Presentation: Reactive: Do. Or do not. There is no try.

Transcript

1. Reactive DOs and DONTs
   Sergei Egorov, Pivotal
   @bsideup
   

   View Slide

2. • Staff Engineer at Pivotal, Project Reactor
   team
   
   • Oracle Groundbreakers Ambassador
   
   • Berlin Spring User Group co-organizer
   
   • Testcontainers co-maintainer
   About me
   @bsideup
   

   View Slide

3. My first experience with
   reactive programming was like…
   

   View Slide

4. @bsideup
   

   View Slide

5. @bsideup
   

   View Slide

6. 1 week later…
   

   View Slide

7. I agree with Heinrich Apfelmus that the essence
   of functional reactive programming is to specify
   the dynamic behavior of a value completely at
   the time of declaration.
   “Reactive Sir”
   @bsideup
   

   View Slide

8. 1 week of production later…
   

   View Slide

9. @bsideup
   

   View Slide

10. So I went to the Gods…
    

    View Slide

11. Stephane @smaldini Maldini,
    
    Project Reactor’s team lead @bsideup
    Ex-
    

    View Slide

12. He was my manager, lol
    Stephane @smaldini Maldini,
    
    Project Reactor’s team lead @bsideup
    Ex-
    

    View Slide

13. Stephane @smaldini Maldini,
    
    Project Reactor’s team lead @bsideup
    Ex-
    

    View Slide

14. The Reactive Lord has given
    unto you these fifteen…
    

    View Slide

15. oy…
    

    View Slide

16. Ten! Ten commandments,
    for all to obey.
    

    View Slide

17. Learn functional programming
    DO
    @bsideup
    

    View Slide

18. @bsideup
    Haskell?
    

    View Slide

19. @bsideup
    Haskell?
    

    View Slide

20. @bsideup
    https://twitter.com/mariofusco/status/571999216039542784
    

    View Slide

21. Lazy evaluation
    

    View Slide

22. Mono.fromRunnable()
    @bsideup
    

    View Slide

23. Mono.fromRunnable()
    My code
    @bsideup
    

    View Slide

24. Mono.fromRunnable()
    My code My code
    @bsideup
    

    View Slide

25. Mono.fromRunnable()
    My code My code My code
    Work!
    @bsideup
    

    View Slide

26. Mono.fromRunnable().subscribe()
    @bsideup
    

    View Slide

27. An ideal app
    subscribes only once
    @bsideup
    

    View Slide

28. Immutability
    

    View Slide

29. Quiz
    static void addLogging(Flux flux) {
    flux.doOnNext(it -> println("Received " + it))
    .doOnError(e -> e.printStackTrace());
    }
    // ...
    Flux items = getFlux();
    addLogging(items);
    items.subscribe();
    @bsideup
    

    View Slide

30. Quiz
    static void addLogging(Flux flux) {
    flux.doOnNext(it -> println("Received " + it))
    .doOnError(e -> e.printStackTrace());
    }
    // ...
    Flux items = getFlux();
    addLogging(items);
    items.subscribe();
    1. Will print both items and errors
    @bsideup
    

    View Slide

31. Quiz
    static void addLogging(Flux flux) {
    flux.doOnNext(it -> println("Received " + it))
    .doOnError(e -> e.printStackTrace());
    }
    // ...
    Flux items = getFlux();
    addLogging(items);
    items.subscribe();
    1. Will print both items and errors
    2. Will only print items, not errors
    @bsideup
    

    View Slide

32. Quiz
    static void addLogging(Flux flux) {
    flux.doOnNext(it -> println("Received " + it))
    .doOnError(e -> e.printStackTrace());
    }
    // ...
    Flux items = getFlux();
    addLogging(items);
    items.subscribe();
    1. Will print both items and errors
    2. Will only print items, not errors
    3. Will not print anything
    @bsideup
    

    View Slide

33. Quiz
    static void addLogging(Flux flux) {
    flux.doOnNext(it -> println("Received " + it))
    .doOnError(e -> e.printStackTrace());
    }
    // ...
    Flux items = getFlux();
    addLogging(items);
    items.subscribe();
    1. Will print both items and errors
    
    2. Will only print items, not errors
    
    3. Will not print anything
    @bsideup
    

    View Slide

34. public Mono map(Function super T, ? extends R> mapper) {
    return new MonoMap<>(this, mapper);
    }
    Immutability of reactive operators
    @bsideup
    Mono.just("Hello")
    .map(it -> it + " World!")
    .map(String::length)
    

    View Slide

35. public Mono map(Function super T, ? extends R> mapper) {
    return new MonoMap<>(this, mapper);
    }
    Immutability of reactive operators
    @bsideup
    Mono.just("Hello")
    .map(it -> it + " World!")
    .map(String::length)
    

    View Slide

36. static Flux addLogging(Flux flux) {
    return flux
    .doOnNext(it -> println("Received " + it))
    .doOnError(e -> e.printStackTrace());
    }
    // ...
    getFlux()
    .transform(flux -> addLogging(flux))
    .subscribe();
    @bsideup
    Always return…
    

    View Slide

37. static Flux addLogging(Flux flux) {
    return flux
    .doOnNext(it -> println("Received " + it))
    .doOnError(e -> e.printStackTrace());
    }
    // ...
    getFlux()
    .transform(flux -> addLogging(flux))
    .subscribe();
    … and use the returned value!
    @bsideup
    

    View Slide

38. static Flux addLogging(Flux flux) {
    return flux
    .doOnNext(it -> println("Received " + it))
    .doOnError(e -> e.printStackTrace());
    }
    // ...
    getFlux()
    .transform(flux -> addLogging(flux))
    .subscribe();
    … and use the returned value!
    @bsideup
    

    View Slide

39. Higher-order functions
    

    View Slide

40. Mono userIdMono = getUserId();
    userIdMono.subscribe(userId -> {
    Mono userMono = getUser(userId);
    userMono.subscribe(user -> {
    if (isUserValid(user)) {
    println("User: " + user);
    }
    });
    });
    // vs
    getUserId()
    .flatMap(userId -> getUser(userId))
    .filter(user -> isUserValid(user))
    .subscribe(user -> System.out.println("User: " + user));
    Higher-order functions
    @bsideup
    

    View Slide

41. Mono userIdMono = getUserId();
    userIdMono.subscribe(userId -> {
    Mono userMono = getUser(userId);
    userMono.subscribe(user -> {
    if (isUserValid(user)) {
    println("User: " + user);
    }
    });
    });
    // vs
    getUserId()
    .flatMap(userId -> getUser(userId))
    .filter(user -> isUserValid(user))
    .subscribe(user -> System.out.println("User: " + user));
    Higher-order functions
    @bsideup
    

    View Slide

42. Mono userIdMono = getUserId();
    userIdMono.subscribe(userId -> {
    Mono userMono = getUser(userId);
    userMono.subscribe(user -> {
    if (isUserValid(user)) {
    println("User: " + user);
    }
    });
    });
    // vs
    getUserId()
    .flatMap(userId -> getUser(userId))
    .filter(user -> isUserValid(user))
    .subscribe(user -> System.out.println("User: " + user));
    Higher-order functions
    @bsideup
    

    View Slide

43. Mono userIdMono = getUserId();
    userIdMono.subscribe(userId -> {
    Mono userMono = getUser(userId);
    userMono.subscribe(user -> {
    if (isUserValid(user)) {
    println("User: " + user);
    }
    });
    });
    // vs
    getUserId()
    .flatMap(userId -> getUser(userId))
    .filter(user -> isUserValid(user))
    .subscribe(user -> System.out.println("User: " + user));
    Higher-order functions
    @bsideup
    

    View Slide

44. Mono userIdMono = getUserId();
    userIdMono.subscribe(userId -> {
    Mono userMono = getUser(userId);
    userMono.subscribe(user -> {
    if (isUserValid(user)) {
    println("User: " + user);
    }
    });
    });
    // vs
    getUserId()
    .flatMap(userId -> getUser(userId))
    .filter(user -> isUserValid(user))
    .subscribe(user -> println("User: " + user));
    Higher-order functions
    @bsideup
    

    View Slide

45. Mono userIdMono = getUserId();
    userIdMono.subscribe(userId -> {
    Mono userMono = getUser(userId);
    userMono.subscribe(user -> {
    if (isUserValid(user)) {
    println("User: " + user);
    }
    });
    });
    // vs
    getUserId()
    .flatMap(userId -> getUser(userId))
    .filter(user -> isUserValid(user))
    .subscribe(user -> println("User: " + user));
    Higher-order functions
    Less scopes
    @bsideup
    

    View Slide

46. Mono userIdMono = getUserId();
    userIdMono.subscribe(userId -> {
    Mono userMono = getUser(userId);
    userMono.subscribe(user -> {
    if (isUserValid(user)) {
    println("User: " + user);
    }
    });
    });
    // vs
    getUserId()
    .flatMap(userId -> getUser(userId))
    .filter(user -> isUserValid(user))
    .subscribe(user -> println("User: " + user));
    Higher-order functions
    Composition
    @bsideup
    

    View Slide

47. Side effects are not welcomed
    

    View Slide

48. https:/
    /xkcd.com/1312/
    @bsideup
    

    View Slide

49. Avoid side effects
    Flux users = getUsers();
    return users.doOnNext(user -> {
    storeUser(user);
    });
    @bsideup
    

    View Slide

50. Avoid side effects
    Flux users = getUsers();
    return users.doOnNext(user -> {
    storeUser(user);
    });
    Side effect!
    void storeUser(User user) {
    //
    }
    @bsideup
    

    View Slide

51. Avoid side effects
    Flux users = getUsers();
    return users.concatMap(user -> storeUser(user));
    Mono storeUser(User user) {
    //
    }
    @bsideup
    

    View Slide

52. “things you do in your (async) callbacks
    should never take (significant) time”
    Stephane Maldini, 2019
    @bsideup
    

    View Slide

53. Errors are side effects too
    return fetchUsers().map(json -> {
    try {
    return decodeJSON(json);
    } catch (IOException e) {
    throw new RuntimeException(e);
    }
    });
    @bsideup
    

    View Slide

54. Errors are side effects too
    return fetchUsers().map(json -> {
    try {
    return decodeJSON(json);
    } catch (IOException e) {
    throw new RuntimeException(e);
    }
    });
    @bsideup
    

    View Slide

55. Errors are side effects too
    return fetchUsers().map(json -> {
    try {
    return decodeJSON(json);
    } catch (IOException e) {
    throw new RuntimeException(e);
    }
    });
    return fetchUsers().handle((json, sink) -> {
    try {
    sink.next(decodeJSON(json));
    } catch (IOException e) {
    sink.error(e);
    }
    });
    @bsideup
    

    View Slide

56. Errors are side effects too
    return fetchUsers().map(json -> {
    try {
    return decodeJSON(json);
    } catch (IOException e) {
    throw new RuntimeException(e);
    }
    });
    return fetchUsers().handle((json, sink) -> {
    try {
    sink.next(decodeJSON(json));
    } catch (IOException e) {
    sink.error(e);
    }
    });
    @bsideup
    

    View Slide

57. Errors are side effects too
    return fetchUsers().map(json -> {
    try {
    return decodeJSON(json);
    } catch (IOException e) {
    throw new RuntimeException(e);
    }
    });
    return fetchUsers().handle((json, sink) -> {
    try {
    sink.next(decodeJSON(json));
    } catch (IOException e) {
    sink.error(e);
    }
    }); No need to wrap with RuntimeException
    @bsideup
    

    View Slide

58. use it for heavy computations
    DON’T
    @bsideup
    

    View Slide

59. ~5x
    Overhead
    

    View Slide

60. .flatMap
    .concatMap(this::calculateHash)
    .delayUntil(this::squareRoot)
    

    View Slide

61. .flatMap
    .concatMap
    .delayUntil(this::squareRoot)
    

    View Slide

62. .flatMap
    .concatMap
    .publishOn
    

    View Slide

63. .flatMap
    .concatMap
    .publishOn
    Queue
    
    Queue
    
    Queue
    

    View Slide

64. View Slide

65. View Slide

66. check various operators
    DO
    @bsideup
    

    View Slide

67. “Reactive starter pack”
    • map
    
    • filter
    
    • flatMap
    
    • take
    
    • subscribeOn/publishOn
    @bsideup
    

    View Slide

68. @bsideup
    

    View Slide

69. @bsideup
    

    View Slide

70. “flatMaps”
    • flatMap - transform every item concurrently into a sub-stream, and join
    the current and the sub-stream.
    @bsideup
    

    View Slide

71. “flatMaps”
    • flatMap - transform every item concurrently into a sub-stream, and join
    the current and the sub-stream.
    • concatMap - same as flatMap, but one-by-one
    @bsideup
    

    View Slide

72. “flatMaps”
    • flatMap - transform every item concurrently into a sub-stream, and join
    the current and the sub-stream.
    • concatMap - same as flatMap, but one-by-one
    • switchMap - same as concatMap, but will cancel the previous sub-
    stream when a new item arrives
    @bsideup
    

    View Slide

73. “flatMaps”
    • flatMap - transform every item concurrently into a sub-stream, and join
    the current and the sub-stream.
    • concatMap - same as flatMap, but one-by-one
    • switchMap - same as concatMap, but will cancel the previous sub-
    stream when a new item arrives
    • flatMapSequential - same as flatMap, but preserves the order of sub-
    stream items according to the original stream’s order
    @bsideup
    

    View Slide

74. Which operator to use?
    

    View Slide

75. View Slide

76. block non-blocking threads
    DON’T
    @bsideup
    

    View Slide

77. block non-blocking threads
    DON’T
    @bsideup
    

    View Slide

78. How reactive frameworks
    schedule tasks
    @bsideup
    

    View Slide

79. Default thread pools
    Schedulers.parallel() - N threads, where N matches the CPUs count.
    
    Schedulers.single() - 1 thread handling all submitted tasks
    
    Schedulers.boundedElastic() - dynamic, thread caching capped pool
    @bsideup
    

    View Slide

80. Mono.delay(ofSeconds(1))
    @bsideup
    

    View Slide

81. Mono.delay(ofSeconds(1))
    Mono.delay(ofSeconds(1), Schedulers.parallel())
    @bsideup
    

    View Slide

82. Mono.delay(ofSeconds(1))
    Mono.delay(ofSeconds(1), Schedulers.parallel())
    Every non-instant operation 

    runs on the parallel scheduler

    by default!
    @bsideup
    

    View Slide

83. parallel-1
    parallel-2
    parallel-3
    parallel-4
    Non-blocking execution
    @bsideup
    

    View Slide

84. parallel-1
    parallel-2
    parallel-3
    parallel-4
    Non-blocking execution
    @bsideup
    

    View Slide

85. parallel-1
    parallel-2
    parallel-3
    parallel-4
    Non-blocking execution
    @bsideup
    

    View Slide

86. parallel-1
    parallel-2
    parallel-3
    parallel-4
    Non-blocking execution
    @bsideup
    

    View Slide

87. parallel-1
    parallel-2
    parallel-3
    parallel-4
    
    Non-blocking execution
    @bsideup
    

    View Slide

88. parallel-1
    parallel-2
    parallel-3
    parallel-4
    Blocking execution
    @bsideup
    

    View Slide

89. parallel-1
    parallel-2
    parallel-3
    parallel-4
    Blocking execution
    @bsideup
    

    View Slide

90. parallel-1
    parallel-2
    parallel-3
    parallel-4
    Blocking execution
    @bsideup
    

    View Slide

91. parallel-1
    parallel-2
    parallel-3
    parallel-4
    Blocking execution
    @bsideup
    

    View Slide

92. parallel-1
    parallel-2
    parallel-3
    parallel-4
    Blocking execution
    @bsideup
    

    View Slide

93. parallel-1
    parallel-2
    parallel-3
    parallel-4
    Blocking execution
    @bsideup
    

    View Slide

94. parallel-1
    parallel-2
    parallel-3
    parallel-4
    Blocking execution
    @bsideup
    

    View Slide

95. parallel-1
    parallel-2
    parallel-3
    parallel-4
    Blocking execution
    @bsideup
    

    View Slide

96. parallel-1
    parallel-2
    parallel-3
    parallel-4
    
    Blocking execution
    @bsideup
    

    View Slide

97. parallel-1
    parallel-2
    parallel-3
    parallel-4
    
    Blocking execution
    ⚠ no more tasks scheduled 

    until this task returns
    @bsideup
    

    View Slide

98. How to fix?
    

    View Slide

99. Project Loom
    

    View Slide

100. @bsideup
     

     View Slide

101. @bsideup
     

     View Slide

102. Use non-blocking APIs, or…
     

     View Slide

103. parallel-1
     parallel-2
     parallel-3
     parallel-4
     Custom thread pool!
     custom-1
     custom-2
     @bsideup
     

     View Slide

104. parallel-1
     parallel-2
     parallel-3
     parallel-4
     Custom thread pool!
     custom-1
     custom-2
     @bsideup
     

     View Slide

105. parallel-1
     parallel-2
     parallel-3
     parallel-4
     Custom thread pool!
     custom-1
     custom-2
     @bsideup
     

     View Slide

106. parallel-1
     parallel-2
     parallel-3
     parallel-4
     Custom thread pool!
     custom-1
     custom-2
     @bsideup
     

     View Slide

107. parallel-1
     parallel-2
     parallel-3
     parallel-4
     Custom thread pool!
     custom-1
     custom-2
     @bsideup
     

     View Slide

108. parallel-1
     parallel-2
     parallel-3
     parallel-4
     Custom thread pool!
     custom-1
     custom-2
     @bsideup
     

     View Slide

109. parallel-1
     parallel-2
     parallel-3
     parallel-4
     Custom thread pool!
     custom-1
     custom-2
     @bsideup
     

     View Slide

110. parallel-1
     parallel-2
     parallel-3
     parallel-4
     Custom thread pool!
     custom-1
     custom-2
     @bsideup
     

     View Slide

111. parallel-1
     parallel-2
     parallel-3
     parallel-4
     Custom thread pool!
     custom-1
     custom-2
     
     @bsideup
     

     View Slide

112. Demo
     http://eskipaper.com/homer-jay-simpson-cartoon.html
     

     View Slide

113. Demo outcomes
     • Blocking calls are bad, m’kay?
     
     • They may sneak into your production system
     • Use https://github.com/reactor/BlockHound to detect them
     
     • Supports multiple frameworks (Reactor, RxJava, etc)
     
     • … and maybe even Kotlin: 

     https://github.com/Kotlin/kotlinx.coroutines/issues/1031
     
     • Use a dedicated pool for the necessary blocking calls, or schedule them on
     the Schedulers.boundedElastic() built-in pool if they happen rarely
     @bsideup
     

     View Slide

114. “I use async APIs and I am safe!”
     

     View Slide

115. Yeah… sure.
     

     View Slide

116. Or…
     

     View Slide

117. Are you sure?
     

     View Slide

118. KafkaProducer#send(ProducerRecord,Callback)
     @bsideup
     

     View Slide

119. KafkaProducer#send(ProducerRecord,Callback)
     “Asynchronously send a record to a topic and invoke the provided
     callback when the send has been acknowledged.” - Javadoc
     @bsideup
     

     View Slide

120. KafkaProducer#send(ProducerRecord,Callback)
     “Asynchronously send a record to a topic and invoke the provided
     callback when the send has been acknowledged.” - Javadoc
     java.lang.Error: Blocking call! java.lang.Object#wait
     
     at reactor.BlockHound$Builder.lambda$new$0(BlockHound.java:154)
     
     at reactor.BlockHound$Builder.lambda$install$8(BlockHound.java:254)
     
     at reactor.BlockHoundRuntime.checkBlocking(BlockHoundRuntime.java:43)
     
     at java.lang.Object.wait(Object.java)
     
     at org.apache.kafka.clients.Metadata.awaitUpdate(Metadata.java:181)
     
     at org.apache.kafka.clients.producer.KafkaProducer.waitOnMetadata(KafkaProducer.java:938)
     
     at org.apache.kafka.clients.producer.KafkaProducer.doSend(KafkaProducer.java:823)
     
     at org.apache.kafka.clients.producer.KafkaProducer.send(KafkaProducer.java:803)
     @bsideup
     

     View Slide

121. KafkaProducer#send(ProducerRecord,Callback)
     “Asynchronously send a record to a topic and invoke the provided
     callback when the send has been acknowledged.” - Javadoc
     java.lang.Error: Blocking call! java.lang.Object#wait
     
     at reactor.BlockHound$Builder.lambda$new$0(BlockHound.java:154)
     
     at reactor.BlockHound$Builder.lambda$install$8(BlockHound.java:254)
     
     at reactor.BlockHoundRuntime.checkBlocking(BlockHoundRuntime.java:43)
     
     at java.lang.Object.wait(Object.java)
     
     at org.apache.kafka.clients.Metadata.awaitUpdate(Metadata.java:181)
     
     at org.apache.kafka.clients.producer.KafkaProducer.waitOnMetadata(KafkaProducer.java:938)
     
     at org.apache.kafka.clients.producer.KafkaProducer.doSend(KafkaProducer.java:823)
     
     at org.apache.kafka.clients.producer.KafkaProducer.send(KafkaProducer.java:803)
     https://issues.apache.org/jira/browse/KAFKA-3539
     @bsideup
     

     View Slide

122. long remainingWaitMs = maxWaitMs;
     long elapsed;
     // Issue metadata requests until we have metadata for the topic or maxWaitTimeMs is exceeded.
     // In case we already have cached metadata for the topic, but the requested partition is greater
     // than expected, issue an update request only once. This is necessary in case the metadata
     // is stale and the number of partitions for this topic has increased in the meantime.
     do {
     log.trace("Requesting metadata update for topic {}.", topic);
     metadata.add(topic);
     int version = metadata.requestUpdate();
     sender.wakeup();
     try {
     metadata.awaitUpdate(version, remainingWaitMs);
     } catch (TimeoutException ex) {
     // Rethrow with original maxWaitMs to prevent logging exception with remainingWaitMs
     throw new TimeoutException("Failed to update metadata after " + maxWaitMs + " ms.");
     }
     cluster = metadata.fetch();
     elapsed = time.milliseconds() - begin;
     if (elapsed >= maxWaitMs)
     throw new TimeoutException("Failed to update metadata after " + maxWaitMs + " ms.");
     if (cluster.unauthorizedTopics().contains(topic))
     throw new TopicAuthorizationException(topic);
     remainingWaitMs = maxWaitMs - elapsed;
     partitionsCount = cluster.partitionCountForTopic(topic);
     } while (partitionsCount == null);
     waitOnMetadata(record.topic(), record.partition(), maxBlockTimeMs);
     Default is “60 seconds”
     @bsideup
     

     View Slide

123. @bsideup
     

     View Slide

124. Start gradually
     DO
     @bsideup
     

     View Slide

125. class UsersController {
     UsersService usersService;
     @GetMapping("/users")
     List getUsers() {
     return usersService.getUsers().stream()
     .map(UserDTO::new)
     .collect(Collectors.toList());
     }
     }
     class UsersService {
     UsersRepository usersRepository;
     List getUsers() {
     return usersRepository.findAll();
     }
     }
     interface UsersRepository {
     List findAll();
     }
     @bsideup
     

     View Slide

126. class UsersController {
     UsersService usersService;
     @GetMapping("/users")
     List getUsers() {
     return usersService.getUsers().stream()
     .map(UserDTO::new)
     .collect(Collectors.toList());
     }
     }
     class UsersService {
     UsersRepository usersRepository;
     List getUsers() {
     return usersRepository.findAll();
     }
     }
     interface UsersRepository {
     List findAll();
     }
     @bsideup
     

     View Slide

127. class UsersController {
     UsersService usersService;
     @GetMapping("/users")
     List getUsers() {
     return usersService.getUsers().stream()
     .map(UserDTO::new)
     .collect(Collectors.toList());
     }
     }
     class UsersService {
     UsersRepository usersRepository;
     List getUsers() {
     return usersRepository.findAll();
     }
     }
     interface UsersRepository {
     Flux findAll();
     }
     @bsideup
     

     View Slide

128. class UsersController {
     UsersService usersService;
     @GetMapping("/users")
     List getUsers() {
     return usersService.getUsers().stream()
     .map(UserDTO::new)
     .collect(Collectors.toList());
     }
     }
     class UsersService {
     UsersRepository usersRepository;
     List getUsers() {
     return usersRepository.findAll();
     }
     }
     interface UsersRepository {
     Flux findAll();
     }
     @bsideup
     

     View Slide

129. class UsersController {
     UsersService usersService;
     @GetMapping("/users")
     List getUsers() {
     return usersService.getUsers().stream()
     .map(UserDTO::new)
     .collect(Collectors.toList());
     }
     }
     class UsersService {
     UsersRepository usersRepository;
     List getUsers() {
     return usersRepository.findAll()
     .collectList()
     .block();
     }
     }
     interface UsersRepository {
     Flux findAll();
     }
     @bsideup
     

     View Slide

130. class UsersController {
     UsersService usersService;
     @GetMapping("/users")
     List getUsers() {
     return usersService.getUsers().stream()
     .map(UserDTO::new)
     .collect(Collectors.toList());
     }
     }
     class UsersService {
     UsersRepository usersRepository;
     List getUsers() {
     return usersRepository.findAll()
     .collectList()
     .timeout(Mono.delay(ofSeconds(10)))
     .retry(5)
     .block();
     }
     }
     interface UsersRepository {
     Flux findAll();
     }
     @bsideup
     

     View Slide

131. class UsersController {
     UsersService usersService;
     @GetMapping("/users")
     List getUsers() {
     return usersService.getUsers().stream()
     .map(UserDTO::new)
     .collect(Collectors.toList());
     }
     }
     class UsersService {
     UsersRepository usersRepository;
     Flux getUsers() {
     return usersRepository.findAll()
     .timeout(Mono.delay(ofSeconds(10)))
     .retry(5);
     }
     }
     interface UsersRepository {
     Flux findAll();
     }
     @bsideup
     

     View Slide

132. class UsersController {
     UsersService usersService;
     @GetMapping("/users")
     List getUsers() {
     return usersService.getUsers().stream()
     .map(UserDTO::new)
     .collect(Collectors.toList());
     }
     }
     class UsersService {
     UsersRepository usersRepository;
     Flux getUsers() {
     return usersRepository.findAll()
     .timeout(Mono.delay(ofSeconds(10)))
     .retry(5);
     }
     }
     interface UsersRepository {
     Flux findAll();
     }
     @bsideup
     

     View Slide

133. class UsersController {
     UsersService usersService;
     @GetMapping("/users")
     Flux getUsers() {
     return usersService.getUsers()
     .map(UserDTO::new);
     }
     }
     class UsersService {
     UsersRepository usersRepository;
     Flux getUsers() {
     return usersRepository.findAll()
     .timeout(Mono.delay(ofSeconds(10)))
     .retry(5);
     }
     }
     interface UsersRepository {
     Flux findAll();
     }
     @bsideup
     

     View Slide

134. Before:
     class UsersController {
     UsersService usersService;
     @GetMapping("/users")
     List getUsers() {
     return usersService.getUsers()
     .stream()
     .map(UserDTO::new)
     .collect(toList());
     }
     }
     class UsersService {
     UsersRepository usersRepository;
     List getUsers() {
     return usersRepository.findAll();
     }
     }
     interface UsersRepository {
     List findAll();
     }
     class UsersController {
     UsersService usersService;
     @GetMapping("/users")
     Flux getUsers() {
     return usersService.getUsers()
     .map(UserDTO::new);
     }
     }
     class UsersService {
     UsersRepository usersRepository;
     Flux getUsers() {
     return usersRepository.findAll()
     .timeout(Mono.delay(ofSeconds(10)))
     .retry(5);
     }
     }
     interface UsersRepository {
     Flux findAll();
     }
     After:
     

     View Slide

135. use ThreadLocals
     DON’T
     @bsideup
     

     View Slide

136. Synchronous programming
     • 1 request == 1 thread
     
     • Everything is blocking, the execution continues on the same thread
     
     • ThreadLocals work just fine
     @bsideup
     

     View Slide

137. Asynchronous programming
     • 1 request will most probably be handled by multiple threads
     
     • Everything is non-blocking, an accepted request may be returned to the
     client from another thread
     
     • ThreadLocals must be propagated from one thread to another
     @bsideup
     

     View Slide

138. Solution?
     

     View Slide

139. Context!
     

     View Slide

140. Mono
     .deferWithContext(ctx -> {
     return this.getUser(ctx.get("userId"));
     })
     // Later in the framework (e.g. Spring Security)
     .subscriberContext(Context.of("userId", "bsideup"))
     .subscribe();
     @bsideup
     

     View Slide

141. Mono
     .deferWithContext(ctx -> {
     return this.getUser(ctx.get("userId"));
     })
     // Later in the framework (e.g. Spring Security)
     .subscriberContext(Context.of("userId", "bsideup"))
     .subscribe();
     @bsideup
     

     View Slide

142. Mono
     .deferWithContext(ctx -> {
     return this.getUser(ctx.get("userId"));
     })
     // Later in the framework (e.g. Spring Security)
     .subscriberContext(Context.of("userId", "bsideup"))
     .subscribe();
     @bsideup
     

     View Slide

143. Legacy?
     

     View Slide

144. Schedulers.onScheduleHook(fn)
     @bsideup
     

     View Slide

145. Schedulers.onScheduleHook(fn)
     @bsideup
     Schedulers.onScheduleHook("myHook", runnable -> {
     println("Before every scheduled runnable");
     return () -> {
     println("Before execution");
     runnable.run();
     println("After execution");
     };
     });
     

     View Slide

146. Schedulers.onScheduleHook(fn)
     @bsideup
     Schedulers.onScheduleHook("mdc", runnable -> {
     String userId = MDC.get("userId");
     return () -> {
     MDC.put("userId", userId);
     try {
     runnable.run();
     } finally {
     MDC.remove("userId");
     }
     };
     });
     

     View Slide

147. think about the resiliency
     DO
     @bsideup
     

     View Slide

148. “Error… is a signal.”
     Stephane Maldini,
     year unknown
     @bsideup
     

     View Slide

149. Resiliency with Reactor
     @bsideup
     

     View Slide

150. Resiliency with Reactor
     • .timeout(Duration) - cancel the subscription and fail if no items emitted
     @bsideup
     

     View Slide

151. Resiliency with Reactor
     • .timeout(Duration) - cancel the subscription and fail if no items emitted
     • .retry()/retryWithBackoff() - retry the subscription on failure
     @bsideup
     

     View Slide

152. Resiliency with Reactor
     • .timeout(Duration) - cancel the subscription and fail if no items emitted
     • .retry()/retryWithBackoff() - retry the subscription on failure
     • .repeatWhenEmpty() - repeat the subscription when it completes without
     values
     @bsideup
     

     View Slide

153. Resiliency with Reactor
     • .timeout(Duration) - cancel the subscription and fail if no items emitted
     • .retry()/retryWithBackoff() - retry the subscription on failure
     • .repeatWhenEmpty() - repeat the subscription when it completes without
     values
     • .defaultIfEmpty() - fallback when empty
     @bsideup
     

     View Slide

154. Resiliency with Reactor
     • .timeout(Duration) - cancel the subscription and fail if no items emitted
     • .retry()/retryWithBackoff() - retry the subscription on failure
     • .repeatWhenEmpty() - repeat the subscription when it completes without
     values
     • .defaultIfEmpty() - fallback when empty
     • .onErrorResume() - fallback on error
     @bsideup
     

     View Slide

155. Distributed?
     

     View Slide

156. https://github.com/resilience4j/resilience4j
     RateLimiter rateLimiter = RateLimiter.ofDefaults(“name");
     Mono.fromCallable(backendService::doSomething)
     .transformDeferred(RateLimiterOperator.of(rateLimiter))
     

     View Slide

157. care about the threads
     DON’T
     @bsideup
     

     View Slide

158. You may ask…
     • “How do I get the current Scheduler?”
     
     • “Why does flatMap changes the thread?”
     
     • …”Why it does not?”
     @bsideup
     

     View Slide

159. The answer is…
     

     View Slide

160. You.
     

     View Slide

161. You. Should.
     

     View Slide

162. You. Should. Not.
     

     View Slide

163. You. Should. Not. Care.
     

     View Slide

164. Prepare for day 2
     DO
     @bsideup
     

     View Slide

165. Demo
     http://eskipaper.com/homer-jay-simpson-cartoon.html
     

     View Slide

166. Demo outcomes
     • Use .checkpoint(“something”) to “mark” reactive “milestones”
     
     • Read about Hooks.onOperatorDebug()…
     
     • … but use reactor-tools’ ReactorDebugAgent (works in prod too)
     
     • https://spring.io/blog/2019/03/06/flight-of-the-flux-1-assembly-vs-
     subscription - great article from Simon Basle about the internals
     @bsideup
     

     View Slide

167. Be afraid of reactive programming ;)
     DON’T
     @bsideup
     

     View Slide

168. @bsideup
     

     View Slide

169. Spring Webflux
     @bsideup
     

     View Slide

170. Developer experience
     is one of the main focuses
     in Reactor 3.3…
     

     View Slide

171. … and we just started
     

     View Slide

172. Summary
     • Learn functional programming
     • Check various operators
     • Think about the resiliency
     • Start gradually
     • Prepare for day 2
     • Use it for heavy computations
     • Care about the threads
     • Block non-blocking threads
     • Use ThreadLocals
     • Be afraid of it ;)
     DO… DON’T…
     @bsideup
     

     View Slide

173. @bsideup
     bsideup
     

     View Slide