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Ragunath Jawahar @ragunathjawahar https://ragunath.xyz Building Robust Software Episode 2: Exceptions, side-e ff ects and boundaries

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Why? • People • Code • Process • Tooling

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Why? • People • Code • Process • Tooling

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Code • Less bureaucracy • Simple ideas • Starting point to hone your craft

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Recap • Robustness principle + tweaked version • Algebraic data types with Kotlin’s sealed classes • Total & partial functions • Using types, • To prevent defensive programming overhead • To avoid primitive obsession

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Episode 1 https://bit.ly/30zwgTy

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Pendulum swings

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// Primitive obsession val password: String = "super secret stuff!" PasswordValidator.validate(password) // Encapsulated type val password: Password = Password("super secret stuff!") password.validate(password)

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// Primitive obsession val password: String = "super secret stuff!" PasswordValidator.validate(password) // Encapsulated type val password: Password = Password("super secret stuff!") password.validate(password)

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// Primitive obsession val password: String = "super secret stuff!" PasswordValidator.validate(password) // Encapsulated type val password: Password = Password("super secret stuff!") password.validate(password)

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// Primitive obsession val password: String = "super secret stuff!" PasswordValidator.validate(password) // Encapsulated type val password: Password = Password("super secret stuff!") password.validate(password)

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Pendulum swings example Primitive obsession • Built-in operators (+, -, !, & & , || , [], etc.,) • value or inline classes • ORM and serialisation/deserialisation libraries • Don’t have enough clarity about the problem domain

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Exceptions

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https://docs.oracle.com/javase/tutorial/essential/exceptions/de fi nition.html The term exception is shorthand for the phrase “exceptional event.”

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Treat exceptions as first class citizens.

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Exceptions Anti-patterns

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try { / / make a network call } catch (e: IOException) { / / Gulp !! 🥤 } catch (e: HttpRetryException) { / / Delicious 😋 } 1. Swallowing exceptions

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try { / / make a network call } catch (e: Exception) { showSomethingWentWrong() } 2. Catch-all

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private fun isNumber(candidate: String): Boolean { return try { Integer.parseInt(candidate) true } catch (e: NumberFormatException) { false } } 3. Using exceptions for control flow

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… Result "xyz.ragunath.benchmark.ReturnValues.returnValueTest": 224747599.794 ±(99.9%) 1251327.222 ops/s [Average] (min, avg, max) = (220755995.007, 224747599.794, 227672182.229), stdev = 1670486.041 CI (99.9%): [223496272.572, 225998927.016] (assumes normal distribution) … Result "xyz.ragunath.benchmark.StackTraces.stackTraceTest": 5830.292 ±(99.9%) 31.441 ops/s [Average] (min, avg, max) = (5676.655, 5830.292, 5889.489), stdev = 41.973 CI (99.9%): [5798.851, 5861.733] (assumes normal distribution) …

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… Result "xyz.ragunath.benchmark.ReturnValues.returnValueTest": 224747599.794 ±(99.9%) 1251327.222 ops/s [Average] (min, avg, max) = (220755995.007, 224747599.794, 227672182.229), stdev = 1670486.041 CI (99.9%): [223496272.572, 225998927.016] (assumes normal distribution) … Result "xyz.ragunath.benchmark.StackTraces.stackTraceTest": 5830.292 ±(99.9%) 31.441 ops/s [Average] (min, avg, max) = (5676.655, 5830.292, 5889.489), stdev = 41.973 CI (99.9%): [5798.851, 5861.733] (assumes normal distribution) …

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… Result "xyz.ragunath.benchmark.ReturnValues.returnValueTest": 224747599.794 ±(99.9%) 1251327.222 ops/s [Average] (min, avg, max) = (220755995.007, 224747599.794, 227672182.229), stdev = 1670486.041 CI (99.9%): [223496272.572, 225998927.016] (assumes normal distribution) … Result "xyz.ragunath.benchmark.StackTraces.stackTraceTest": 5830.292 ±(99.9%) 31.441 ops/s [Average] (min, avg, max) = (5676.655, 5830.292, 5889.489), stdev = 41.973 CI (99.9%): [5798.851, 5861.733] (assumes normal distribution) …

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… Result "xyz.ragunath.benchmark.ReturnValues.returnValueTest": 224747599.794 ±(99.9%) 1251327.222 ops/s [Average] (min, avg, max) = (220755995.007, 224747599.794, 227672182.229), stdev = 1670486.041 CI (99.9%): [223496272.572, 225998927.016] (assumes normal distribution) … Result "xyz.ragunath.benchmark.StackTraces.stackTraceTest": 5830.292 ±(99.9%) 31.441 ops/s [Average] (min, avg, max) = (5676.655, 5830.292, 5889.489), stdev = 41.973 CI (99.9%): [5798.851, 5861.733] (assumes normal distribution) …

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… Result "xyz.ragunath.benchmark.ReturnValues.returnValueTest": 224747599.794 ±(99.9%) 1251327.222 ops/s [Average] (min, avg, max) = (220755995.007, 224747599.794, 227672182.229), stdev = 1670486.041 CI (99.9%): [223496272.572, 225998927.016] (assumes normal distribution) … Result "xyz.ragunath.benchmark.StackTraces.stackTraceTest": 5830.292 ±(99.9%) 31.441 ops/s [Average] (min, avg, max) = (5676.655, 5830.292, 5889.489), stdev = 41.973 CI (99.9%): [5798.851, 5861.733] (assumes normal distribution) …

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… Result "xyz.ragunath.benchmark.ReturnValues.returnValueTest": 224747599.794 ±(99.9%) 1251327.222 ops/s [Average] (min, avg, max) = (220755995.007, 224747599.794, 227672182.229), stdev = 1670486.041 CI (99.9%): [223496272.572, 225998927.016] (assumes normal distribution) … Result "xyz.ragunath.benchmark.StackTraces.stackTraceTest": 5830.292 ±(99.9%) 31.441 ops/s [Average] (min, avg, max) = (5676.655, 5830.292, 5889.489), stdev = 41.973 CI (99.9%): [5798.851, 5861.733] (assumes normal distribution) …

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for (candidate in potentialNumbers) { if (isNumber(candidate)) { println("It's a number!") } else { println("NaN") } } 3. Using exceptions for control flow (contd…)

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Tests

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assertThat(false).isTrue() Tests

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Exception in thread "main" expected to be true at CanaryTestKt.main(CanaryTest.kt:15) at CanaryTestKt.main(CanaryTest.kt) Tests

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if (balance < requestedAmount) { throw Exception("Meh… 😒") } 4. Throwing Exception

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Exception handling in Kotlin • No checked exceptions • try is an expression

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var guardDogResult: Result try { guardDogResult = getGuardDog() } catch (e: NoGuardDogException) { guardDogResult = Result.failure(e) } try expression

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val guardDogResult: Result = try { getGuardDog() } catch (e: NoGuardDogException) { Result.failure(e) } try expression

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Exception handling patterns

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1. Tester-Doer pattern • Tester - a member that is used to test a condition • Doer - a member that performs a potentially throwing operation

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stack.pop()

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java.util.EmptyStackException at java.base/java.util.Stack.peek(Stack.java:102) at java.base/java.util.Stack.pop(Stack.java:84) at MainKt.main(Main.kt:8) at MainKt.main(Main.kt)

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if (stack.isNotEmpty()) { stack.pop() }

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2. Try-Parse pattern • Unvalidated input data • tryParse - veri fi es if the input is valid and can be parsed • parse - performs the actual parsing

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if (SerialNumber.tryParse(possiblySerialNumber)) { val serialNumber = SerialNumber.parse(possiblySerialNumber) save(serialNumber) }

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3. Converting exceptions to values • Non-trivial exceptional conditions • Exception has domain meaning • Has to be communicated to the consumer • Use Kotlin’s sealed classes, data classes and object to provide meaning

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Registering a new user from a client • Successfully creates a user account • Email already registered • Validation errors • Connection errors • 5xx server errors • Unknown errors

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Registering a new user from a client • Successfully creates a user account • Email already registered • Validation errors • Connection errors • 5xx server errors • Unknown errors

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SignupResponse.kt sealed class SignupResponse { data class AccountCreated(val userId: String) : SignupResponse() data class EmailAlreadyRegistered(val registrationEmail: String) : SignupResponse() data class InputValidationFailed(val errors: List) : SignupResponse() object ConnectionError : SignupResponse() data class ServerError(val statusCode: Int) : SignupResponse() data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse() }

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Usage val response = accountsApi.signup("John Doe", "[email protected]") when (response) { is AccountCreated -> saveUserId(response.userId) is EmailAlreadyRegistered - > showEmailAlreadyRegistered(response.registrationEmail) is InputValidationFailed -> showValidationErrors(response.errors) ConnectionError -> showCheckConnectionMessage() is ServerError, is UnknownError -> showTryAgainInSometimeMessage() }

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Exceptions summary • Exception handling summary • Kotlin’s try expression • Tester-Doer pattern • Try-Parse pattern • Converting exceptions to values

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Boundaries

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Objective • Find attendees that have RSVP’d “Yes” or “Maybe” • Remind them about an upcoming event

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { val attendees = dao.getAttendees() val potentialAttendees = attendees .filter { it.rsvp == Rsvp.YES | | it.rsvp == Rsvp.MAYBE } potentialAttendees .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { val attendees = dao.getAttendees() val potentialAttendees = attendees .filter { it.rsvp == Rsvp.YES | | it.rsvp == Rsvp.MAYBE } potentialAttendees .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { val attendees = dao.getAttendees() val potentialAttendees = attendees .filter { it.rsvp == Rsvp.YES | | it.rsvp == Rsvp.MAYBE } potentialAttendees .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { val attendees = dao.getAttendees() val potentialAttendees = attendees .filter { it.rsvp == Rsvp.YES | | it.rsvp == Rsvp.MAYBE } potentialAttendees .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { val attendees = dao.getAttendees() val potentialAttendees = attendees .filter { it.rsvp == Rsvp.YES | | it.rsvp == Rsvp.MAYBE } potentialAttendees .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { val attendees = dao.getAttendees() val potentialAttendees = attendees .filter { it.rsvp == Rsvp.YES | | it.rsvp == Rsvp.MAYBE } potentialAttendees .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { val attendees = dao.getAttendees() val potentialAttendees = attendees .filter { it.rsvp == Rsvp.YES | | it.rsvp == Rsvp.MAYBE } potentialAttendees .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { val attendees = dao.getAttendees() val potentialAttendees = attendees .filter { it.rsvp == Rsvp.YES | | it.rsvp == Rsvp.MAYBE } potentialAttendees .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { val attendees = dao.getAttendees() val potentialAttendees = attendees .filter { it.rsvp == Rsvp.YES | | it.rsvp == Rsvp.MAYBE } potentialAttendees .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { val attendees = dao.getAttendees() val potentialAttendees = attendees .filter { it.rsvp == Rsvp.YES | | it.rsvp == Rsvp.MAYBE } potentialAttendees .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { val attendees = dao.getAttendees() val potentialAttendees = attendees .filter { it.rsvp == Rsvp.YES | | it.rsvp == Rsvp.MAYBE } potentialAttendees .onEach { mailer.mail(it) } } }

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Values as boundaries

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fun List.attending(): List { return filter { it.rsvp == Rsvp.YES | | it.rsvp == Rsvp.MAYBE } }

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fun List.attending(): List { return filter { it.rsvp == Rsvp.YES | | it.rsvp == Rsvp.MAYBE } }

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fun List.attending(): List { return filter { it.rsvp == Rsvp.YES | | it.rsvp == Rsvp.MAYBE } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { dao.getAttendees() .attending() .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { dao.getAttendees() .attending() .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { dao.getAttendees() .attending() .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { dao.getAttendees() .attending() .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { dao.getAttendees() .attending() .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { dao.getAttendees() .attending() .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { dao.getAttendees() .attending() .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { dao.getAttendees() .attending() .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { dao.getAttendees() .attending() .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { dao.getAttendees() .attending() .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { dao.getAttendees() .attending() .onEach { mailer.mail(it) } } }

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { dao.getAttendees() .attending() .onEach { mailer.mail(it) } } }

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Testing (Isolated for logic & Integrated for effects) • No attendees • All RSVP’d “Yes” • All RSVP’d “No” • All RSVP’d “Maybe” • Mix of “Yes”, “No”, “Maybe” • Query database, Send email

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { val attendees = dao.getAttendees() val potentialAttendees = attendees .filter { it.rsvp == Rsvp.YES | | it.rsvp == Rsvp.MAYBE } potentialAttendees .onEach { mailer.mail(it) } } } Testing 😱

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Testing (Isolated for logic & Integrated for effects) • No attendees • All RSVP’d “Yes” • All RSVP’d “No” • All RSVP’d “Maybe” • Mix of “Yes”, “No”, “Maybe” • Query database, Send email

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class EventManager( private val dao: EventDao, private val mailer: EventMailer ) { fun sendReminder() { dao.getAttendees() .attending() .onEach { mailer.mail(it) } } } Testing 🥳

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Core • Path + • Dependencies - • Isolated tests Shell • Path - • Dependencies + • Integrated tests

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https://github.com/redgreenio/ fl uid

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Value as boundaries in the wild • HTTP • IPC • RPC • Actor-based systems • Event-driven systems • Message queues • Event streams

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Architectures • Functional core & imperative shell • Hexagonal architecture • Ports & adapters • Redux

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References • https://docs.oracle.com/javase/tutorial/essential/exceptions/index.html • https://kotlinlang.org/docs/exceptions.html • https://radio-weblogs.com/0122027/stories/2003/04/01/JavasCheckedExceptionsWereAMistake.html • https://www.artima.com/articles/the-trouble-with-checked-exceptions • https://www.destroyallsoftware.com/talks/boundaries • https://github.com/spotify/mobius • https://github.com/redgreenio/fluid

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Questions? @ragunathjawahar • https: / / ragunath.xyz