The document discusses the challenges of using reactive programming in Java, particularly focusing on the nuances of using the Reactor framework. It highlights issues like exceptions caused by emissions and encourages the use of debugging tools such as `reactordebugagent` for production scenarios. The author also shares lessons learned from practical experiences and includes code examples to illustrate common pitfalls.

1. Don’t be Homer Simpson
with your Reactor!
Sergei Egorov, Pivotal
@bsideup
https://simpsonswiki.com/wiki/File:Springfield_Nuclear_Power_Plant_6.png

2. • Staff Engineer at Pivotal
• Berlin Spring User Group co-organizer
• Testcontainers co-maintainer 🐳
About me
@bsideup

5. Remember Peter…

6. With great performance comes…
Remember Peter…

7. With great performance comes…
great PITA*.
Remember Peter…
* PITA - performance improvement to apps

8. My first experience with
reactive programming was like…

9. @bsideup

10. @bsideup

11. Mono.fromRunnable()

12. Mono.fromRunnable()
@bsideup

13. Mono.fromRunnable()
My code
@bsideup

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

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

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

17. 1 week later…

18. 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

19. 1 week of production later…

20. @bsideup

21. What could go wrong

22. Whoa #1: exceptions

23. // com/example/demo/Example.java
Flux.range(0, 5)
.single()
.subscribeOn(Schedulers.parallel())
.subscribe();
@bsideup

24. // com/example/demo/Example.java
Flux.range(0, 5)
.single()
.subscribeOn(Schedulers.parallel())
.subscribe();
java.lang.IndexOutOfBoundsException: Source emitted more than one item
at reactor.core.publisher.MonoSingle$SingleSubscriber.onNext(MonoSingle.java:129)
at reactor.core.publisher.FluxRange$RangeSubscription.fastPath(FluxRange.java:129)
at reactor.core.publisher.FluxRange$RangeSubscription.request(FluxRange.java:107)
at reactor.core.publisher.MonoSingle$SingleSubscriber.request(MonoSingle.java:94)
at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.trySchedule(MonoSubscribeOn.java:186)
at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.onSubscribe(MonoSubscribeOn.java:131)
at reactor.core.publisher.MonoSingle$SingleSubscriber.onSubscribe(MonoSingle.java:114)
at reactor.core.publisher.FluxRange.subscribe(FluxRange.java:68)
at reactor.core.publisher.MonoSingle.subscribe(MonoSingle.java:58)
at reactor.core.publisher.Mono.subscribe(Mono.java:3711)
at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.run(MonoSubscribeOn.java:123)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:84)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:37)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
@bsideup

25. // com/example/demo/Example.java
Flux.range(0, 5)
.single()
.subscribeOn(Schedulers.parallel())
.subscribe();
java.lang.IndexOutOfBoundsException: Source emitted more than one item
at reactor.core.publisher.MonoSingle$SingleSubscriber.onNext(MonoSingle.java:129)
at reactor.core.publisher.FluxRange$RangeSubscription.fastPath(FluxRange.java:129)
at reactor.core.publisher.FluxRange$RangeSubscription.request(FluxRange.java:107)
at reactor.core.publisher.MonoSingle$SingleSubscriber.request(MonoSingle.java:94)
at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.trySchedule(MonoSubscribeOn.java:186)
at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.onSubscribe(MonoSubscribeOn.java:131)
at reactor.core.publisher.MonoSingle$SingleSubscriber.onSubscribe(MonoSingle.java:114)
at reactor.core.publisher.FluxRange.subscribe(FluxRange.java:68)
at reactor.core.publisher.MonoSingle.subscribe(MonoSingle.java:58)
at reactor.core.publisher.Mono.subscribe(Mono.java:3711)
at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.run(MonoSubscribeOn.java:123)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:84)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:37)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
@bsideup

26. // com/example/demo/Example.java
Flux.range(0, 5)
.single()
.subscribeOn(Schedulers.parallel())
.subscribe();
java.lang.IndexOutOfBoundsException: Source emitted more than one item
at reactor.core.publisher.MonoSingle$SingleSubscriber.onNext(MonoSingle.java:129)
at reactor.core.publisher.FluxRange$RangeSubscription.fastPath(FluxRange.java:129)
at reactor.core.publisher.FluxRange$RangeSubscription.request(FluxRange.java:107)
at reactor.core.publisher.MonoSingle$SingleSubscriber.request(MonoSingle.java:94)
at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.trySchedule(MonoSubscribeOn.java:186)
at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.onSubscribe(MonoSubscribeOn.java:131)
at reactor.core.publisher.MonoSingle$SingleSubscriber.onSubscribe(MonoSingle.java:114)
at reactor.core.publisher.FluxRange.subscribe(FluxRange.java:68)
at reactor.core.publisher.MonoSingle.subscribe(MonoSingle.java:58)
at reactor.core.publisher.Mono.subscribe(Mono.java:3711)
at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.run(MonoSubscribeOn.java:123)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:84)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:37)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Which source?
@bsideup

27. // com/example/demo/Example.java
Flux.range(0, 5)
.single()
.subscribeOn(Schedulers.parallel())
.subscribe();
java.lang.IndexOutOfBoundsException: Source emitted more than one item
at reactor.core.publisher.MonoSingle$SingleSubscriber.onNext(MonoSingle.java:129)
at reactor.core.publisher.FluxRange$RangeSubscription.fastPath(FluxRange.java:129)
at reactor.core.publisher.FluxRange$RangeSubscription.request(FluxRange.java:107)
at reactor.core.publisher.MonoSingle$SingleSubscriber.request(MonoSingle.java:94)
at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.trySchedule(MonoSubscribeOn.java:186)
at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.onSubscribe(MonoSubscribeOn.java:131)
at reactor.core.publisher.MonoSingle$SingleSubscriber.onSubscribe(MonoSingle.java:114)
at reactor.core.publisher.FluxRange.subscribe(FluxRange.java:68)
at reactor.core.publisher.MonoSingle.subscribe(MonoSingle.java:58)
at reactor.core.publisher.Mono.subscribe(Mono.java:3711)
at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.run(MonoSubscribeOn.java:123)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:84)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:37)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Which source?
Where is “com.example.demo” package?
@bsideup

28. // com/example/demo/Example.java
Flux.range(0, 5)
.single()
.subscribeOn(Schedulers.parallel())
.subscribe();
java.lang.IndexOutOfBoundsException: Source emitted more than one item
at reactor.core.publisher.MonoSingle$SingleSubscriber.onNext(MonoSingle.java:129)
at reactor.core.publisher.FluxRange$RangeSubscription.fastPath(FluxRange.java:129)
at reactor.core.publisher.FluxRange$RangeSubscription.request(FluxRange.java:107)
at reactor.core.publisher.MonoSingle$SingleSubscriber.request(MonoSingle.java:94)
at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.trySchedule(MonoSubscribeOn.java:186)
at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.onSubscribe(MonoSubscribeOn.java:131)
at reactor.core.publisher.MonoSingle$SingleSubscriber.onSubscribe(MonoSingle.java:114)
at reactor.core.publisher.FluxRange.subscribe(FluxRange.java:68)
at reactor.core.publisher.MonoSingle.subscribe(MonoSingle.java:58)
at reactor.core.publisher.Mono.subscribe(Mono.java:3711)
at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.run(MonoSubscribeOn.java:123)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:84)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:37)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Which source?
Where is “com.example.demo” package?
Why they closed Google Inbox? 😭
@bsideup

29. Assembly vs Subscription

30. Flux.range(0, 5)
Assembly
@bsideup

31. Flux.range(0, 5)
.filter(it -> it > 3)
Assembly
@bsideup

32. Flux.range(0, 5)
.filter(it -> it > 3)
.map(it -> it.toString())
Assembly
@bsideup

33. Flux.range(0, 5)
.filter(it -> it > 3)
.map(it -> it.toString())
.single()
Assembly
@bsideup

34. Flux.range(0, 5)
.filter(it -> it > 3)
.map(it -> it.toString())
.single()
.subscribeOn(Schedulers.parallel())
Assembly
@bsideup

35. Flux.range(0, 5)
.filter(it -> it > 3)
.map(it -> it.toString())
.single()
.subscribeOn(Schedulers.parallel())
.subscribe()
Assembly
@bsideup

36. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Subscription
current thread parallel-1
@bsideup

37. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Subscription
🐈
current thread parallel-1
* 🐈 = subscriber @bsideup

38. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Subscription
current thread parallel-1
🐈
@bsideup

39. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Subscription
current thread parallel-1
🐈
@bsideup

40. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Subscription
current thread parallel-1
🐈
@bsideup

41. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Subscription
current thread parallel-1
🐈
@bsideup

42. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Subscription
current thread parallel-1
🐈.onSubscribe( 🐟)
* 🐟= subscription @bsideup

43. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Subscription
current thread parallel-1
🐟
@bsideup

44. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Subscription
current thread parallel-1
🐟
@bsideup

45. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Subscription
current thread parallel-1
🐟
@bsideup

46. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Subscription
current thread parallel-1
🐟
@bsideup

47. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Subscription
current thread parallel-1
🐟.request(max)
@bsideup

48. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Subscription
current thread parallel-1
🐟.request(max)
@bsideup

49. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Subscription
current thread parallel-1
🐟.request(max)
@bsideup

50. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Subscription
current thread parallel-1
🐟.request(max)
@bsideup

51. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Subscription
current thread parallel-1
🐟.request(max)
@bsideup

52. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Subscription
current thread parallel-1
🐟.request(max)
@bsideup

53. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Execution
current thread parallel-1
🐈.onNext(0)
@bsideup

54. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Execution
current thread parallel-1
🐈.onNext(0)
@bsideup

55. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Execution
current thread parallel-1
🐈.onNext(0)
@bsideup

56. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Execution
current thread parallel-1
🐈.onNext(0)
@bsideup

57. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Execution
current thread parallel-1
🐈.onNext(0)
@bsideup

58. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Execution
current thread parallel-1
🐈.onNext(1)
@bsideup

59. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Execution
current thread parallel-1
🐈.onNext(1)
@bsideup

60. FluxRange
FluxFilter
FluxMap
MonoSingle
MonoSubscribeOn
Execution
current thread parallel-1
🐈.onNext(1)
💥java.lang.IndexOutOfBoundsException: Source emitted more than one item
at reactor.core.publisher.MonoSingle$SingleSubscriber.onNext(Mono
at reactor.core.publisher.FluxRange$RangeSubscription.fastPath(Flux
at reactor.core.publisher.FluxRange$RangeSubscription.request(FluxR
at reactor.core.publisher.MonoSingle$SingleSubscriber.request(Mono
at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.
at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.
at reactor.core.publisher.MonoSingle$SingleSubscriber.onSubscribe(M
at reactor.core.publisher.FluxRange.subscribe(FluxRange.java:68)
at reactor.core.publisher.MonoSingle.subscribe(MonoSingle.java:58)
at reactor.core.publisher.Mono.subscribe(Mono.java:3711)
at reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.
@bsideup

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

62. How ReactorDebugAgent works?
@bsideup

63. // com/example/demo/Example.java
Flux.range(0, 5)
.single()
.subscribeOn(Schedulers.parallel())
.subscribe();
@bsideup

64. // com/example/demo/Example.java
Flux.range(0, 5)
.single()
.subscribeOn(Schedulers.parallel())
.subscribe();
@bsideup

65. // com/example/demo/Example.java
Flux.range(0, 5)
.checkpoint("Example.java:16")
.single()
.checkpoint("Example.java:17")
.subscribeOn(Schedulers.parallel())
.checkpoint(“Example.java:18")
.subscribe();
@bsideup

66. 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

67. Whoa #2: Blocking calls!
https://blog.tfd.co.uk/2010/10/15/jackrabbit-performance/ @bsideup

68. How Reactor schedules tasks
@bsideup

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

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

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

72. Mono.delay(ofSeconds(1))
Mono.delay(ofSeconds(1), Schedulers.parallel())
💡Every non-instant operation
runs on the parallel scheduler
by default!
@bsideup

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

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

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

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

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

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

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

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

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

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

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

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

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

86. parallel-1
parallel-2
parallel-3
parallel-4
😭
Blocking execution
@bsideup

87. parallel-1
parallel-2
parallel-3
parallel-4
😭
Blocking execution
⚠ no more tasks scheduled
until this task returns
@bsideup

88. How to fix?

89. Project Loom

90. @bsideup

91. @bsideup

92. Use non-blocking APIs, or…

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

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

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

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

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

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

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

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

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

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

103. How BlockHound works?
@bsideup

104. public class Thread implements Runnable {
public static native void sleep(long millis);
}
@bsideup

105. public class Thread implements Runnable {
private static native void $$BlockHound$$_sleep(long millis);
public static void sleep(long millis) {
$$BlockHound$$_sleep(millis);
}
}
@bsideup

106. public class Thread implements Runnable {
private static native void $$BlockHound$$_sleep(long millis);
public static void sleep(long millis) {
reactor.BlockHoundRuntime.checkBlocking(
"java.lang.Thread",
"sleep",
/*method modifiers*/
);
$$BlockHound$$_sleep(millis);
}
}
@bsideup

107. public class Thread implements Runnable {
private static native void $$BlockHound$$_sleep(long millis);
public static void sleep(long millis) {
reactor.BlockHoundRuntime.checkBlocking(
"java.lang.Thread",
"sleep",
/*method modifiers*/
);
$$BlockHound$$_sleep(millis);
}
}
@bsideup

108. Blocking check
@bsideup

109. Blocking check
1. Get or tag the current thread
• JVMTI’s built-in mechanism to tag objects, calls nonBlockingThreadPredicate on
creation and caches the result forever
@bsideup

110. Blocking check
1. Get or tag the current thread
• JVMTI’s built-in mechanism to tag objects, calls nonBlockingThreadPredicate on
creation and caches the result forever
2. Is it non-blocking? (the predicate returns “true”)
• Very fast, O(1) check
@bsideup

111. Blocking check
1. Get or tag the current thread
• JVMTI’s built-in mechanism to tag objects, calls nonBlockingThreadPredicate on
creation and caches the result forever
2. Is it non-blocking? (the predicate returns “true”)
• Very fast, O(1) check
3. Walk the stacktrace until a frame marked with (dis)allowBlockingCallsInside
• Makes it possible to whitelist loggers and other non-harmful blocking calls
@bsideup

112. Stack
@bsideup
at sun.misc.Unsafe.park(Unsafe.java)
at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(AbstractQueuedSynchronizer.java:997)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1304)
at java.util.concurrent.CountDownLatch.await(CountDownLatch.java:231)
at com.example.demo.BlockingCodeTest.lambda$testBlockingCode$1(BlockingCodeTest.java:24)
at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:100)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.onNext(FluxSubscribeOn.java:151)
at reactor.core.publisher.FluxRange$RangeSubscription.fastPath(FluxRange.java:129)
at reactor.core.publisher.FluxRange$RangeSubscription.request(FluxRange.java:107)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.requestUpstream(FluxSubscribeOn.java:131)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.onSubscribe(FluxSubscribeOn.java:124)
at reactor.core.publisher.FluxRange.subscribe(FluxRange.java:68)
at reactor.core.publisher.Flux.subscribe(Flux.java:7800)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.run(FluxSubscribeOn.java:194)
at reactor.core.scheduler.ReactorBlockHoundIntegration$Wrapper.run(ReactorBlockHoundIntegration.java:56)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:84)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:37)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)

113. Stack
@bsideup
at sun.misc.Unsafe.park(Unsafe.java)
at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(AbstractQueuedSynchronizer.java:997)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1304)
at java.util.concurrent.CountDownLatch.await(CountDownLatch.java:231)
at com.example.demo.BlockingCodeTest.lambda$testBlockingCode$1(BlockingCodeTest.java:24)
at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:100)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.onNext(FluxSubscribeOn.java:151)
at reactor.core.publisher.FluxRange$RangeSubscription.fastPath(FluxRange.java:129)
at reactor.core.publisher.FluxRange$RangeSubscription.request(FluxRange.java:107)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.requestUpstream(FluxSubscribeOn.java:131)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.onSubscribe(FluxSubscribeOn.java:124)
at reactor.core.publisher.FluxRange.subscribe(FluxRange.java:68)
at reactor.core.publisher.Flux.subscribe(Flux.java:7800)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.run(FluxSubscribeOn.java:194)
at reactor.core.scheduler.ReactorBlockHoundIntegration$Wrapper.run(ReactorBlockHoundIntegration.java:56)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:84)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:37)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)

114. Stack
@bsideup
at sun.misc.Unsafe.park(Unsafe.java)
at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(AbstractQueuedSynchronizer.java:997)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1304)
at java.util.concurrent.CountDownLatch.await(CountDownLatch.java:231)
at com.example.demo.BlockingCodeTest.lambda$testBlockingCode$1(BlockingCodeTest.java:24)
at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:100)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.onNext(FluxSubscribeOn.java:151)
at reactor.core.publisher.FluxRange$RangeSubscription.fastPath(FluxRange.java:129)
at reactor.core.publisher.FluxRange$RangeSubscription.request(FluxRange.java:107)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.requestUpstream(FluxSubscribeOn.java:131)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.onSubscribe(FluxSubscribeOn.java:124)
at reactor.core.publisher.FluxRange.subscribe(FluxRange.java:68)
at reactor.core.publisher.Flux.subscribe(Flux.java:7800)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.run(FluxSubscribeOn.java:194)
at reactor.core.scheduler.ReactorBlockHoundIntegration$Wrapper.run(ReactorBlockHoundIntegration.java:56)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:84)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:37)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)

115. Stack
@bsideup
at sun.misc.Unsafe.park(Unsafe.java)
at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(AbstractQueuedSynchronizer.java:997)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1304)
at java.util.concurrent.CountDownLatch.await(CountDownLatch.java:231)
at com.example.demo.BlockingCodeTest.lambda$testBlockingCode$1(BlockingCodeTest.java:24)
at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:100)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.onNext(FluxSubscribeOn.java:151)
at reactor.core.publisher.FluxRange$RangeSubscription.fastPath(FluxRange.java:129)
at reactor.core.publisher.FluxRange$RangeSubscription.request(FluxRange.java:107)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.requestUpstream(FluxSubscribeOn.java:131)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.onSubscribe(FluxSubscribeOn.java:124)
at reactor.core.publisher.FluxRange.subscribe(FluxRange.java:68)
at reactor.core.publisher.Flux.subscribe(Flux.java:7800)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.run(FluxSubscribeOn.java:194)
at reactor.core.scheduler.ReactorBlockHoundIntegration$Wrapper.run(ReactorBlockHoundIntegration.java:56)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:84)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:37)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)

116. Stack
@bsideup
at sun.misc.Unsafe.park(Unsafe.java)
at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(AbstractQueuedSynchronizer.java:997)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1304)
at java.util.concurrent.CountDownLatch.await(CountDownLatch.java:231)
at com.example.demo.BlockingCodeTest.lambda$testBlockingCode$1(BlockingCodeTest.java:24)
at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:100)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.onNext(FluxSubscribeOn.java:151)
at reactor.core.publisher.FluxRange$RangeSubscription.fastPath(FluxRange.java:129)
at reactor.core.publisher.FluxRange$RangeSubscription.request(FluxRange.java:107)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.requestUpstream(FluxSubscribeOn.java:131)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.onSubscribe(FluxSubscribeOn.java:124)
at reactor.core.publisher.FluxRange.subscribe(FluxRange.java:68)
at reactor.core.publisher.Flux.subscribe(Flux.java:7800)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.run(FluxSubscribeOn.java:194)
at reactor.core.scheduler.ReactorBlockHoundIntegration$Wrapper.run(ReactorBlockHoundIntegration.java:56)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:84)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:37)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
💥
Schedulers.onScheduleHook("BlockHound", Wrapper::new);
builder.disallowBlockingCallsInside(Wrapper.class.getName(), "run");

117. Stack
@bsideup
at java.io.FileOutputStream.writeBytes(FileOutputStream.java)

118. Stack
@bsideup
at java.io.FileOutputStream.writeBytes(FileOutputStream.java)
at java.io.FileOutputStream.write(FileOutputStream.java:326)
at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:82)
at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:140)
at java.io.PrintStream.write(PrintStream.java:482)
at java.io.FilterOutputStream.write(FilterOutputStream.java:97)

119. Stack
@bsideup
at java.io.FileOutputStream.writeBytes(FileOutputStream.java)
at java.io.FileOutputStream.write(FileOutputStream.java:326)
at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:82)
at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:140)
at java.io.PrintStream.write(PrintStream.java:482)
at java.io.FilterOutputStream.write(FilterOutputStream.java:97)
at ch.qos.logback.core.joran.spi.ConsoleTarget$1.write(ConsoleTarget.java:37)
at ch.qos.logback.core.OutputStreamAppender.writeBytes(OutputStreamAppender.java:199)
at ch.qos.logback.core.OutputStreamAppender.subAppend(OutputStreamAppender.java:231)
at ch.qos.logback.core.OutputStreamAppender.append(OutputStreamAppender.java:102)
at ch.qos.logback.core.UnsynchronizedAppenderBase.doAppend(UnsynchronizedAppenderBase.java:84)
at ch.qos.logback.core.spi.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:51)
at ch.qos.logback.classic.Logger.appendLoopOnAppenders(Logger.java:270)
at ch.qos.logback.classic.Logger.callAppenders(Logger.java:257)
👍
builder.allowBlockingCallsInside(
“ch.qos.logback.classic.Logger",
“callAppenders"
);

120. 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.elastic() built-in pool if they happen rarely
@bsideup

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

122. Yeah… sure.

123. Or…

124. Are you sure?

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

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

127. 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

128. 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

129. 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

130. @bsideup

131. How to instrument Reactor

132. Schedulers.onScheduleHook(fn)
@bsideup

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

134. 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");
}
};
});

135. Hooks.onEachOperator(fn)
@bsideup
Flux.range(0, 5)
.map(it -> it.toString())
.single()
.subscribeOn(Schedulers.parallel())
.subscribe();

136. Hooks.onEachOperator(fn)
@bsideup
Flux.range(0, 5)
.map(it -> it.toString())
.single()
.subscribeOn(Schedulers.parallel())
.subscribe();

137. Hooks.onEachOperator(fn)
@bsideup
Flux.range(0, 5)
.map(it -> it.toString())
.single()
.subscribeOn(Schedulers.parallel())
.subscribe();
.transform(fn)
.transform(fn)
.transform(fn)
.transform(fn)

138. Hooks.onLastOperator(fn)
@bsideup
Flux.range(0, 5)
.map(it -> it.toString())
.single()
.subscribeOn(Schedulers.parallel())
.subscribe();

139. Hooks.onLastOperator(fn)
@bsideup
Flux.range(0, 5)
.map(it -> it.toString())
.single()
.subscribeOn(Schedulers.parallel())
.transform(fn)
.subscribe();

140. Your Reactor should be like:

141. https://9gag.com/gag/aqKWPwY

142. Takeaways
• https://github.com/reactor/reactor-tools
• Flux#checkpoint() / Flux#log()
• Hooks class
• https://github.com/reactor/BlockHound
• Async APIs - trust, but verify
@bsideup

143. @bsideup
bsideup