): Processes

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WHY AM I TALKING ABOUT THIS? def database: Resource[Database] def businessLogic(db: Database): BusinessLogic def server(logic: BusinessLogic): Resource[Server] def backgroundProcesses(logic: BusinessLogic): Resource[Processes] def build: Resource[(Server, Processes)] = database.flatMap { db !=> val logic = businessLogic(db) server(logic).flatMap { srv !=> backgroundProcesses(logic).map(p !=> (srv, p)) } }

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WHY AM I TALKING ABOUT THIS? def database: Resource[Database] def businessLogic(db: Database): BusinessLogic def server(logic: BusinessLogic): Resource[Server] def backgroundProcesses(logic: BusinessLogic): Resource[Processes] def build: Resource[(Server, Processes)] = for { db !<- database logic = businessLogic(db) srv !<- server(logic) processes !<- backgroundProcesses(logic) } yield (srv, processes)

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HOW IS A BACKGROUND PROCESS A RESOURCE? Watch this space: yt.kubukoz.com -> "Background processing in functional Scala" playlist

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DEPENDENCY GRAPH AS A RESOURCE

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ALGEBRAS / CAPABILITY TRAITS

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ALGEBRAS / CAPABILITY TRAITS Tagless Final style

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ALGEBRAS / CAPABILITY TRAITS Tagless Final style Interfaces parameterised by an eﬀect

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ALGEBRAS / CAPABILITY TRAITS Tagless Final style Interfaces parameterised by an eﬀect Capability traits - lawless type classes

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RULES OF THUMB

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RULES OF THUMB Prefer capability traits (Files[F], Network[F], Console[F]) over Sync/Async

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RULES OF THUMB Prefer capability traits (Files[F], Network[F], Console[F]) over Sync/Async Implicit or explicit?

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RULES OF THUMB Prefer capability traits (Files[F], Network[F], Console[F]) over Sync/Async Implicit or explicit? - 1 instance per type: implicit definition, pass implicitly

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RULES OF THUMB Prefer capability traits (Files[F], Network[F], Console[F]) over Sync/Async Implicit or explicit? - 1 instance per type: implicit definition, pass implicitly - has possible test instance: explicit definition, pass implicitly

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IMAGE PROCESSING APP CASE STUDY

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IMAGE PROCESSING APP CASE STUDY Project Goals

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IMAGE PROCESSING APP CASE STUDY Project Goals Search images from a datasource by the text on them (OCR)

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IMAGE PROCESSING APP CASE STUDY Project Goals Search images from a datasource by the text on them (OCR) Live OCR is too slow, so we'll index ahead of time

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IMAGE PROCESSING APP CASE STUDY Project Goals Search images from a datasource by the text on them (OCR) Live OCR is too slow, so we'll index ahead of time github.com/kubukoz/dropbox-demo

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DATA FLOW (LINEAR)

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TANGENT: HEXAGONAL ARCHITECTURE Hexagonal Architecture by Cth027, licensed under CC BY-SA 4.0

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TANGENT: HEXAGONAL ARCHITECTURE Hexagonal Architecture by Cth027, licensed under CC BY-SA 4.0 Or... just sensible architecture.

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TANGENT: HEXAGONAL ARCHITECTURE Hexagonal Architecture by Cth027, licensed under CC BY-SA 4.0 Or... just sensible architecture. Keep vendor/implementation-specific details hidden and away from core logic

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TANGENT: HEXAGONAL ARCHITECTURE Hexagonal Architecture by Cth027, licensed under CC BY-SA 4.0 Jakub Nabrdalik - Hexagonal Architecture in practice https://www.youtube.com/watch?v=sOaS83Ir8Ck Or... just sensible architecture. Keep vendor/implementation-specific details hidden and away from core logic Only talk to these via adapters with a simple API

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DEPENDENCY GRAPH

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PROJECT STRUCTURE shared - contains common vocabulary Used by adapters and core logic imagesource, ocr, indexer - modules root - contains core logic + http module Standard sbt pattern for "main" sources

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OCR MODULE

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OCR MODULE ProcessRunner - capability trait for running system processes

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OCR MODULE ProcessRunner - capability trait for running system processes Tesseract - runs a Tesseract process

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OCR MODULE ProcessRunner - capability trait for running system processes Tesseract - runs a Tesseract process OCR - wraps Tesseract and specifies config options (languages)

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OCR MODULE ProcessRunner - capability trait for running system processes Tesseract - runs a Tesseract process OCR - wraps Tesseract and specifies config options (languages) TestOCRInstances - contains test fakes for OCR for usage in tests of higher-level components (processes)

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PROCESS RUNNER package com.kubukoz.process trait ProcessRunner[F[_]] { def run(program: List[String]): Resource[F, ProcessRunner.Running[F]] } object ProcessRunner { def apply[F[_]](implicit F: ProcessRunner[F]): ProcessRunner[F] = F implicit def instance[F[_]: Async]: ProcessRunner[F] = !!... }

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TESSERACT package com.kubukoz.ocr.tesseract private[ocr] trait Tesseract[F[_]] { def decode(input: fs2.Stream[F, Byte], languages: List[String]): F[String] } object Tesseract { def apply[F[_]](implicit F: Tesseract[F]): Tesseract[F] = F def instance[F[_]: ProcessRunner: Logger: Concurrent](implicit SC: fs2.Compiler[F, F]): Tesseract[F] = !!... }

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OCR

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package com.kubukoz.ocr trait OCR[F[_]] { def decodeText(file: fs2.Stream[F, Byte]): F[DecodedText] } object OCR { def apply[F[_]](implicit F: OCR[F]): OCR[F] = F } OCR

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package com.kubukoz.ocr trait OCR[F[_]] { def decodeText(file: fs2.Stream[F, Byte]): F[DecodedText] } object OCR { def apply[F[_]](implicit F: OCR[F]): OCR[F] = F } OCR final case class Config(languages: List[String]) def config[F[_]]: ConfigValue[F, Config] = !!... private[ocr] def tesseractInstance[F[_]: Tesseract: Functor](config: Config): OCR[F] = new OCR[F] { def decodeText(file: fs2.Stream[F, Byte]): F[DecodedText] = Tesseract[F].decode(file, config.languages).map(DecodedText(_)) }

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package com.kubukoz.ocr object TestOCRInstances { !// decodeText("hello".getBytes) !== "hello" def simple[F[_]: Functor](implicit SC: fs2.Compiler[F, F]): OCR[F] = _.through(fs2.text.utf8Decode[F]).compile.string.map(DecodedText(_)) } TEST INSTANCE

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WIRING IT ALL UP

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WIRING IT ALL UP

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object Application { final case class Config( indexer: Indexer.Config, imageSource: ImageSource.Config, processQueue: ProcessQueue.Config, ocr: OCR.Config, http: HttpServer.Config, ) }

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object Application { final case class Config( indexer: Indexer.Config, imageSource: ImageSource.Config, processQueue: ProcessQueue.Config, ocr: OCR.Config, http: HttpServer.Config, ) } def config[F[_]: ApplicativeThrow]: ConfigValue[F, Config] = ( Indexer.config[F], ImageSource.config[F], ProcessQueue.config[F], OCR.config[F], HttpServer.config[F], ).parMapN(Config) def run[F[_]: Async: Logger](config: Config): Resource[F, Server] = for { implicit0(client: Client[F]) !<- HttpClient.instance[F] } implicit0(imageSource: ImageSource[F]) !<- ImageSource.module[F](config.imageSource).toResource implicit0(indexer: Indexer[F]) !<- Indexer.module[F](config.indexer) implicit0(ocr: OCR[F]) !<- OCR.module[F](config.ocr).pure[Resource[F, *]] processQueue !<- ProcessQueue.instance(config.processQueue)

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A COUPLE GUIDELINES TESTING

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A COUPLE GUIDELINES TESTING Test the contract, not the implementation

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A COUPLE GUIDELINES TESTING Test the contract, not the implementation Prefer fakes over mocks/stubs

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A COUPLE GUIDELINES TESTING Test the contract, not the implementation Prefer fakes over mocks/stubs Test your fakes with the same suite as the real things

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TESTING

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TESTING index.schedule(Path("/hello"))

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TESTING index.schedule(Path("/hello")) val file = fakeFile("hello world", "/hello/world") imageSource.uploadFile(file.fileData) !*>

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TESTING index.schedule(Path("/hello")) val file = fakeFile("hello world", "/hello/world") imageSource.uploadFile(file.fileData) !*> !*> indexer.search("hello").compile.toList

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TESTING index.schedule(Path("/hello")) val file = fakeFile("hello world", "/hello/world") imageSource.uploadFile(file.fileData) !*> !*> indexer.search("hello").compile.toList { }.map { results !=> expect(results !== List(file.fileDocument)) }

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TESTING Await blogpost for more ;) index.schedule(Path("/hello")) val file = fakeFile("hello world", "/hello/world") imageSource.uploadFile(file.fileData) !*> !*> indexer.search("hello").compile.toList { }.map { results !=> expect(results !== List(file.fileDocument)) }

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SUMMARY

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TIPS

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TIPS Use Resource + IO for stateful dependencies

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TIPS Use Resource + IO for stateful dependencies Define clear responsibilities for modules

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TIPS Use Resource + IO for stateful dependencies Define clear responsibilities for modules Design for replacement

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TIPS Use Resource + IO for stateful dependencies Define clear responsibilities for modules Design for replacement Look for abstractions

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TIPS Use Resource + IO for stateful dependencies Define clear responsibilities for modules Design for replacement Look for abstractions Prototype early

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TIPS Use Resource + IO for stateful dependencies Define clear responsibilities for modules Design for replacement Look for abstractions Prototype early Draw some diagrams, see if you have too many arrows ;)

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LEARN MORE Check out the sources: github.com/kubukoz/dropbox-demo Read "Practical FP in Scala" by Gabriel Volpe github.com/scala-steward-org/scala-steward github.com/branchtalk-io/backend github.com/kubukoz/spotify-next github.com/pitgull/pitgull leanpub.com/pfp-scala

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THANK YOU 📰 blog.kubukoz.com 🐦 @kubukoz Slides: speakerdeck.com/kubukoz Code: git.io/JONEj Find me on YouTube! (yt.kubukoz.com)