Docker Image Layers

Refer here for image creation & understanding layers

Experiments

• Lets pull a docker image docker pull alpine
• Lets examine Layers. Execute command docker image inspect alpine
• Make a note of Rootfs

"RootFS": {
            "Type": "layers",
            "Layers": [
                "sha256:03901b4a2ea88eeaad62dbe59b072b28b6efa00491962b8741081c5df50c65e0"
            ]
        }

• Lets do the same with different image docker pull tomcat:8 && docker image inspect tomcat:8. Make note of Rootfs

"RootFS": {
            "Type": "layers",
            "Layers": [
                "sha256:55e6b89812f369277290d098c1e44c9e85a5ab0286c649f37e66e11074f8ebd1",
                "sha256:26b1991f37bd5b798e1523f65d7f6aa6961b75515f465cf44123fa0ad3b8961b",
                "sha256:8bacec4e34468110538ebf108ca8ec0d880a37018a55be91b9670b8e900c593a",
                "sha256:2e669e0134f53839293f077f21dd124634f92aa5f7e3fd013c530779dd41d1c5",
                "sha256:5a30999619d793135da20f07cef97d34e741dbae20364ad87803e7a26906b365",
                "sha256:1690af51cb08c01e45b1ad3ea2d6e6ad68e6eaf88bd01b8725d21cb782849dac",
                "sha256:57e6a3d20ce900546bb12f793c7c4057f87b7b509488f42c68d1772380e5e6e0",
                "sha256:5161954b2663c24f662d3b8f7dd404821bf8ba77fdd405a327be76061490bbe1",
                "sha256:bee27ee72bbdce8cf0d487ea3ccd8b6b208ebc1d1ae6bcfd92da9e18381a460f",
                "sha256:3d7c5e9cb860fcb040f7df0331c460dae60e3fe03ce075837ccc3f1b1176aede"
            ]
        }

• Lets do the same for tomcat:9

"RootFS": {
            "Type": "layers",
            "Layers": [
                "sha256:55e6b89812f369277290d098c1e44c9e85a5ab0286c649f37e66e11074f8ebd1",
                "sha256:26b1991f37bd5b798e1523f65d7f6aa6961b75515f465cf44123fa0ad3b8961b",
                "sha256:8bacec4e34468110538ebf108ca8ec0d880a37018a55be91b9670b8e900c593a",
                "sha256:2e669e0134f53839293f077f21dd124634f92aa5f7e3fd013c530779dd41d1c5",
                "sha256:5a30999619d793135da20f07cef97d34e741dbae20364ad87803e7a26906b365",
                "sha256:4e8bbb91df0400bfa8e435e3aceddeb4fa69c633570b8d93a863d08ed4e41721",
                "sha256:57f260974b764bbcf9ddea8ad1e14a91ecfa1be54d03f5f61d699bd25d815598",
                "sha256:80dc36c9ba8c710a238be61b68403b854940248bd2d8b31e39e6a2de63a0e76e",
                "sha256:13cb70b6919782948114761573401a60ea70f68af9fa36f371005ff88f2b32c1",
                "sha256:720d011cc35aaa8dc148522e49bbb4c4567e23840f1a2ab50a1e3103b0e0a818"
            ]
        }

• Layer Creation are done during image creation. Simple Logic is in Dockerfile instructions (RUN, ADD, COPY) lead to image layer creation

• Lets try this

FROM alpine
RUN echo "hello to new layer" >> test.txt 

• Create a image by execute docker image build -t testlayer .
• Now inspect this images docker image inspect testlayer and make note of rootfs

"RootFS": {
            "Type": "layers",
            "Layers": [
                "sha256:03901b4a2ea88eeaad62dbe59b072b28b6efa00491962b8741081c5df50c65e0",
                "sha256:f5ca6f6050faa8316489da066e4b7d760af71635568be558254d929d5a04f289"
            ]
        }

• Union File System:

  • Docker uses union file system to make things work with image layers

• Imapact of Layers on Image

• Impact of Layers on Container