Dynamic Code With Zipline

Video: [coming soon]

As products grow, teams tend to move business logic to the backend. Keeping clients dumb avoids duplication and allows behavior changes without app releases. But it comes with significant downsides: limited interactivity, difficult development, and inflexible APIs.

Zipline is a new library from Cash App that takes a new approach. Instead of moving logic to the backend, Zipline runs dynamic Kotlin/JS code in your Android and iOS apps. It lets you ship behavior changes without an app release!

This talk advises when to use dynamic code and how to adopt it in your apps. It also goes deep on Zipline internals:

- Interface bridging
- Coroutines & Flows
- Fast launches
- Debugging features

If your apps are getting dumber, or you’re using server-driven UI, don’t miss this talk for a great alternative.

Transcript

1. @jakewha on
   @jessewilson
   https://github.com/cashapp/zipline/
   Dynamic Code
   with Zipline
   

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2. Dynamic Code
   

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3. SEPT
   2022
   DEC
   2022
   MAR
   2023
   JUNE
   2023
   SEPT
   2023
   

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4. CHANGE
   PRICINGe
   SEPT
   2022
   DEC
   2022
   MAR
   2023
   JUNE
   2023
   SEPT
   2023
   

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5. CHANGE
   PRICINGe
   BECOMEe
   AD-FUNDED
   SEPT
   2022
   DEC
   2022
   MAR
   2023
   JUNE
   2023
   SEPT
   2023
   

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6. CHANGE
   PRICINGe
   LAUNCHe
   ‘STORIES’
   PIVOT
   TO VIDEOe
   BECOMEe
   AD-FUNDED
   ACQUIRED
   BY GOOGLE
   NEW PRIVACY
   LAWe
   SEPT
   2022
   DEC
   2022
   MAR
   2023
   JUNE
   2023
   SEPT
   2023
   

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7. We Don’t Know The Future
   • So our apps prepare for many possibilities
   • Feature flags
   • Server-driven UI
   • Loose & Lenient APIs
   

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8. Backends are Stuck in the Past
   • Testing old client versions? So much work
   • Changing an API? So much work
   • Testing server-driven UI? So much work
   SEPT
   2021
   DEC
   2021
   MAR
   2022
   JUNE
   2022
   SEPT
   2022
   

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9. The Web Solved This in the ’90s
   Every user is always on the latest version
   It’s awesome for customers, engineers, velocity
   N
   

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10. Demo
    

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11. JS
    

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12. QuickJS
    • An embeddable JavaScript engine
    • 65,560 lines of C
    • 1.2 MiB native library in your APK
    • Authored by Fabrice Bellard and Charlie Gordon
    

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13. JAVASCRIPT SOURCE
    function sayHello() {
    return ['Hello', 'JavaScript']
    .join(' ');
    }
    IN:
    QuickJS
    

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14. JAVASCRIPT SOURCE
    function sayHello() {
    return ['Hello', 'JavaScript']
    .join(' ');
    }
    hello.js:1: function: sayHello
    mode: strict
    stack_size: 3
    opcodes:
    push_atom_value "Hello"
    push_atom_value "JavaScript"
    array_from 2
    get_field2 join
    push_atom_value " "
    tail_call_method 1
    IN: OUT:
    QUICKJS BYTECODE
    QuickJS Compiler
    

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15. hello.js:1: function: sayHello
    mode: strict
    stack_size: 3
    opcodes:
    push_atom_value "Hello"
    push_atom_value "JavaScript"
    array_from 2
    get_field2 join
    push_atom_value " "
    tail_call_method 1
    IN: OUT:
    QUICKJS BYTECODE
    "Hello JavaScript"
    EVALUATION RESULT
    QuickJS Interpreter
    

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16. JAVASCRIPT SOURCE
    function sayHello() {
    return ['Hello', 'JavaScript']
    .join(' ');
    }
    IN: OUT:
    "Hello JavaScript"
    EVALUATION RESULT
    QuickJS
    

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17. JNI Bindings for QuickJS
    • JNI: Java Native Inte ace
    • Lets you use C and C++ libraries from the JVM or Android!
    

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18. https://speakerdeck.com/swankjesse/jni-hello-world
    

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19. @Test fun runJavaScript() {
    val quickJs = QuickJs.create()
    assertEquals(
    "Hello JavaScript",
    quickJs.evaluate("['Hello', 'JavaScript'].join(' ')"),
    )
    quickJs.close()
    }
    

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20. Kotlin/JS
    • Run Kotlin in your browser
    • Or any modern JavaScript engine, like QuickJS
    

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21. Kotlin/JS is Kotlin
    • Syntax: nullability, data classes, named & default parameters,
    extension functions, annotations, lambdas
    • Stdlib: strings, collections, transforms, sequences, ranges,
    regexes, firstNotNullOfOrNull
    • Tools: IntelliJ, Gradle
    • Libraries: Compose, Coroutines, Serialization, Okio
    

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22. Kotlin/JS is JS
    • Syntax: dynamic, js("...")
    • Stdlib: Console.log, JSON.stringify()
    • Tools: Webpack, Terser
    • Libraries: either NPM or Kotlin/Multiplatform
    

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23. package com.example
    @JsExport
    fun sayHello(): String {
    return listOf("Hello", "Kotlin/JS")
    .joinToString(separator = " ")
    }
    src / jsMain / kotlin / com / example / hello.kt
    

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24. @Test fun sayHello() {
    val quickJs = QuickJs.create()
    quickJs.loadJsModule("./kotlin-kotlin-stdlib-js-ir.js")
    quickJs.loadJsModule("./hello-kotlin-js.js")
    assertEquals(
    "Hello Kotlin/JS",
    quickJs.evaluate(
    "require('./hello-kotlin-js.js').com.example.sayHello()"
    )
    )
    quickJs.close()
    }
    src / jvmTest / kotlin / com / example / HelloTest.kt
    

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25. Inte ace Bridging
    

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26. Idea
    • Running Kotlin/JS in your app is neat, but clumsy
    • What if it worked like Retrofit?
    

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27. https://jakewharton.com/using-kotlin-js-and-native-on-android/
    

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28. API
    class Zipline {
    fun bind(name: String, instance: T)
    fun take(name: String): T
    ...
    companion object {
    fun create(...): Zipline
    }
    }
    

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29. interface WorldClockService {
    fun formatTime(location: Location): String
    }x
    src / commonMain / kotlin / com / example / WorldClockService.kt
    

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30. interface WorldClockService : ZiplineService {
    fun formatTime(location: Location): String
    }x
    src / commonMain / kotlin / com / example / WorldClockService.kt
    

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31. fun prepare(zipline: Zipline) {
    zipline.bind("clock", RealWorldClockService())
    }
    class RealWorldClockService : WorldClockService {
    override fun formatTime(location: Location): String {
    ...
    }
    }
    src / jsMain / kotlin / com / example / RealWorldClockService.kt
    

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32. src / androidMain / kotlin / com / example / ClockPresenter.kt
    class ClockPresenter(
    zipline: Zipline
    ) {
    val service = zipline.take("clock")
    fun displayTime(): String {
    return service.formatTime(Location(...))
    }
    }
    

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33. Implementation
    • Zipline has a Kotlin compiler plugin
    • It generates an adapter to implement bridging
    • It rewrites bind() and take() calls to pass that adapter
    • Outbound: implements the inte ace & encodes calls as JSON
    • Inbound: decodes JSON calls and calls the real implementation
    

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34. interface WorldClockService : ZiplineService {ab
    fun formatTime(location: Location): String
    }x
    src / commonMain / kotlin / com / example / WorldClockService.kt
    

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35. companion object {
    internal object Adapter
    : ZiplineServiceAdapter() {yyy
    interface WorldClockService : ZiplineService {
    fun formatTime(location: Location): String
    src / commonMain / kotlin / com / example / WorldClockService.kt
    

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36. companion object {
    internal object Adapter
    : ZiplineServiceAdapter() {yyy
    interface WorldClockService : ZiplineService {
    fun formatTime(location: Location): String
    src / commonMain / kotlin / com / example / WorldClockService.kt
    override fun outboundService(handler: Handler) =
    object : WorldClockService {
    override fun formatTime(location: Location) =
    handler.call(this, functionIndex = 0, location)
    as String
    }
    

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37. companion object {
    internal object Adapter
    : ZiplineServiceAdapter() {yyy
    interface WorldClockService : ZiplineService {
    fun formatTime(location: Location): String
    src / commonMain / kotlin / com / example / WorldClockService.kt
    override val functions = listOf(
    object : ZiplineFunction(
    "fun formatTime(location: Location): String",
    ) {
    override fun call(
    service: WorldClockService, args: List<*>
    ) = service.formatTime(args[0] as Location)
    },
    

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38. Kotlin Serialization
    • Bridged calls use JSON as an intermediate representation
    • And kotlinx.serialization to get that JSON
    

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39. Fast-enough JSON
    fun Json.encodeToStringFast(
    serializer: KSerializer,
    value: T,
    ): String {
    val jsValue = encodeToDynamic(serializer, value)
    return JSON.stringify(jsValue)
    }
    

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40. Suspension Bridges
    interface NetworkTimeService : ZiplineService {
    }
    suspend fun now( :
    ) Instant
    

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41. Suspension Bridges
    interface NetworkTimeService : ZiplineService {
    }
    fun now( )
    Instant
    callback: SuspendCallback< >
    

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42. Pass-by-Reference
    • How do we encode a SuspendCallback as JSON?
    • x
    

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43. Pass-by-Reference
    • How do we encode a SuspendCallback as JSON?
    • By building another bridge!
    

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44. Example
    • To encode:
    1. Generate an ID
    2. Call bind(id, service)
    3. Encode the ID
    • To decode:
    1. Decode the ID
    2. Call take(id)
    3. Return the service
    

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45. Flows
    interface WorldClockService : ZiplineService {
    fun formatNow(): Flow
    }
    

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46. Flows Pass-By-Reference
    • Returning a flow? Pass-by-reference of Flow
    • Collecting a flow? Pass-by-reference back of FlowCollector
    • Emitting to a collector? Uses the suspending emit() function,
    which is implemented as pass-by-reference of SuspendCallback
    

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47. Bridging Is Powe ul...
    • Call Kotlin/JS mostly like a regular function call
    • Offer data & services to Kotlin JS using the same mechanism
    • Capable concurrency
    • You never see the JavaScript
    • You never see the JSON
    

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48. ...But There’s Gotchas!
    • You need to close() everything you take()
    • Exceptions are wrapped in a ZiplineException
    • Code versions might be different!
    • Be mindful of parameter & response data size
    

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49. Packaging
    

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50. @Test fun sayHello() {
    val quickJs = QuickJs.create()
    quickJs.loadJsModule("./kotlin-kotlin-stdlib-js-ir.js")
    quickJs.loadJsModule("./hello-kotlin-js.js")
    assertEquals(
    "Hello Kotlin/JS",
    quickJs.evaluate(
    "require('./hello-kotlin-js.js').com.example.sayHello()"
    )
    )
    quickJs.close()
    }
    src / jvmTest / kotlin / com / example / HelloTest.kt
    

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51. @Test fun sayHello() {
    val quickJs = QuickJs.create()
    quickJs.loadJsModule("./kotlin-kotlin-stdlib-js-ir.js")
    quickJs.loadJsModule("./hello-kotlin-js.js")
    assertEquals(
    "Hello Kotlin/JS",
    quickJs.evaluate(
    "require('./hello-kotlin-js.js').com.example.sayHello()"
    )
    )
    quickJs.close()
    }
    src / jvmTest / kotlin / com / example / HelloTest.kt
    YUCK
    

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52. Kotlin/JS
    Source Code
    Running in
    QuickJS
    ? ?
    ?
    

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53. Kotlin/JS
    Source Code
    Running in
    QuickJS
    .js files
    build
    Kotlin/JS
    Gradle
    Plugin
    

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54. Kotlin/JS
    Source Code
    Running in
    QuickJS
    .js files .js files
    CDN
    build
    Kotlin/JS
    Gradle
    Plugin
    Your
    Upload
    Task
    

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55. Kotlin/JS
    Source Code
    Running in
    QuickJS
    .js files .js files
    CDN
    build
    Kotlin/JS
    Gradle
    Plugin
    Your
    Upload
    Task
    Zipline
    Loader
    

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56. .js files
    build
    

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57. compileSync
    main
    productionExecutable
    kotlin
    atomicfu.js
    kotlin-stdlib.js
    kotlinx-serialization.js
    kotlinx-coroutines-core.js
    hello-kotlin-js.js
    zipline.js
    build
    

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58. build / compileSync / main / productionExecutable / kotlin / manifest.zipline.json
    {
    "modules": [
    "atomicfu.js",
    "kotlin-stdlib.js",
    "kotlinx-serialization.js",
    "kotlinx-coroutines-core.js",
    "hello-kotlin-js.js",
    "zipline.js"
    ]
    }
    

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59. Kotlin/JS
    Source Code
    Running in
    QuickJS
    .js files
    .js files +
    manifest
    CDN
    build
    Kotlin/JS
    Gradle
    Plugin
    Your
    Upload
    Task
    Zipline
    Loader
    .js files +
    manifest
    build
    Zipline
    Gradle
    Plugin
    

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60. Remember how
    QuickJS Works?
    

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61. SOURCE
    function sayHello() {
    return ['Hello', 'JavaScript']
    .join(' ');
    }
    hello.js:1: function: sayHello
    mode: strict
    stack_size: 3
    opcodes:
    push_atom_value "Hello"
    push_atom_value "JavaScript"
    array_from 2
    get_field2 join
    push_atom_value " "
    tail_call_method 1
    BYTECODE
    "Hello JavaScript"
    RESULT
    QuickJS
    Compiler
    QuickJS
    Interpreter
    

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62. .js source
    + manifest
    .js source
    + manifest
    Kotlin/JS
    Source Code
    Running in
    QuickJS
    .js files
    CDN
    build
    Kotlin/JS
    Gradle
    Plugin
    Your
    Upload
    Task
    Zipline
    Loader
    build
    Zipline
    Gradle
    Plugin
    

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63. bytecode
    + manifest
    bytecode
    + manifest
    Kotlin/JS
    Source Code
    Running in
    QuickJS
    .js files
    CDN
    build
    Kotlin/JS
    Gradle
    Plugin
    Your
    Upload
    Task
    Zipline
    Loader
    build
    Zipline
    Gradle
    Plugin
    

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64. ZiplineLoader
    • Takes the URL of a manifest
    • Returns a running Zipline (QuickJS + bridges)
    

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65. Loading Strategy
    1. Fetch a manifest
    2. Fetch all the bytecode, in parallel
    3. Load bytecode into QuickJS as it arrives
    

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66. Caching
    • We don’t update our kotlin stdlib every day
    • Only download it when it changes!
    • Same for all modules
    

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67. build / compileSync / main / productionExecutable / kotlin / manifest.zipline.json
    {
    "modules": {
    "atomicfu.js": {},
    "kotlin-stdlib.js": {},
    "kotlinx-serialization.js": {},
    "kotlinx-coroutines-core.js": {},
    "hello-kotlin-js.js": {},
    "zipline.js": {}
    }
    }
    

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68. build / compileSync / main / productionExecutable / kotlin / manifest.zipline.json
    {
    "modules": {
    "atomicfu.js": {
    "sha256": "5eb408f3363d431344133ef5"
    },
    "kotlin-stdlib.js": {
    "sha256": "f3363d431344133ef55eb408"
    },
    "kotlinx-serialization.js": {
    "sha256": "4133ef55eb408f3363d43134"
    },
    "kotlinx-coroutines-core.js": {
    "sha256": "d431344133ef55eb408f3363"
    },
    

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69. Embedded Code
    • Embed the latest bytecode + manifest in the APK
    • Minimize downloads on most launches
    • Something to fall back to when running offline
    

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70. Flows of Code
    val ziplineFlow: Flow = ziplineLoader.load(
    applicationName = "world-clock",
    manifestUrlFlow = repeatFlow(manifestUrl, 500L),
    )
    

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71. Always Fresh Code
    • In Development:
    • Load on save
    • In Production:
    • Load cached/embedded code if launched offline
    • Update to fresh code once connected
    

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72. Zipline On iOS
    

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73. Multiplatformization
    • Zipline is a regular Kotlin/Multiplatform library
    • QuickJS runs natively on iOS, no JNI required!
    

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74. Android App
    Multiplatform Libraries
    Downloadable Code
    

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75. Downloadable Code
    iOS App
    Multiplatform Libraries
    

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76. httpClient: OkHttpClient,
    fun ZiplineLoader(
    dispatcher: CoroutineDispatcher,
    manifestVerifier: ManifestVerifier,
    eventListener: EventListener = EventListener.NONE,
    nowEpochMs: () -> Long = systemEpochMsClock,
    ): ZiplineLoader
    

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77. urlSession: NSURLSession,
    fun ZiplineLoader(
    dispatcher: CoroutineDispatcher,
    manifestVerifier: ManifestVerifier,
    eventListener: EventListener = EventListener.NONE,
    nowEpochMs: () -> Long = systemEpochMsClock,
    ): ZiplineLoader
    

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78. context: Context,
    fileSystem: FileSystem,
    directory: Path,
    maxSizeInBytes: Long,
    ): ZiplineCache
    fun ZiplineCache(
    

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79. fileSystem: FileSystem,
    directory: Path,
    maxSizeInBytes: Long,
    ): ZiplineCache
    fun ZiplineCache(
    

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80. Using Zipline + MPP
    • Shared inte aces and types in commonMain
    • Implementations in hostMain, or androidMain + iosMain
    

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81. COOL THINGS
    5
    

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82. java.lang.Exception: unexpected timezone: America/Gotham
    at captureStack (runtime/coreRuntime.kt:86)
    at IllegalStateException_init (kotlin-kotlin-stdlib-js-ir.js)
    at (app.cash.worldclock/TimeFormatter.kt:98)
    at (app.cash.worldclock/WorldClockJs.kt:615)
    at (InboundService.kt:837)
    at (zipline-root-zipline.js)
    at app.cash.zipline.OutboundService.runJob(platform.kt:37)
    at app.cash.zipline.EventLoop.run(CoroutineEventLoop.kt:57)
    at j.u.concurrent.Executor.runWorker(ThreadPoolExecutor.java:1167)
    at j.u.concurrent.Executor$Worker.run(ThreadPoolExecutor.java:641)
    at java.lang.Thread.run(Thread.java:920)
    Kotlin Line Numbers
    #1
    

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83. java.lang.Exception: unexpected timezone: America/Gotham
    at captureStack (runtime/coreRuntime.kt:86)
    at IllegalStateException_init (kotlin-kotlin-stdlib-js-ir.js)
    at (app.cash.worldclock/TimeFormatter.kt:98)
    at (app.cash.worldclock/WorldClockJs.kt:615)
    at (InboundService.kt:837)
    at (zipline-root-zipline.js)
    at app.cash.zipline.OutboundService.runJob(platform.kt:37)
    at app.cash.zipline.EventLoop.run(CoroutineEventLoop.kt:57)
    at j.u.concurrent.Executor.runWorker(ThreadPoolExecutor.java:1167)
    at j.u.concurrent.Executor$Worker.run(ThreadPoolExecutor.java:641)
    at java.lang.Thread.run(Thread.java:920)
    Kotlin Line Numbers
    #1
    

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84. Kotlin Line Numbers
    #1
    bytecode
    files
    .kt files .js files
    Kotlin/JS
    Gradle
    Plugin
    Zipline
    Gradle
    Plugin
    

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85. Kotlin Line Numbers
    #1
    bytecode
    files
    .kt files
    .js files
    .js.map files
    Kotlin/JS
    Gradle
    Plugin
    Zipline
    Gradle
    Plugin
    

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86. Kotlin Line Numbers
    #1
    bytecode
    files
    .kt files
    .js files
    .js.map files
    Kotlin/JS
    Gradle
    Plugin
    Zipline
    Gradle
    Plugin
    

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87. val hprofFile = File("fibonacci.hprof")
    zipline.quickJs.startCpuSampling(hprofFile).use {
    // ...use Zipline...
    }
    Sampling Profiler
    #2
    

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88. Sampling Profiler
    #2
    

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89. java.lang.Exception: helloService created here, was not closed
    at EventListener.takeService(LoggingEventListener.kt:36)
    at Endpoint.take(Endpoint.kt:127)
    at Zipline.take(Zipline.kt:132)
    at ServiceLeakingTest.test(ServiceLeakingTest.kt:67)
    Not closed Detection
    #3
    

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90. Code Signing
    #4
    zipline {
    ...
    signingKeys {
    create("key1") {
    privateKeyHex.set("ae4737d95df505eac2424000559d0")
    algorithmId.set(Ed25519)
    }
    create("key2") {
    privateKeyHex.set("6207b6f19c9d7dfa8af31ed5d9789")
    algorithmId.set(EcdsaP256)
    }
    }
    }
    

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91. "modules": {
    "atomicfu.js": {
    "sha256": "5eb408f3363d431344133ef5"
    },
    "kotlin-stdlib.js": {
    "sha256": "f3363d431344133ef55eb408"
    },
    "kotlinx-serialization.js": {
    "sha256": "4133ef55eb408f3363d43134"
    },
    "kotlinx-coroutines-core.js": {
    "sha256": "d431344133ef55eb408f3363"
    },
    "hello-kotlin-js.js": {
    "sha256": "5eb408f3363d431344133ef5"
    Code Signing
    #4
    {
    

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92. "unsigned": {
    "signatures": {
    "key1": "0f91508b8451a8ed4eedf723f22613fe",
    "key2": "ff105ef4a5bc691614c3b8b472808e2b"
    }
    },
    "modules": {
    "atomicfu.js": {
    "sha256": "5eb408f3363d431344133ef5"
    },
    "kotlin-stdlib.js": {
    "sha256": "f3363d431344133ef55eb408"
    },
    "kotlinx-serialization.js": {
    "sha256": "4133ef55eb408f3363d43134"
    Code Signing
    #4
    {
    

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93. Code Signing
    #4
    val verifier = ManifestVerifier.Builder()
    .addEd25519("key1", "6207b6f19c9d7dfa8af3d5d".decodeHex())
    .addEcdsaP256("key2", "3041020100301306072a88c".decodeHex())
    .build()
    val ziplineLoader = ZiplineLoader(
    manifestVerifier = verifier,
    dispatcher = ...,
    httpClient = ...,
    )
    

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94. No Eval
    #5
    fun getMessage(): String {
    return eval(
    "['Hello', 'JavaScript'].join(' ')"
    ) as String
    }
    

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95. No Eval
    #5
    fun getMessage(): String {
    return eval(
    "['Hello', 'JavaScript'].join(' ')"
    ) as String
    }
    app.cash.zipline.QuickJsException: eval is not supported
    at JavaScript.(test.kt)
    at app.cash.zipline.QuickJs.execute(Native Method)
    at app.cash.zipline.QuickJs.execute(QuickJs.kt:147)
    at app.cash.zipline.QuickJs.evaluate(QuickJs.kt:111)
    at app.cash.zipline.Test.getMessage(Test.kt:123)
    

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96. COOL THINGS
    5
    Kotlin Line Numbers
    Sampling Profiler
    Not Closed Detection
    1.
    2.
    3.
    4.
    5.
    Code Signing
    No Eval
    

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97. Concerns
    

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98. Bigger Alternatives Exist
    JS ecosystem
    Works best when you go all-in
    JS ecosystem
    Strada is a rad idea
    Flutter ecosystem
    Go all-in
    React Native
    Web Views
    Flutter
    

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99. No Debugging
    • Zipline doesn’t (yet) have a step-debugger
    • You have to build for Kotlin/JVM and debug there
    

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100. Rules are Rules
     • Zipline lets you change behavior of already-released apps
     • Follow Apple’s and Google’s rules
     

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101. Powered by JavaScript?
     • But you never see the JavaScript
     • We’re gonna experiment with a Kotlin/WASM backend
     

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102. Next Steps
     

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103. Is This You?
     • Too much speculative code?
     • Too many feature flags?
     • Slow feedback loops?
     • Backwards-compatibility hell?
     

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104. Zipline Status
     • We’re close to shipping 1.0
     • In Cash App this fall!
     

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105. Thanks
     @jakewha on
     @jessewilson
     https://github.com/cashapp/zipline/
     

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