1 00:00:02,120 --> 00:00:04,610 When it comes to the Docker core concepts, 2 00:00:04,610 --> 00:00:07,020 there are essentially two core concepts 3 00:00:07,020 --> 00:00:08,760 you have to be aware of. 4 00:00:08,760 --> 00:00:11,460 Containers and images. 5 00:00:11,460 --> 00:00:15,840 Now, containers are these isolated boxes, 6 00:00:15,840 --> 00:00:17,630 which contain our code, 7 00:00:17,630 --> 00:00:20,700 and the environment needed to run that code. 8 00:00:20,700 --> 00:00:21,970 And every container, 9 00:00:21,970 --> 00:00:23,410 and that's really important, 10 00:00:23,410 --> 00:00:26,480 is focused one task typically, 11 00:00:26,480 --> 00:00:29,290 which means we don't do multiple things 12 00:00:29,290 --> 00:00:30,400 in one container. 13 00:00:30,400 --> 00:00:32,890 We either run a web server, 14 00:00:32,890 --> 00:00:34,970 or we run a front end, 15 00:00:34,970 --> 00:00:37,080 or we run a database. 16 00:00:37,080 --> 00:00:40,730 But we don't all these things in one container. 17 00:00:40,730 --> 00:00:42,430 The idea of course is 18 00:00:42,430 --> 00:00:45,800 that we have small, lightweight packages, 19 00:00:45,800 --> 00:00:49,440 which are easy to share and reproduce, therefore. 20 00:00:49,440 --> 00:00:52,270 We have a reproducible environment locked 21 00:00:52,270 --> 00:00:54,340 into that container, after all. 22 00:00:54,340 --> 00:00:56,240 That is the main asset, 23 00:00:56,240 --> 00:00:57,410 the main selling point 24 00:00:57,410 --> 00:00:59,373 of Docker containers, in the end. 25 00:01:00,340 --> 00:01:01,790 Now, we also learned 26 00:01:01,790 --> 00:01:05,250 that these containers are kind of stateless. 27 00:01:05,250 --> 00:01:07,030 It's a term I didn't really use, 28 00:01:07,030 --> 00:01:09,600 but it means the data which is written 29 00:01:09,600 --> 00:01:12,040 in a container and that is possible, 30 00:01:12,040 --> 00:01:14,000 containers can write data, 31 00:01:14,000 --> 00:01:17,930 will be lost whenever a container is shut down. 32 00:01:17,930 --> 00:01:20,940 With the exception of volumes, of course. 33 00:01:20,940 --> 00:01:24,700 We can use volumes to kind of mirror 34 00:01:24,700 --> 00:01:28,770 or copy certain folders in our container 35 00:01:28,770 --> 00:01:31,830 to folders on our localhost machine. 36 00:01:31,830 --> 00:01:33,130 And therefore the data 37 00:01:33,130 --> 00:01:35,740 in those volumes will survive, 38 00:01:35,740 --> 00:01:38,990 even if a container is shut down. 39 00:01:38,990 --> 00:01:40,460 Now containers, as you learned, 40 00:01:40,460 --> 00:01:43,040 are created with images. 41 00:01:43,040 --> 00:01:46,890 The images are created with Dockerfiles 42 00:01:46,890 --> 00:01:49,490 or pulled from Docker Hub, 43 00:01:49,490 --> 00:01:52,800 but they are of course someone else created them 44 00:01:52,800 --> 00:01:54,100 with a Dockerfile, 45 00:01:54,100 --> 00:01:57,410 and they contain the code and the environment, 46 00:01:57,410 --> 00:02:00,120 as described in the Dockerfile. 47 00:02:00,120 --> 00:02:03,730 And then the container is just extra thin layer 48 00:02:03,730 --> 00:02:05,080 on top of the image, 49 00:02:05,080 --> 00:02:07,280 and we can run multiple containers 50 00:02:07,280 --> 00:02:08,850 on one of the same image. 51 00:02:08,850 --> 00:02:10,380 That's indeed the core idea 52 00:02:10,380 --> 00:02:12,960 behind images, that they are shareable, 53 00:02:12,960 --> 00:02:17,670 and then multiple containers can be created based on them. 54 00:02:17,670 --> 00:02:20,050 Now, therefore, they act as blueprints 55 00:02:20,050 --> 00:02:21,230 for containers, 56 00:02:21,230 --> 00:02:24,260 as I said, they contain the code and environment. 57 00:02:24,260 --> 00:02:25,580 They are read-only, 58 00:02:25,580 --> 00:02:27,870 which means that the image itself does not run. 59 00:02:27,870 --> 00:02:30,270 It does not write any code inside of it. 60 00:02:30,270 --> 00:02:32,480 If you run a container based off an image, 61 00:02:32,480 --> 00:02:35,530 and that container generates some data, 62 00:02:35,530 --> 00:02:38,230 that data is not written into the image, 63 00:02:38,230 --> 00:02:42,410 it's just written in that container read-write filesystem 64 00:02:42,410 --> 00:02:45,370 in that thin layer on top of the image. 65 00:02:45,370 --> 00:02:47,660 And therefore, the image will never change 66 00:02:47,660 --> 00:02:50,697 and can easily be built and shared with others. 67 00:02:51,610 --> 00:02:54,810 We create the image by defining these instructions, 68 00:02:54,810 --> 00:02:58,170 and that in turn creates these different layers, 69 00:02:58,170 --> 00:02:59,350 as it's called, 70 00:02:59,350 --> 00:03:00,810 which basically ensures 71 00:03:00,810 --> 00:03:03,420 that if only one instruction changes, 72 00:03:03,420 --> 00:03:06,060 not everything needs to be executed again, 73 00:03:06,060 --> 00:03:08,720 but instead these layers are cached 74 00:03:08,720 --> 00:03:10,200 and can be reused. 75 00:03:10,200 --> 00:03:14,410 And hence, building these images is efficient process 76 00:03:14,410 --> 00:03:16,540 and can be very quick. 77 00:03:16,540 --> 00:03:20,100 And then the container is just an extra thin layer 78 00:03:20,100 --> 00:03:21,460 on top of the image, 79 00:03:21,460 --> 00:03:22,990 which is created a new, 80 00:03:22,990 --> 00:03:25,520 for every new container we run. 81 00:03:25,520 --> 00:03:28,570 That, in a nutshell, is what Docker is about 82 00:03:28,570 --> 00:03:30,750 and what containers are about. 83 00:03:30,750 --> 00:03:33,713 Images and containers working together.