Kotlin Coroutines in Practice @ KotlinConf 2018

Let's see how Kotlin Coroutines are used to solve real-life concurrency and coordination problems. With coroutines we don't have to worry about shared mutable state and synchronization. We can solve the problems we face using a number of communicating coroutines, where each piece of state is confined to a single coroutine.

Transcript

1. elizarov@
   Roman Elizarov
   Kotlin Coroutines in
   Practice
   @relizarov
   

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2. Coroutines recap
   

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3. Coroutines are like
   light-weight threads
   

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4. suspend fun main() {
   val jobs = List(100_000) {
   GlobalScope.launch {
   delay(5000)
   print(".")
   }
   }
   jobs.forEach { it.join() }
   }
   Coroutines are like
   light-weight threads
   

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5. suspend fun main() {
   val jobs = List(100_000) {
   GlobalScope.launch {
   delay(5000)
   print(".")
   }
   }
   jobs.forEach { it.join() }
   }
   Coroutines are like
   light-weight threads
   

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6. suspend fun main() {
   val jobs = List(100_000) {
   GlobalScope.launch {
   delay(5000)
   print(".")
   }
   }
   jobs.forEach { it.join() }
   }
   Coroutines are like
   light-weight threads
   

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7. Quantity
   

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8. Quantity → Quality
   

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9. A practical challenge
   suspend fun downloadContent(location: Location): Content
   

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10. fun processReferences(refs: List)
    References
    

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11. References
    fun processReferences(refs: List) {
    for (ref in refs) {
    …
    }
    }
    

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12. References Locations
    resolve
    fun processReferences(refs: List) {
    for (ref in refs) {
    val location = ref.resolveLocation()
    …
    }
    }
    

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13. References Locations Contents
    resolve download
    fun processReferences(refs: List) {
    for (ref in refs) {
    val location = ref.resolveLocation()
    val content = downloadContent(location)
    processContent(ref, content)
    }
    }
    

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14. References Locations Contents
    resolve download
    suspend fun processReferences(refs: List) {
    for (ref in refs) {
    val location = ref.resolveLocation()
    val content = downloadContent(location)
    processContent(ref, content)
    }
    }
    

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15. References Locations Contents
    resolve download
    suspend fun processReferences(refs: List) {
    for (ref in refs) {
    val location = ref.resolveLocation()
    val content = downloadContent(location)
    processContent(ref, content)
    }
    }
    Sequential
    

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16. References Locations Contents
    resolve download
    fun processReferences(refs: List) {
    for (ref in refs) {
    val location = ref.resolveLocation()
    val content = downloadContent(location)
    processContent(ref, content)
    }
    }
    Parallel
    

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17. References Locations Contents
    resolve download
    fun processReferences(refs: List) {
    for (ref in refs) {
    val location = ref.resolveLocation()
    GlobalScope.launch {
    val content = downloadContent(location)
    processContent(ref, content)
    }
    }
    }
    Parallel
    

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18. fun processReferences(refs: List) {
    for (ref in refs) {
    val location = ref.resolveLocation()
    GlobalScope.launch {
    val content = downloadContent(location)
    processContent(ref, content)
    }
    }
    }
    Coroutines are cheap! What could go wrong?
    

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19. fun processReferences(refs: List) {
    for (ref in refs) {
    val location = ref.resolveLocation()
    GlobalScope.launch {
    val content = downloadContent(location)
    processContent(ref, content)
    }
    }
    } ref1 location1 Launch download
    ref2 location2 Launch download
    ref3
    Crash!
    Crash!
    

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20. ref1 location1 Launch download
    ref2 location2 Launch download
    ref3
    Crash!
    Crash! Leak!
    fun processReferences(refs: List) {
    for (ref in refs) {
    val location = ref.resolveLocation()
    GlobalScope.launch {
    val content = downloadContent(location)
    processContent(ref, content)
    }
    }
    }
    

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21. Structured
    concurrency
    

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22. fun processReferences(refs: List)
    

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23. suspend fun processReferences(refs: List)
    

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24. suspend fun processReferences(refs: List) =
    coroutineScope {
    …
    }
    

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25. suspend fun processReferences(refs: List) =
    coroutineScope {
    for (ref in refs) {
    val location = ref.resolveLocation()
    GlobalScope.launch {
    val content = downloadContent(location)
    processContent(ref, content)
    }
    }
    }
    

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26. suspend fun processReferences(refs: List) =
    coroutineScope {
    for (ref in refs) {
    val location = ref.resolveLocation()
    launch {
    val content = downloadContent(location)
    processContent(ref, content)
    }
    }
    }
    

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27. suspend fun processReferences(refs: List) =
    coroutineScope {
    for (ref in refs) {
    val location = ref.resolveLocation()
    launch {
    val content = downloadContent(location)
    processContent(ref, content)
    }
    }
    }
    Child
    

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28. Crash?
    suspend fun processReferences(refs: List) =
    coroutineScope {
    for (ref in refs) {
    val location = ref.resolveLocation()
    launch {
    val content = downloadContent(location)
    processContent(ref, content)
    }
    }
    }
    

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29. suspend fun processReferences(refs: List) =
    coroutineScope {
    for (ref in refs) {
    val location = ref.resolveLocation()
    launch {
    val content = downloadContent(location)
    processContent(ref, content)
    }
    }
    }
    Crash?
    

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30. suspend fun processReferences(refs: List) =
    coroutineScope {
    for (ref in refs) {
    val location = ref.resolveLocation()
    launch {
    val content = downloadContent(location)
    processContent(ref, content)
    }
    }
    }
    Crash?
    cancels
    

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31. Crash?
    cancels
    Waits for completion
    suspend fun processReferences(refs: List) =
    coroutineScope {
    for (ref in refs) {
    val location = ref.resolveLocation()
    launch {
    val content = downloadContent(location)
    processContent(ref, content)
    }
    }
    }
    

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32. suspend fun processReferences(refs: List) =
    coroutineScope {
    for (ref in refs) {
    val location = ref.resolveLocation()
    launch {
    val content = downloadContent(location)
    processContent(ref, content)
    }
    }
    }
    Never leaks jobs
    

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33. The state
    

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34. References Contents
    Download process
    

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35. Reference 1
    Content
    Reference 2
    Location
    Download process
    State
    

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36. class Downloader {
    }
    

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37. class Downloader {
    private val requested = mutableSetOf()
    }
    

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38. class Downloader {
    private val requested = mutableSetOf()
    fun downloadReference(ref: Reference) {
    val location = ref.resolveLocation()
    …
    }
    }
    

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39. class Downloader {
    private val requested = mutableSetOf()
    fun downloadReference(ref: Reference) {
    val location = ref.resolveLocation()
    if (requested.add(location)) {
    // schedule download
    }
    }
    }
    

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40. class Downloader {
    private val requested = mutableSetOf()
    fun downloadReference(ref: Reference) {
    val location = ref.resolveLocation()
    if (requested.add(location)) {
    // schedule download
    }
    // ... wait for result ...
    processContent(ref, content)
    }
    }
    

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41. class Downloader {
    private val requested = mutableSetOf()
    fun downloadReference(ref: Reference) {
    val location = ref.resolveLocation()
    if (requested.add(location)) {
    // schedule download
    }
    // ... wait for result ...
    processContent(ref, content)
    }
    }
    Concurrent
    

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42. class Downloader {
    private val requested = mutableSetOf()
    fun downloadReference(ref: Reference) {
    val location = ref.resolveLocation()
    if (requested.add(location)) {
    // schedule download
    }
    // ... wait for result ...
    processContent(ref, content)
    }
    }
    Concurrent
    

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43. class Downloader {
    private val requested = mutableSetOf()
    fun downloadReference(ref: Reference) {
    val location = ref.resolveLocation()
    if (requested.add(location)) {
    // schedule download
    }
    // ... wait for result ...
    processContent(ref, content)
    }
    }
    Shared mutable state
    

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44. class Downloader {
    private val requested = mutableSetOf()
    fun downloadReference(ref: Reference) {
    val location = ref.resolveLocation()
    if (requested.add(location)) {
    // schedule download
    }
    // ... wait for result ...
    processContent(ref, content)
    }
    }
    Shared mutable state
    Needs Synchronization
    Shared + Mutable =
    

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45. Shared
    Mutable State
    Share by
    Communicating
    

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46. Synchronization
    Primitives
    Communication
    Primitives
    

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47. classes coroutines
    

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48. launch {
    val requested = mutableSetOf()
    …
    }
    Does not share mutable state
    

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49. launch {
    val requested = mutableSetOf()
    for (ref in references) {
    val location = ref.resolveLocation()
    if (requested.add(location)) {
    // schedule download
    }
    // ... wait for result ...
    processContent(ref, content)
    }
    }
    

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50. launch {
    val requested = mutableSetOf()
    for (ref in references) {
    val location = ref.resolveLocation()
    if (requested.add(location)) {
    // schedule download
    }
    // ... wait for result ...
    processContent(ref, content)
    }
    }
    

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51. Channel
    References
    Downloader
    

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52. fun CoroutineScope.downloader(
    references: ReceiveChannel,
    ) =
    launch {
    …
    }
    

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53. fun CoroutineScope.downloader(
    references: ReceiveChannel,
    ) =
    launch {
    …
    }
    

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54. fun CoroutineScope.downloader(
    references: ReceiveChannel,
    ) =
    launch {
    …
    }
    

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55. fun CoroutineScope.downloader(
    references: ReceiveChannel,
    ) =
    launch {
    …
    }
    Convention
    

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56. fun CoroutineScope.downloader(
    references: ReceiveChannel,
    ) =
    launch {
    val requested = mutableSetOf()
    for (ref in references) {
    val location = ref.resolveLocation()
    if (requested.add(location)) {
    // schedule download
    }
    // ... wait for result ...
    processContent(ref, content)
    }
    }
    

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57. fun CoroutineScope.downloader(
    references: ReceiveChannel,
    ) =
    launch {
    val requested = mutableSetOf()
    for (ref in references) {
    val location = ref.resolveLocation()
    if (requested.add(location)) {
    // schedule download
    }
    // ... wait for result ...
    processContent(ref, content)
    }
    }
    

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58. fun CoroutineScope.downloader(
    references: ReceiveChannel,
    ) =
    launch {
    val requested = mutableSetOf()
    for (ref in references) {
    val location = ref.resolveLocation()
    if (requested.add(location)) {
    // schedule download
    }
    // ... wait for result ...
    processContent(ref, content)
    }
    }
    

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59. fun CoroutineScope.downloader(
    references: ReceiveChannel,
    ) =
    launch {
    val requested = mutableSetOf()
    for (ref in references) {
    val location = ref.resolveLocation()
    if (requested.add(location)) {
    // schedule download
    }
    // ... wait for result ...
    processContent(ref, content)
    }
    }
    

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60. fun CoroutineScope.downloader(
    references: ReceiveChannel,
    ) =
    launch {
    val requested = mutableSetOf()
    for (ref in references) {
    val location = ref.resolveLocation()
    if (requested.add(location)) {
    launch { … }
    }
    // ... wait for result ...
    processContent(ref, content)
    }
    }
    Coroutines are cheap! What could go wrong?
    

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61. fun CoroutineScope.downloader(
    references: ReceiveChannel,
    ) =
    launch {
    val requested = mutableSetOf()
    for (ref in references) {
    val location = ref.resolveLocation()
    if (requested.add(location)) {
    launch { … }
    }
    // ... wait for result ...
    processContent(ref, content)
    }
    }
    Child
    

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62. fun CoroutineScope.downloader(
    references: ReceiveChannel,
    ) =
    launch {
    val requested = mutableSetOf()
    for (ref in references) {
    val location = ref.resolveLocation()
    if (requested.add(location)) {
    launch { … }
    }
    // ... wait for result ...
    processContent(ref, content)
    }
    }
    Coroutines are cheap! But the work they do…
    

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63. Limiting
    concurrency
    

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64. Worker 1
    Worker 2
    Worker 3
    Worker N
    Worker pool
    …
    References
    Downloader
    references locations
    

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65. fun CoroutineScope.downloader(
    references: ReceiveChannel,
    locations: SendChannel
    )
    

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66. fun CoroutineScope.downloader(
    references: ReceiveChannel,
    locations: SendChannel
    ) =
    launch {
    val requested = mutableSetOf()
    for (ref in references) {
    val location = ref.resolveLocation()
    if (requested.add(location)) {
    locations.send(location)
    }
    }
    }
    

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67. fun CoroutineScope.worker(
    locations: ReceiveChannel
    )
    

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68. fun CoroutineScope.worker(
    locations: ReceiveChannel
    )
    

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69. fun CoroutineScope.worker(
    locations: ReceiveChannel
    ) =
    launch {
    for (loc in locations) {
    val content = downloadContent(loc)
    processContent(ref, content)
    }
    }
    

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70. fun CoroutineScope.worker(
    locations: ReceiveChannel
    ) =
    launch {
    for (loc in locations) {
    val content = downloadContent(loc)
    processContent(ref, content)
    }
    }
    Fan-out
    

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71. fun CoroutineScope.worker(
    locations: ReceiveChannel
    ) =
    launch {
    for (loc in locations) {
    val content = downloadContent(location)
    processContent(ref, content)
    }
    }
    

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72. fun CoroutineScope.worker(
    locations: ReceiveChannel
    ) =
    launch {
    for (loc in locations) {
    val content = downloadContent(loc)
    processContent(ref, content)
    }
    }
    

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73. fun CoroutineScope.worker(
    locations: ReceiveChannel
    ) =
    launch {
    for (loc in locations) {
    val content = downloadContent(loc)
    processContent(ref, content)
    }
    }
    

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74. Worker 1
    Worker 2
    Worker 3
    Worker N
    …
    References
    Downloader
    

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75. Worker 1
    Worker 2
    Worker 3
    Worker N
    …
    References
    Downloader
    Refs ↔ Locs
    location &
    content
    

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76. data class LocContent(val loc: Location, val content: Content)
    

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77. data class LocContent(val loc: Location, val content: Content)
    fun CoroutineScope.worker(
    locations: ReceiveChannel,
    contents: SendChannel
    )
    

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78. data class LocContent(val loc: Location, val content: Content)
    fun CoroutineScope.worker(
    locations: ReceiveChannel,
    contents: SendChannel
    ) =
    launch {
    for (loc in locations) {
    val content = downloadContent(loc)
    contents.send(LocContent(loc, content))
    }
    }
    

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79. Worker 1
    Worker 2
    Worker 3
    Worker N
    …
    References
    Downloader
    locations
    contents
    

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80. fun CoroutineScope.downloader(
    references: ReceiveChannel,
    locations: SendChannel,
    contents: ReceiveChannel
    )
    

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81. fun CoroutineScope.downloader(
    references: ReceiveChannel,
    locations: SendChannel,
    contents: ReceiveChannel
    ) =
    launch {
    val requested = mutableSetOf()
    for (ref in references) {
    val location = ref.resolveLocation()
    if (requested.add(location)) {
    locations.send(location)
    }
    }
    }
    Hmm….
    

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82. Select
    

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83. select {
    references.onReceive { ref ->
    …
    }
    contents.onReceive { (loc, content) ->
    …
    }
    }
    

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84. select {
    references.onReceive { ref ->
    …
    }
    contents.onReceive { (loc, content) ->
    …
    }
    }
    

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85. select {
    references.onReceive { ref ->
    …
    }
    contents.onReceive { (loc, content) ->
    …
    }
    }
    

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86. launch {
    val requested = mutableMapOf>()
    …
    }
    

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87. launch {
    val requested = mutableMapOf>()
    while (true) {
    select {
    references.onReceive { ref -> … }
    contents.onReceive { (loc, content) -> … }
    }
    }
    }
    

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88. launch {
    val requested = mutableMapOf>()
    while (true) {
    select {
    references.onReceive { ref -> … }
    contents.onReceive { (loc, content) -> … }
    }
    }
    }
    

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89. launch {
    val requested = mutableMapOf>()
    while (true) {
    select {
    references.onReceive { ref ->
    val loc = ref.resolveLocation()
    …
    }
    contents.onReceive { (loc, content) -> … }
    }
    }
    }
    

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90. launch {
    val requested = mutableMapOf>()
    while (true) {
    select {
    references.onReceive { ref ->
    val loc = ref.resolveLocation()
    val refs = requested[loc]
    …
    }
    contents.onReceive { (loc, content) -> … }
    }
    }
    }
    

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91. launch {
    val requested = mutableMapOf>()
    while (true) {
    select {
    references.onReceive { ref ->
    val loc = ref.resolveLocation()
    val refs = requested[loc]
    if (refs == null) {
    requested[loc] = mutableListOf(ref)
    locations.send(loc)
    }
    }
    contents.onReceive { (loc, content) -> … }
    }
    }
    }
    

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92. launch {
    val requested = mutableMapOf>()
    while (true) {
    select {
    references.onReceive { ref ->
    val loc = ref.resolveLocation()
    val refs = requested[loc]
    if (refs == null) {
    requested[loc] = mutableListOf(ref)
    locations.send(loc)
    } else {
    refs.add(ref)
    }
    }
    contents.onReceive { (loc, content) -> … }
    }
    }
    }
    

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93. launch {
    val requested = mutableMapOf>()
    while (true) {
    select {
    references.onReceive { ref ->
    val loc = ref.resolveLocation()
    val refs = requested[loc]
    if (refs == null) {
    requested[loc] = mutableListOf(ref)
    locations.send(loc)
    } else {
    refs.add(ref)
    }
    }
    contents.onReceive { (loc, content) -> … }
    }
    }
    }
    No concurrency
    No synchronization
    

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94. launch {
    val requested = mutableMapOf>()
    while (true) {
    select {
    references.onReceive { ref -> … }
    contents.onReceive { (loc, content) ->
    val refs = requested.remove(loc)!!
    for (ref in refs) {
    processContent(ref, content)
    }
    }
    }
    }
    }
    

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95. Putting it all
    together
    

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96. Worker 1
    Worker 2
    Worker 3
    Worker N
    …
    References
    Downloader
    locations
    contents
    references
    

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97. Worker 1
    Worker 2
    Worker 3
    Worker N
    …
    References
    Downloader
    locations
    contents
    references
    

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98. fun CoroutineScope.processReferences(
    references: ReceiveChannel
    )
    

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99. fun CoroutineScope.processReferences(
    references: ReceiveChannel
    )
    

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100. fun CoroutineScope.processReferences(
     references: ReceiveChannel
     ) {
     val locations = Channel()
     val contents = Channel()
     repeat(N_WORKERS) { worker(locations, contents) }
     downloader(references, locations, contents)
     }
     

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101. fun CoroutineScope.processReferences(
     references: ReceiveChannel
     ) {
     val locations = Channel()
     val contents = Channel()
     repeat(N_WORKERS) { worker(locations, contents) }
     downloader(references, locations, contents)
     }
     

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102. Worker 1
     Worker 2
     Worker 3
     Worker N
     …
     References
     Downloader
     locations
     contents
     references
     

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103. Worker 1
     Worker 2
     Worker 3
     Worker N
     …
     References
     Downloader
     locations
     contents
     references
     processReferences
     

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104. Worker 1
     Worker 2
     Worker 3
     Worker N
     …
     References
     Downloader
     locations
     contents
     references
     processReferences
     Patterns
     everywhere Worker pool
     Actor
     

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105. fun CoroutineScope.processReferences(…)
     

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106. Root
     CoroutineScope
     

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107. class SomethingWithLifecycle {
     }
     

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108. class SomethingWithLifecycle : CoroutineScope {
     }
     

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109. class SomethingWithLifecycle : CoroutineScope {
     override val coroutineContext: CoroutineContext
     get() = …
     }
     

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110. class SomethingWithLifecycle : CoroutineScope {
     private val job = Job()
     override val coroutineContext: CoroutineContext
     get() = …
     }
     

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111. class SomethingWithLifecycle : CoroutineScope {
     private val job = Job()
     fun dispose() { … }
     override val coroutineContext: CoroutineContext
     get() = …
     }
     

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112. class SomethingWithLifecycle : CoroutineScope {
     private val job = Job()
     fun close() { … }
     override val coroutineContext: CoroutineContext
     get() = …
     }
     

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113. class SomethingWithLifecycle : CoroutineScope {
     private val job = Job()
     fun close() {
     job.cancel()
     }
     override val coroutineContext: CoroutineContext
     get() = …
     }
     

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114. class SomethingWithLifecycle : CoroutineScope {
     private val job = Job()
     fun close() {
     job.cancel()
     }
     override val coroutineContext: CoroutineContext
     get() = job
     }
     

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115. class SomethingWithLifecycle : CoroutineScope {
     private val job = Job()
     fun close() {
     job.cancel()
     }
     override val coroutineContext: CoroutineContext
     get() = job + Dispatchers.Main
     }
     

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116. class SomethingWithLifecycle : CoroutineScope {
     …
     override val coroutineContext: CoroutineContext
     get() = job + Dispatchers.Main
     fun doSomething() {
     }
     }
     

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117. class SomethingWithLifecycle : CoroutineScope {
     …
     override val coroutineContext: CoroutineContext
     get() = job + Dispatchers.Main
     fun doSomething() {
     launch { … }
     }
     }
     

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118. class SomethingWithLifecycle : CoroutineScope {
     …
     override val coroutineContext: CoroutineContext
     get() = job + Dispatchers.Main
     fun doSomething() {
     processReferences(references)
     }
     }
     Never leak any coroutines
     

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119. suspend vs scope
     

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120. suspend fun downloadContent(location: Location): Content
     

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121. suspend fun downloadContent(location: Location): Content
     Does something long &
     waits for it to complete without blocking
     

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122. suspend fun downloadContent(location: Location): Content
     fun CoroutineScope.processReferences(…)
     

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123. suspend fun downloadContent(location: Location): Content
     fun CoroutineScope.processReferences(…)
     Launches new coroutines &
     quickly returns, does not wait for them
     

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124. Takeaway
     

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125. Coroutines are like
     light-weight threads
     

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126. Coroutines are NOT like
     threads
     

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127. Coroutines are NOT like
     threads
     Rethink the way you
     structure your code
     

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128. Thank you
     Any questions?
     elizarov@
     Roman Elizarov
     @relizarov
     

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