1 00:00:02,240 --> 00:00:04,030 Now we know the problem. 2 00:00:04,030 --> 00:00:05,740 What's the solution? 3 00:00:05,740 --> 00:00:09,170 Docker has a built in feature called volumes. 4 00:00:09,170 --> 00:00:12,320 And volumes help us with persisting data, 5 00:00:12,320 --> 00:00:16,940 and solving the problem I outlined in the last lecture. 6 00:00:16,940 --> 00:00:21,170 Now how do we utilize volumes in our application? 7 00:00:21,170 --> 00:00:24,830 In this Docker application, for example. 8 00:00:24,830 --> 00:00:28,410 First of all, we have to understand what exactly volumes are 9 00:00:28,410 --> 00:00:30,250 and how they work. 10 00:00:30,250 --> 00:00:34,310 Volumes are folders on your host machine, 11 00:00:34,310 --> 00:00:35,740 so not in the container, 12 00:00:35,740 --> 00:00:36,880 not in the image, 13 00:00:36,880 --> 00:00:39,620 but on your host machine hard drive, 14 00:00:39,620 --> 00:00:41,150 which are mounted, 15 00:00:41,150 --> 00:00:44,670 which basically means, made available or mapped, 16 00:00:44,670 --> 00:00:47,080 into containers. 17 00:00:47,080 --> 00:00:51,840 So volumes are folders on your host machine, 18 00:00:51,840 --> 00:00:53,530 on your computer, 19 00:00:53,530 --> 00:00:56,430 which you make Docker aware of 20 00:00:56,430 --> 00:00:57,920 and which are then mapped 21 00:00:57,920 --> 00:01:02,180 to folders inside of a Docker container. 22 00:01:02,180 --> 00:01:05,340 Now that might sound a bit like the COPY instruction 23 00:01:05,340 --> 00:01:06,770 from the Docker file. 24 00:01:06,770 --> 00:01:09,400 But keep in mind that this instruction, 25 00:01:09,400 --> 00:01:13,700 really just takes a snapshot of the path and files 26 00:01:13,700 --> 00:01:15,483 you would tell it to copy, 27 00:01:15,483 --> 00:01:19,420 and then it copies these files and folders into the image, 28 00:01:19,420 --> 00:01:20,440 and that's it. 29 00:01:20,440 --> 00:01:23,730 There's no on-going relation or connection, 30 00:01:23,730 --> 00:01:26,060 it's just a one time snapshot, 31 00:01:26,060 --> 00:01:28,860 which is copied into the image. 32 00:01:28,860 --> 00:01:31,130 With volumes that's different. 33 00:01:31,130 --> 00:01:33,730 Here you can really connect a folder, 34 00:01:33,730 --> 00:01:35,180 inside of the container, 35 00:01:35,180 --> 00:01:39,610 to a folder outside of the container, on your host machine. 36 00:01:39,610 --> 00:01:42,610 And changes in either folder, 37 00:01:42,610 --> 00:01:44,830 will be reflected in the other one. 38 00:01:44,830 --> 00:01:47,700 So if you add a file on your host machine, 39 00:01:47,700 --> 00:01:49,907 it is accessible inside of the container, 40 00:01:49,907 --> 00:01:54,100 and if the container adds a file in that mapped path, 41 00:01:54,100 --> 00:01:56,130 it is available outside of the container, 42 00:01:56,130 --> 00:01:57,823 in the host machine as well. 43 00:01:58,740 --> 00:02:02,140 And therefore, because of this mechanism, 44 00:02:02,140 --> 00:02:05,270 volumes allow you to persist data. 45 00:02:05,270 --> 00:02:08,090 Volumes persist and continue to exist 46 00:02:08,090 --> 00:02:10,430 even if a container is shut down. 47 00:02:10,430 --> 00:02:11,750 That's important. 48 00:02:11,750 --> 00:02:14,630 If you add a volume to a container, 49 00:02:14,630 --> 00:02:17,830 the volume will not be removed, 50 00:02:17,830 --> 00:02:20,010 when a container is removed, 51 00:02:20,010 --> 00:02:21,370 it survives, 52 00:02:21,370 --> 00:02:24,483 and therefore the data, in a volume survives. 53 00:02:25,340 --> 00:02:26,930 And containers can both 54 00:02:26,930 --> 00:02:31,140 read and write data from and to a volume. 55 00:02:31,140 --> 00:02:33,580 And that's of course a powerful feature. 56 00:02:33,580 --> 00:02:37,020 Which we can use for folders, 57 00:02:37,020 --> 00:02:40,620 which we want to access from outside our container, 58 00:02:40,620 --> 00:02:43,490 and/or simply for data 59 00:02:43,490 --> 00:02:46,230 that should survive container shutdown 60 00:02:46,230 --> 00:02:48,950 and the removal of a container. 61 00:02:48,950 --> 00:02:52,170 Because if the data is also saved outside of the container, 62 00:02:52,170 --> 00:02:55,513 of course it survives the removal of the container.