1 00:00:00,990 --> 00:00:07,980 Hi there if you don't understand that conker and see how much better resource utilization which in turn 2 00:00:07,980 --> 00:00:10,140 helps in improving that throughput. 3 00:00:10,770 --> 00:00:16,140 But at the beginning of the chapter we also say that concurrent programming is more complex compared 4 00:00:16,140 --> 00:00:17,930 to sequential programming. 5 00:00:18,000 --> 00:00:21,080 So now let's understand that aspect of concurrent programming. 6 00:00:21,260 --> 00:00:24,750 That is why it is complex compared to sequential programming. 7 00:00:25,080 --> 00:00:30,270 While it is nice to learn about this advance stuff related to multi-threaded programming we should also 8 00:00:30,270 --> 00:00:33,130 be aware of certain safety hazards. 9 00:00:33,210 --> 00:00:38,940 There are certain concurrency hazards and this isn't the goal to get a concurrency has called a risk 10 00:00:38,940 --> 00:00:40,550 condition. 11 00:00:40,590 --> 00:00:47,130 The main complexity of multithreaded programming comes when the same object is shared across multiple 12 00:00:47,130 --> 00:00:54,170 threads and that object has some speed that is that data which can be modified by the text. 13 00:00:54,210 --> 00:00:57,360 Let's look at an example here let's consider these two users. 14 00:00:57,390 --> 00:01:00,900 John and Anita on their side they are a happily married couple. 15 00:01:01,020 --> 00:01:06,480 Here's the example scenario is that they have a joint bank account and let's see how that balance off 16 00:01:06,590 --> 00:01:08,680 only a hundred dollars. 17 00:01:08,770 --> 00:01:15,390 Now when customers use this banking application dig it up isn't it by looks like we see here and there 18 00:01:15,390 --> 00:01:17,630 is also a task called bank account. 19 00:01:17,910 --> 00:01:23,790 Since it is a task it implements that are noble edifice on the bank can cross has one instance variable 20 00:01:23,790 --> 00:01:29,400 called balance and also one instance method called Make withdrawn which is invoked when the user is 21 00:01:29,400 --> 00:01:36,080 making it with done the make with document that simply checks if there is enough balance for withdrawn 22 00:01:36,080 --> 00:01:36,560 . 23 00:01:36,840 --> 00:01:42,900 And if there is only then it will be that the withdrawal amount from the balance amount on would then 24 00:01:42,960 --> 00:01:45,800 allow the user to withdraw the amount. 25 00:01:45,840 --> 00:01:51,120 So this balance check is very critical so that there is no onward with of money. 26 00:01:51,270 --> 00:01:56,790 Now when the user accesses the banking application then a bank account object will be created for that 27 00:01:56,790 --> 00:01:57,290 to users. 28 00:01:57,290 --> 00:02:04,380 It can and will be passed us input with a third that represents that user non-theism that book John 29 00:02:04,490 --> 00:02:08,980 on it happened to use the banking application at the same time. 30 00:02:09,210 --> 00:02:15,580 Then since they represent the Simek on only a single bank account object will be created for them as 31 00:02:15,670 --> 00:02:18,360 does input to the threats that are present. 32 00:02:18,370 --> 00:02:18,950 John. 33 00:02:18,990 --> 00:02:22,500 Anita just like the BBC here. 34 00:02:23,210 --> 00:02:24,240 No let's consider that. 35 00:02:24,270 --> 00:02:30,090 But John I don't want to get draw an amount of dollar I got on the same time. 36 00:02:30,420 --> 00:02:37,170 That is they are using the banking system at the same time to get created on Lettice's should jewler 37 00:02:37,240 --> 00:02:39,530 bakes Johnston at first. 38 00:02:39,740 --> 00:02:46,360 And so Johnston that would invoke the make withdrawal method under the make with Dolemite that the balance 39 00:02:46,360 --> 00:02:47,280 is checked. 40 00:02:47,460 --> 00:02:52,760 And since John only wants 75 bucks the check will succeed. 41 00:02:52,800 --> 00:02:58,680 However there is room that wretched Guler decides to send John Buddha on noble estate before the new 42 00:02:58,680 --> 00:03:00,800 balances computer. 43 00:03:01,710 --> 00:03:06,840 So Jon Stewart is an honorable straight on tourch a jeweler will then bring an industrial to running 44 00:03:06,840 --> 00:03:13,260 state Undun understood it will and does make withdrawal method and real check if there is enough balance 45 00:03:13,260 --> 00:03:13,920 . 46 00:03:13,920 --> 00:03:20,310 Since John did not withdraw any money the balance cheque will succeed and the new balances computed 47 00:03:21,060 --> 00:03:23,450 which will be dollar fine. 48 00:03:24,330 --> 00:03:31,140 Next John will be brought to running's do it as unattested it has completed execution and assuming that 49 00:03:31,230 --> 00:03:34,950 the balance is still $100 John will order all. 50 00:03:35,310 --> 00:03:38,340 Which brings the balance to minus $50. 51 00:03:38,800 --> 00:03:45,750 So the system is in a bad state now so that data got corrupted and this all happened because somebody 52 00:03:45,750 --> 00:03:49,210 was not careful enough when quoting this class. 53 00:03:49,350 --> 00:03:56,680 That is a bank account object is not safe and what it means is that the bank account object had stayed 54 00:03:56,700 --> 00:03:57,840 that is mutable. 55 00:03:58,110 --> 00:04:03,570 That is the instance variable balance is mutable and the bank account object was being shared between 56 00:04:03,570 --> 00:04:09,810 threads which were accessing a method that can change the mutable state changing the mutable state is 57 00:04:09,810 --> 00:04:16,620 fine but the process of changing the state was not properly managed due to which the bank account object 58 00:04:16,620 --> 00:04:21,660 was not to when in not properly managed. 59 00:04:21,690 --> 00:04:28,610 What it means is that was a particular concurrency hasn't called risk race condition so the make withdrawn 60 00:04:28,610 --> 00:04:34,640 method hazardous condition under the type of race condition it exemplifies is called as check. 61 00:04:34,680 --> 00:04:40,620 Then act which is a very common type of race condition under just got his check done act as we are first 62 00:04:40,620 --> 00:04:43,460 making a check if there is enough balance. 63 00:04:43,530 --> 00:04:48,650 Only then we are performing some action which is to compute the new balance. 64 00:04:49,080 --> 00:04:55,580 The problem is the desirable outcome which is not overdraw and that is dependent on the wretched behavior 65 00:04:55,600 --> 00:04:56,330 . 66 00:04:56,460 --> 00:05:02,230 In this case after Jones thread observes the balance it is suspended briefly so that Anita's thread 67 00:05:02,250 --> 00:05:04,800 finishes to completion undaunted. 68 00:05:04,910 --> 00:05:11,520 Earlier observation is no longer valid so the correctness of the program depends on luck. 69 00:05:11,520 --> 00:05:17,040 If Jon Stewart is not suspended and is allowed to compute the new balance right after performing the 70 00:05:17,040 --> 00:05:19,170 badness check then everything is good. 71 00:05:19,390 --> 00:05:20,710 OK so life is good. 72 00:05:21,880 --> 00:05:28,690 So the solution is that make the document that should be made to run as one single economic unit that 73 00:05:28,690 --> 00:05:30,740 is once a threat and does make withdrawal. 74 00:05:30,790 --> 00:05:37,090 It must be a lot to finish before any other threat can and does make withdrawing matter and this behavior 75 00:05:37,080 --> 00:05:39,410 is also referred to as a mutual exclusion. 76 00:05:39,630 --> 00:05:45,390 You must have heard of it that is only one threat is being able to enter the method at a given instance 77 00:05:45,380 --> 00:05:52,090 of by and the next lesson we will look at locking which is a job loss but it didn't mechanism for ensuring 78 00:05:52,140 --> 00:05:54,490 atomic behavior. 79 00:05:54,730 --> 00:06:01,330 So non-threat if object means an incorrect program and an incorrect program can lead to Dudack option 80 00:06:02,280 --> 00:06:08,200 sometimes programs that are not threats they might work on pro forma for years but they may fail at 81 00:06:08,190 --> 00:06:09,040 any moment. 82 00:06:09,120 --> 00:06:10,860 So they are like timebombs. 83 00:06:10,950 --> 00:06:17,070 So basically we are relying on luck but not that the problem is mainly because of how mutable state 84 00:06:17,170 --> 00:06:18,430 that can be shared. 85 00:06:18,940 --> 00:06:23,330 But if the object does not have any state at all then object destructive. 86 00:06:23,520 --> 00:06:28,290 That is if it does not have any instance variables then it means that there is no state in the first 87 00:06:28,300 --> 00:06:35,210 place and program would have and the methods that operate on local variables. 88 00:06:35,280 --> 00:06:39,670 No let's look at another interesting example that there is a race condition here. 89 00:06:39,670 --> 00:06:45,420 The ID generator class is responsible for generating unique IDs in this system. 90 00:06:45,730 --> 00:06:53,440 So IDs are long variable and the method get new ID which simply written that next ID unfought that all 91 00:06:53,440 --> 00:06:57,300 it is doing is incrementing the current ID value. 92 00:06:57,310 --> 00:07:04,170 Now we would assume that this increment operation is a single action because it is only a single statement 93 00:07:04,360 --> 00:07:07,480 unlike our make withdrawal example. 94 00:07:07,560 --> 00:07:09,590 However that is not the case. 95 00:07:09,740 --> 00:07:14,390 And there are three discrete operations going on in the background. 96 00:07:14,440 --> 00:07:20,190 The first operation is to first the current value of ID then increment it. 97 00:07:20,470 --> 00:07:24,100 And finally we write the new value back. 98 00:07:24,220 --> 00:07:30,020 So due to the fact that this single statement has multiple operations we have a risk condition. 99 00:07:30,100 --> 00:07:33,490 For instance let's add the current value of ID is to. 100 00:07:34,140 --> 00:07:40,470 Then let's say there is a threat a which invokes this method get your ID so it could read the current 101 00:07:40,480 --> 00:07:44,140 value of ID then it performs the increment operation. 102 00:07:44,230 --> 00:07:50,350 But before that writes back the new value for Treacher you look and suspended and bring and under threat 103 00:07:50,570 --> 00:07:51,020 etc.. 104 00:07:51,020 --> 00:07:57,430 Be in good running state which would involve this method and treat the value for ID as the previous 105 00:07:57,460 --> 00:07:58,160 threat threat. 106 00:07:58,180 --> 00:08:00,830 He did not write it back. 107 00:08:00,870 --> 00:08:04,140 Subtract B or increment it and write the value back. 108 00:08:04,500 --> 00:08:07,650 So now the ideal value is for next right. 109 00:08:07,650 --> 00:08:13,500 It would raise you on it would continue by performing only the last operation because that's when it 110 00:08:13,500 --> 00:08:17,970 got suspended or no it would write the value back which is also 4. 111 00:08:18,460 --> 00:08:21,580 So essentially ID was incremented only once. 112 00:08:21,570 --> 00:08:25,060 Although the method did you ID was in work twice. 113 00:08:25,060 --> 00:08:27,500 So one of the updates was lost. 114 00:08:27,780 --> 00:08:32,830 This method would work perfectly in a single threaded environment but in a multi-threaded environment 115 00:08:33,150 --> 00:08:35,820 it runs the risk of working incorrectly. 116 00:08:36,370 --> 00:08:42,120 So once again we need to ensure that the increment operations statement executes as a single economic 117 00:08:42,150 --> 00:08:44,830 statement under that see how that can be done. 118 00:08:44,860 --> 00:08:53,180 And the next lesson not have to be fully eclipsed and it cooked them all offer this condition. 119 00:08:53,190 --> 00:08:57,900 This is a quick demonstration of the John an example which we have seen earlier. 120 00:08:58,090 --> 00:09:04,330 So it's going to be the exact same example and how a bank account and both John and thought well how 121 00:09:04,330 --> 00:09:05,370 a joint account. 122 00:09:05,430 --> 00:09:12,100 And they have a balance off $200 and they're going to withdraw an amount of dollars 75 at around that 123 00:09:12,150 --> 00:09:13,070 same time. 124 00:09:13,380 --> 00:09:13,600 OK. 125 00:09:13,600 --> 00:09:19,710 So that was the scenario on we are going to simulate that exact same scenario and we will also see how 126 00:09:19,720 --> 00:09:26,280 it can generate the undesirable effect or undesirable outcome off overdrawing. 127 00:09:26,520 --> 00:09:26,910 Okay. 128 00:09:27,120 --> 00:09:32,220 So here the name of the class the new class is race can be more on your husband I get into the concurrency 129 00:09:32,230 --> 00:09:36,930 package and here in the statement we are creating the bank account object. 130 00:09:36,930 --> 00:09:39,630 So it's a class which implements the Runnable interface. 131 00:09:39,690 --> 00:09:40,860 So it does that to us. 132 00:09:41,140 --> 00:09:47,260 And we are setting the balance of $100 on here we are creating two threads John and Anita and we are 133 00:09:47,250 --> 00:09:50,790 passing the same bank account object to both threats. 134 00:09:51,170 --> 00:09:51,800 Okay. 135 00:09:51,820 --> 00:09:56,340 And we're also setting the names here forward that aspect of threats John and Anita on we are stopping 136 00:09:56,350 --> 00:10:03,030 them here and here we have the bank account object bank account class which implements runnable. 137 00:10:03,210 --> 00:10:07,170 So it has the pirate incidents but it will balance here as a certain matter. 138 00:10:07,510 --> 00:10:11,250 And here is that run matter on that on. 139 00:10:11,350 --> 00:10:16,650 We are invoking the make with documented with the withdrawal amount which is $75. 140 00:10:16,950 --> 00:10:19,490 So here is the make Woodhall method. 141 00:10:19,940 --> 00:10:26,430 And initially we are making that balance check and if it succeeds then we computed our new balance and 142 00:10:26,430 --> 00:10:32,100 we have a bunch of brand statements saying the contract that is executing the method. 143 00:10:32,800 --> 00:10:35,480 Now the balance is less than zero. 144 00:10:35,560 --> 00:10:39,070 Then the money overdrawn text will be Pinkard. 145 00:10:39,350 --> 00:10:39,900 OK. 146 00:10:40,090 --> 00:10:45,200 So that's just not a desirable outcome but it can happen because off the race condition. 147 00:10:45,310 --> 00:10:46,070 OK. 148 00:10:46,120 --> 00:10:48,260 So let's go ahead on this. 149 00:10:48,880 --> 00:10:50,430 So here it is. 150 00:10:50,500 --> 00:10:52,900 So money has been overdrawn. 151 00:10:53,460 --> 00:10:53,800 Okay. 152 00:10:53,800 --> 00:10:55,380 And this is what happened. 153 00:10:55,620 --> 00:11:03,010 So initially John has entered the make with Dortmunder and this statement is getting printed which means 154 00:11:03,000 --> 00:11:05,360 that John is about to withdraw. 155 00:11:05,590 --> 00:11:13,420 After that John has been sent into runnable state and I need to also under this method and unadvised 156 00:11:13,570 --> 00:11:19,410 has also on it also sees that the balance is greater than or equal to the amount which means that there 157 00:11:19,400 --> 00:11:20,610 is enough balance. 158 00:11:20,640 --> 00:11:26,920 So the next statement is about or with guys getting printed and after that and it is going into into 159 00:11:27,140 --> 00:11:32,520 runnable straight I end up most likely John is in the running state and is computing the balance of 160 00:11:32,520 --> 00:11:37,690 New Balance amount and B happened in the statement John has withdrawn 75 bucks. 161 00:11:37,920 --> 00:11:38,430 Okay. 162 00:11:38,640 --> 00:11:46,990 Then Anita makes up and John is being sent into unbelted and Anita is also drawing 75 bucks. 163 00:11:47,050 --> 00:11:47,280 OK. 164 00:11:47,290 --> 00:11:54,630 So but this time with the new balance it is actually it goes in negative okay and it goes into negative 165 00:11:54,730 --> 00:11:56,910 balance as minus 50 now. 166 00:11:56,940 --> 00:12:05,700 So after that it looks like John is coming back into running state and it is being sent back into into 167 00:12:05,700 --> 00:12:06,490 run up Alstad. 168 00:12:06,580 --> 00:12:08,310 Okay after the husband drunk. 169 00:12:08,430 --> 00:12:15,030 So because of that here when John finishes the execution of make withdrawal history sees that balance 170 00:12:15,020 --> 00:12:17,310 is resident zero and access money. 171 00:12:17,350 --> 00:12:18,160 All are wrong. 172 00:12:18,260 --> 00:12:18,660 OK. 173 00:12:18,660 --> 00:12:25,240 On John Street finishes its execution after that on this dusted it is going to resume its execution 174 00:12:25,570 --> 00:12:31,570 unflat for heart rate also the balances less than zero and this thing is getting printed twice. 175 00:12:31,590 --> 00:12:32,490 OK. 176 00:12:32,830 --> 00:12:39,100 So although we see the money overdrawn message to be printed twice in reality the money has been overdrawn 177 00:12:39,150 --> 00:12:41,530 only once and that is based on the. 178 00:12:41,710 --> 00:12:42,060 OK. 179 00:12:42,250 --> 00:12:43,550 So it was fine then. 180 00:12:43,560 --> 00:12:45,820 Jon Stewart has withdrawn the money. 181 00:12:45,890 --> 00:12:46,390 OK. 182 00:12:46,440 --> 00:12:48,270 It was not overdrawn. 183 00:12:48,420 --> 00:12:51,480 It is only when it's us that has withdrawn. 184 00:12:51,490 --> 00:12:53,370 Then the money has been overdrawn. 185 00:12:53,450 --> 00:12:58,950 OK it's just because of the interleaving of that threat we are seeing this Caernarfon output. 186 00:12:59,230 --> 00:13:03,120 So there we can see how the threat should you get us behaving. 187 00:13:03,120 --> 00:13:10,860 So it is interleaving both the threats and we have a risk condition and we can see that undesirable 188 00:13:10,870 --> 00:13:11,800 outcome. 189 00:13:11,800 --> 00:13:16,440 So it's because this particular class is not thread safe. 190 00:13:16,650 --> 00:13:21,040 So we need to make it safe and that is the subject of the next lesson. 191 00:13:21,310 --> 00:13:25,460 So this god is available in the resources section and that's about it. 192 00:13:25,450 --> 00:13:27,130 NC in the next lesson. 193 00:13:27,120 --> 00:13:27,620 Thank you. 194 00:13:27,630 --> 00:13:28,780 And happy coding.