1
00:00:00,320 --> 00:00:02,640
‫So now let's talk about an important concept,

2
00:00:02,640 --> 00:00:05,560
‫which is called the message visibility timeout.

3
00:00:05,560 --> 00:00:08,480
‫So when a message is polled by a consumer,

4
00:00:08,480 --> 00:00:11,350
‫it becomes invisible to other consumers.

5
00:00:11,350 --> 00:00:12,410
‫So let's take an example.

6
00:00:12,410 --> 00:00:14,830
‫We have the time going from left to right,

7
00:00:14,830 --> 00:00:18,160
‫and we have a consumer doing a ReceiveMessage request,

8
00:00:18,160 --> 00:00:22,520
‫and therefore, a message will be returned from the queue.

9
00:00:22,520 --> 00:00:25,460
‫Now the visibility timeout begins.

10
00:00:25,460 --> 00:00:28,430
‫And by default, that message visibility timeout

11
00:00:28,430 --> 00:00:30,160
‫is 30 seconds.

12
00:00:30,160 --> 00:00:32,890
‫That means that during these 30 seconds,

13
00:00:32,890 --> 00:00:35,200
‫the message has to be processed, okay?

14
00:00:35,200 --> 00:00:36,400
‫And if you do so,

15
00:00:36,400 --> 00:00:39,650
‫that means that if the same or other consumers

16
00:00:39,650 --> 00:00:42,930
‫do a message request API call,

17
00:00:42,930 --> 00:00:45,260
‫then the message will not be returned.

18
00:00:45,260 --> 00:00:47,860
‫And again another one during the timeout window,

19
00:00:47,860 --> 00:00:49,020
‫the message will not be returned.

20
00:00:49,020 --> 00:00:51,800
‫So effectively, during the visibility timeout,

21
00:00:51,800 --> 00:00:55,030
‫that message is invisible to other consumers.

22
00:00:55,030 --> 00:00:58,440
‫But after the visibility timeout is elapsed

23
00:00:58,440 --> 00:01:01,140
‫and if the message has not been deleted,

24
00:01:01,140 --> 00:01:02,560
‫then the message will be

25
00:01:02,560 --> 00:01:05,970
‫quote unquote put back into the queue

26
00:01:05,970 --> 00:01:08,990
‫and therefore, another consumer or the same consumer

27
00:01:08,990 --> 00:01:11,570
‫doing a receive message API call

28
00:01:11,570 --> 00:01:15,760
‫will receive the message again, the same message as before.

29
00:01:15,760 --> 00:01:17,870
‫So, this is something that's really important to understand.

30
00:01:17,870 --> 00:01:20,490
‫As you can see, while we receive a message,

31
00:01:20,490 --> 00:01:24,250
‫it becomes invisible during the visibility timeout period.

32
00:01:24,250 --> 00:01:27,740
‫Now, if we take a look at the same diagram,

33
00:01:27,740 --> 00:01:28,640
‫what we notice is that

34
00:01:28,640 --> 00:01:31,280
‫if we don't process a message

35
00:01:31,280 --> 00:01:34,030
‫within the visibility timeout window,

36
00:01:34,030 --> 00:01:37,370
‫then it will be processed twice potentially, right?

37
00:01:37,370 --> 00:01:40,480
‫Because it will be received by two different consumers,

38
00:01:40,480 --> 00:01:42,660
‫or twice by the same consumer.

39
00:01:42,660 --> 00:01:47,160
‫And so, if a consumer is actively processing a message

40
00:01:47,160 --> 00:01:49,560
‫but knows that it needs a bit more time

41
00:01:49,560 --> 00:01:51,900
‫to process the message because otherwise,

42
00:01:51,900 --> 00:01:54,690
‫it will go out of the visibility timeout window,

43
00:01:54,690 --> 00:01:58,370
‫there is an API called ChangeMessageVisibility.

44
00:01:58,370 --> 00:01:59,870
‫And so if a consumer knows

45
00:01:59,870 --> 00:02:02,460
‫that a message needs a bit more time to be processed

46
00:02:02,460 --> 00:02:04,620
‫and you don't want to process that message twice,

47
00:02:04,620 --> 00:02:06,090
‫then the consumer should call

48
00:02:06,090 --> 00:02:08,930
‫the ChangeMessageVisibility API to tell SQS,

49
00:02:08,930 --> 00:02:12,340
‫hey, do not make that message visible for now, okay?

50
00:02:12,340 --> 00:02:14,840
‫I just need a bit more time to process this message.

51
00:02:14,840 --> 00:02:17,840
‫And so, how do you set this message visibility timeout?

52
00:02:17,840 --> 00:02:18,673
‫Well, if you set it to

53
00:02:18,673 --> 00:02:21,720
‫a really, really high value by default, say, hours,

54
00:02:21,720 --> 00:02:23,330
‫and the consumer crashes,

55
00:02:23,330 --> 00:02:26,930
‫then it will take hours until this message reappears,

56
00:02:26,930 --> 00:02:29,730
‫re-becomes visible in your SQS queue,

57
00:02:29,730 --> 00:02:31,780
‫and that will take a lot of time.

58
00:02:31,780 --> 00:02:33,470
‫If you set it to something really, really low,

59
00:02:33,470 --> 00:02:34,760
‫like a few seconds,

60
00:02:34,760 --> 00:02:36,100
‫what happens that if the consumer

61
00:02:36,100 --> 00:02:38,360
‫doesn't end up having enough time to process that message

62
00:02:38,360 --> 00:02:39,470
‫for whatever reason,

63
00:02:39,470 --> 00:02:41,650
‫then it will be read many times by different consumers

64
00:02:41,650 --> 00:02:43,810
‫and you may get duplicate processing.

65
00:02:43,810 --> 00:02:46,880
‫So the idea is that the visibility timeout

66
00:02:46,880 --> 00:02:49,620
‫should be set to something reasonable for your application

67
00:02:49,620 --> 00:02:51,150
‫and your consumer should be programmed

68
00:02:51,150 --> 00:02:53,500
‫that if they know they need a bit more time,

69
00:02:53,500 --> 00:02:56,350
‫then they should call the ChangeMessageVisibility API

70
00:02:56,350 --> 00:02:58,810
‫to get more time and increase the timeout

71
00:02:58,810 --> 00:03:00,250
‫of that visibility window.

72
00:03:00,250 --> 00:03:01,460
‫But understanding this concept

73
00:03:01,460 --> 00:03:03,670
‫is very important from an exam perspective

74
00:03:03,670 --> 00:03:05,990
‫because there will be scenarios on that.

75
00:03:05,990 --> 00:03:07,460
‫So, let's go in the console

76
00:03:07,460 --> 00:03:10,010
‫to see how that works in practice.

77
00:03:10,010 --> 00:03:11,950
‫So I'm going to open

78
00:03:11,950 --> 00:03:16,890
‫two windows of send and receive messages

79
00:03:16,890 --> 00:03:18,920
‫just to show you how that works, so.

80
00:03:18,920 --> 00:03:22,350
‫In the first window, I'm going to enter a hello world

81
00:03:22,350 --> 00:03:26,090
‫and it's going to be sent into the queue.

82
00:03:26,090 --> 00:03:27,170
‫And if you remember,

83
00:03:27,170 --> 00:03:31,840
‫the queue has a default timeout of 30 seconds.

84
00:03:31,840 --> 00:03:34,250
‫So what's going to happen is that I have two consumers,

85
00:03:34,250 --> 00:03:37,390
‫I have the first window and the second window consumer,

86
00:03:37,390 --> 00:03:39,970
‫and so I'm going to read the messages from the first window.

87
00:03:39,970 --> 00:03:41,740
‫So let's poll for messages.

88
00:03:41,740 --> 00:03:43,200
‫Now my message has appeared right here,

89
00:03:43,200 --> 00:03:44,730
‫it has been received.

90
00:03:44,730 --> 00:03:48,820
‫And if I go into my second consumer and poll for messages,

91
00:03:48,820 --> 00:03:51,800
‫as you can see, the message does not appear in here.

92
00:03:51,800 --> 00:03:54,400
‫And the reason it doesn't appear is that we are still within

93
00:03:54,400 --> 00:03:58,240
‫the visibility window timeout of that message

94
00:03:58,240 --> 00:04:00,330
‫and therefore, during these 30 seconds

95
00:04:00,330 --> 00:04:03,080
‫this message is trying to be processed by this consumer,

96
00:04:03,080 --> 00:04:06,170
‫this consumer right here will not see it.

97
00:04:06,170 --> 00:04:08,510
‫But let's assume that we stop polling, okay?

98
00:04:08,510 --> 00:04:11,030
‫So we don't delete the message

99
00:04:11,030 --> 00:04:14,440
‫and we know this message will timeout at some point.

100
00:04:14,440 --> 00:04:16,170
‫What will happen is that yes, I'm already here,

101
00:04:16,170 --> 00:04:18,100
‫in the second window, in the second consumer,

102
00:04:18,100 --> 00:04:19,560
‫this is the second consumer,

103
00:04:19,560 --> 00:04:20,820
‫then the message has been received

104
00:04:20,820 --> 00:04:24,060
‫because it was being put back into the queue.

105
00:04:24,060 --> 00:04:26,670
‫And now let's assume we do something correctly,

106
00:04:26,670 --> 00:04:28,780
‫we delete that message,

107
00:04:28,780 --> 00:04:30,790
‫then we have fully processed that message.

108
00:04:30,790 --> 00:04:33,230
‫But remember, that message was received twice.

109
00:04:33,230 --> 00:04:35,750
‫It said received count was two.

110
00:04:35,750 --> 00:04:37,330
‫So it's important for you to understand

111
00:04:37,330 --> 00:04:38,610
‫how this visibility window works,

112
00:04:38,610 --> 00:04:41,050
‫and this was a good demonstration of it.

113
00:04:41,050 --> 00:04:45,210
‫Now, if you wanted to change this setting by default,

114
00:04:45,210 --> 00:04:47,930
‫what you could be doing is go into Edit,

115
00:04:47,930 --> 00:04:49,750
‫and then for visibility timeout,

116
00:04:49,750 --> 00:04:51,660
‫you could set the default value

117
00:04:51,660 --> 00:04:54,210
‫between zero seconds, which is definitely not recommended,

118
00:04:54,210 --> 00:04:55,510
‫up to 12 hours.

119
00:04:55,510 --> 00:04:58,190
‫I think 30 seconds is fine, but again, remember,

120
00:04:58,190 --> 00:05:02,240
‫if a consumer needs more time to process a message,

121
00:05:02,240 --> 00:05:05,330
‫you should just call the ChangeMessageVisibility API

122
00:05:05,330 --> 00:05:07,720
‫to edit the visibility of that one message

123
00:05:07,720 --> 00:05:08,650
‫and increase the value

124
00:05:08,650 --> 00:05:11,670
‫so that another consumer will not see that message

125
00:05:11,670 --> 00:05:13,420
‫and the first consumer will get enough time

126
00:05:13,420 --> 00:05:15,330
‫to process that message accordingly.

127
00:05:15,330 --> 00:05:16,163
‫So, that's it.

128
00:05:16,163 --> 00:05:18,870
‫I hope you liked it, and I will see you in the next lecture.