WEBVTT 1 00:00:00.480 --> 00:00:01.970 Hi, guys and welcome back. 2 00:00:01.970 --> 00:00:04.360 Before we start working on the project, 3 00:00:04.360 --> 00:00:07.500 we need to talk about what is ACID? 4 00:00:07.500 --> 00:00:11.360 And ACID is something in databases, nothing illegal, 5 00:00:11.360 --> 00:00:13.740 and it stands for Atomicity, Consistency, 6 00:00:13.740 --> 00:00:16.120 Isolation, and Durability. 7 00:00:16.120 --> 00:00:18.890 We've discussed before that SQL operations occur 8 00:00:18.890 --> 00:00:20.390 in transactions. 9 00:00:20.390 --> 00:00:23.130 So, are transactions ACID? 10 00:00:23.130 --> 00:00:24.820 And the answer to that is not really, 11 00:00:24.820 --> 00:00:26.830 although they are a result of ACID, 12 00:00:26.830 --> 00:00:31.830 or, rather, in order to be ACID, we must use transactions. 13 00:00:32.400 --> 00:00:34.220 There's no way around it. 14 00:00:34.220 --> 00:00:38.950 Here's why, the first letter in ACID is Atomicity, 15 00:00:38.950 --> 00:00:42.590 and that means that transactions are indivisible. 16 00:00:42.590 --> 00:00:45.020 Every operation must happen successfully 17 00:00:45.020 --> 00:00:47.350 in a transaction or none do. 18 00:00:47.350 --> 00:00:49.120 Essentially, what Atomicity means 19 00:00:49.120 --> 00:00:52.300 is that you can't divide the unit of work. 20 00:00:52.300 --> 00:00:54.840 And here, the unit of work must be a transaction 21 00:00:54.840 --> 00:00:56.870 because if you want to group to operations 22 00:00:56.870 --> 00:01:00.030 that depend on each other, you must be able to do that 23 00:01:00.030 --> 00:01:03.950 without being able to run one, but not the other. 24 00:01:03.950 --> 00:01:06.900 So, for example, if you try to UPDATE and INSERT 25 00:01:06.900 --> 00:01:11.710 in the same transaction, then you must be able to do both. 26 00:01:11.710 --> 00:01:15.230 And if one fails, then the other one is rolled back, 27 00:01:15.230 --> 00:01:17.180 or just ignored. 28 00:01:17.180 --> 00:01:19.700 This is what Atomicity means. 29 00:01:19.700 --> 00:01:22.520 For Consistency, this means that the rules and constraints 30 00:01:22.520 --> 00:01:25.690 of the database are followed at all times. 31 00:01:25.690 --> 00:01:27.640 For example, with foreign key constraints, 32 00:01:27.640 --> 00:01:29.530 you can't have a foreign key pointing 33 00:01:29.530 --> 00:01:32.400 to a non-existing primary key value. 34 00:01:32.400 --> 00:01:33.720 We've learned about this in Postgres 35 00:01:33.720 --> 00:01:35.790 when we had users and bank accounts. 36 00:01:35.790 --> 00:01:37.520 You couldn't delete a user, 37 00:01:37.520 --> 00:01:39.350 but keep the user's bank account 38 00:01:39.350 --> 00:01:42.313 because that was an inconsistency in the data. 39 00:01:43.370 --> 00:01:46.390 There are more constraints than just foreign keys though, 40 00:01:46.390 --> 00:01:49.800 such as UNIQUE constraints, or NOT NULL, and many more. 41 00:01:49.800 --> 00:01:52.900 We're going to learn about those constraints later on. 42 00:01:52.900 --> 00:01:56.770 Isolation means that data is only visible in the database 43 00:01:56.770 --> 00:02:00.053 when the transaction has ended and been committed. 44 00:02:00.900 --> 00:02:03.290 For example, if one transaction makes 10 INSERTs, 45 00:02:03.290 --> 00:02:06.470 the other transactions won't be able to see any of that data 46 00:02:06.470 --> 00:02:09.410 until the first transaction has been committed. 47 00:02:09.410 --> 00:02:11.670 That means that transactions are essentially isolated. 48 00:02:11.670 --> 00:02:14.960 They work on the database as it exists 49 00:02:14.960 --> 00:02:17.670 at this point in time, but they don't see the results 50 00:02:17.670 --> 00:02:20.960 of any other transactions until they have been committed. 51 00:02:20.960 --> 00:02:24.490 Durability means that data is saved to permanent storage 52 00:02:24.490 --> 00:02:26.500 once a transaction is committed. 53 00:02:26.500 --> 00:02:29.240 So, this is in contrast to some other database systems 54 00:02:29.240 --> 00:02:32.370 that operate either partially or fully in memory. 55 00:02:32.370 --> 00:02:35.350 Operating in memory is faster, but it's also a bit riskier. 56 00:02:35.350 --> 00:02:37.280 For example, there's databases 57 00:02:37.280 --> 00:02:40.490 that hold a lot of the results in memory, 58 00:02:40.490 --> 00:02:44.480 and they only save to disc every now and then. 59 00:02:44.480 --> 00:02:46.090 And if you use a database like that, 60 00:02:46.090 --> 00:02:47.130 it's gonna be much faster. 61 00:02:47.130 --> 00:02:51.260 You can write and read from that memory data very quickly, 62 00:02:51.260 --> 00:02:53.470 but if your database server crashes, 63 00:02:53.470 --> 00:02:55.920 you might lose some work. 64 00:02:55.920 --> 00:02:59.760 The Durability aspect of ACID means that SQL databases, 65 00:02:59.760 --> 00:03:02.650 or databases that follow these ACID properties, 66 00:03:02.650 --> 00:03:04.470 will always save to permanent storage 67 00:03:04.470 --> 00:03:06.280 once a transaction is committed. 68 00:03:06.280 --> 00:03:08.440 Remember, if you join this with Isolation, 69 00:03:08.440 --> 00:03:11.060 then it makes sense to start committing transactions 70 00:03:11.060 --> 00:03:13.400 all the time, otherwise the transactions 71 00:03:13.400 --> 00:03:16.010 that aren't committed don't make the data available 72 00:03:16.010 --> 00:03:16.980 to other transactions, 73 00:03:16.980 --> 00:03:18.880 which means that other transactions can't work 74 00:03:18.880 --> 00:03:21.130 with that data, and so on. 75 00:03:21.130 --> 00:03:24.300 So, you want to be committing your transactions consistently 76 00:03:24.300 --> 00:03:26.160 as that saves them to permanent storage, 77 00:03:26.160 --> 00:03:29.420 you can't lose your work, and also makes the data available 78 00:03:29.420 --> 00:03:31.650 to other transactions as well. 79 00:03:31.650 --> 00:03:34.020 So, as I mentioned down below, you can use a database 80 00:03:34.020 --> 00:03:35.550 that operates in memory for things 81 00:03:35.550 --> 00:03:36.910 that you don't really mind losing, 82 00:03:36.910 --> 00:03:39.080 like, for example, cash data 83 00:03:39.080 --> 00:03:41.850 or the logged in status of users. 84 00:03:41.850 --> 00:03:44.170 If the data for logged in status is lost, 85 00:03:44.170 --> 00:03:45.570 the users may need to log in again, 86 00:03:45.570 --> 00:03:47.200 but it's not a massive deal. 87 00:03:47.200 --> 00:03:50.460 Whereas, for most of the other data, you do want to keep. 88 00:03:50.460 --> 00:03:51.990 All right, guys, thank you for watching this video. 89 00:03:51.990 --> 00:03:54.760 I hope you understand a bit more about what ACID is. 90 00:03:54.760 --> 00:03:56.393 I'll see you in the next one.