From iOS to Distributed SystemsMichele Titolo

Lead Software Engineer, Capital One

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8 Months Ago I Changed Teams

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MobileBUT YOU’RE A

developer

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What Are The Kinds Of Problems I Solve When Writing Mobile Apps?

‣ State

‣ Concurrency

‣ Instrumentation

‣ Testing

‣ Developer Efficiency

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Distributed Systems

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Mostly means “microservices”

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Single Responsibility

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Production Ready Microservices

‣ Stability

‣ Reliability

‣ Scalability

‣ Fault Tolerance

‣ Performance

‣ Monitoring

‣ Documentation

‣ State

‣ Concurrency

‣ Instrumentation

‣ Testing

‣ Developer Efficiency

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State

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iOS Apps Are Large And Stateful

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MassiveViewController

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How To Fix?

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Break Down Into Functional Parts

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Sounds Like A Microservice

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Inflexible Boundaries

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Language Features

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Dynamic Weakly Typed

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Why?

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The Smaller The Codebase, The Fewer Language Features Are Used

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Focus On Simplicity

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Generics: Great For Modeling State

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What If You Have No State 🤔?

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Protocols Nil Checks

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Shared State

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Singletons

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Dependency Injection To The Rescue

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Translates To Service Discovery

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Register And Route To Dependent Services

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Databases!

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Kind Of Like Singletons

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Easy Reads Restrict Writes

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Read Replicas

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Concurrency

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iOS: 1 million users each using 1 instance

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Distributed Systems: 1 million users using n instances

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🙀

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Concurrency Matters

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Don’t Block The Main Thread

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Queues, Threads, Callbacks All Matter

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GCD is 💯

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Other Languages Are Not As Sophisticated

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Instrumentation

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‣ Performance ‣ Usage

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iOS Apps Are Black Boxes

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If It’s Not Tracked, You Can’t Know

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Knowing What To Track Is Key

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Apps Behave Differently In The Wild

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Instrumentation On iOS

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Instruments.App Helps Locally

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AppStore App Analytics

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3rd Party Analytics

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Microservices

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Microservices Are Also Black Boxes

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Developers Shouldn’t Have Prod SSH Access

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Alerts Need Metrics

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Again, Local Dev Is Different

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Simulate Load In Pre-Production

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Imperfect, Imprecise

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Instrumentation Helps Surface And Diagnose Issues

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Testing

Unit

Integration

End

to End

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Testing Is Universal

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Testing On iOS

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Isolated State

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Dependency Injection

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Run Frequently

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Tests For Everything

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Microservices

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But First One Thing I Glossed Over…

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Microservices Are Infrastructure Heavy

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CICD

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If Code Goes To Prod Basically Immediately…

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Automate All The Things

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Codebase Needs Lots Of Tests

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Tests For Microservices Need…

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Isolated State

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Dependency Injection

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All The Testing Layers Covered

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Just Like On iOS

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Developer Efficiency

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Move Fast And Break Things

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Move Fast And Break Things

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Move Fast With Stable Infra

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Developers Like Shipping

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“It Works On My Machine”

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Codesigning 🙀

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Fastlane 👍

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Easy Releases ➡

Frequent Releases

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Same For Microservices

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…If Not More

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Automation ➡ Scaling

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Software Moves Fast

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To Summarize

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Making iOS Apps Requires Specialized Knowledge

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Rooted In Fundamental Concepts

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Different Tools

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You Know More Than You Think 🎉

Thank You@micheletitolo

hi@michele.io

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