Understanding layers

Intermediate Docker

Mike Metzger

Data Engineering Consultant

Docker layers

Docker images are made up of layers
A layer generally references a change or command within a Dockerfile
Layers can be cached / reused
The order of commands within a Dockerfile can affect whether layers are reused

Dockerfile to Layers

Why do we care about layers?

Reusability
- Faster build time
- Smaller builds

docker image inspect

How to determine the layers within an image?
docker image inspect <img id | name>
Provides much information about the content of a Docker image
The RootFS:Layers section provides details about layers in a given Docker image

repl@host:~$ docker image inspect alpine
[
    {        "Id": "sha256:05455a08881ea9cf0e752bc48e61bbd71a34c029bb13df01e40e3e70e0d007bd",
        "RepoTags": [
            "alpine:latest"
        ],
        "Created": "2024-01-27T00:30:48.743965523Z",

docker image inspect example

bash> docker image inspect postgres:latest

"RootFS": {
   "Type": "layers",
   "Layers": [
"sha256:6f2d01c02c30cc1ffac781aff795cba8eeb29cc27756fe37bf525169856369c6",
"sha256:c6ad2d5a3cad837ae66b5560e9c577bfad062556b1f00791d8d733ce44a577ce",
"sha256:2153552a84ccbf7e4a28a50e766b72345072e59f8af0ff068baf98b413132e0c",
"sha256:6c00217b1e4b15c25eb3f6e28b1af8c295f469568014621e31a4c5eb5a8aca6f",
"sha256:167177d78e2a33aa822faebe9f01683c648ae78179059db05cd25737f215c305",
...

jq command-line tool

Sometimes difficult to analyze the results from docker image inspect
jq commandline tool is used to read JSON data, like what's returned from docker image inspect
Can use jq to query data

jq recipes with Docker

Method to see just a specific section, for example the RootFS data:

docker image inspect <id> | jq '.[0] | .RootFS'

{
"Type": "layers",
"Layers": [ "sha256:0f5c115c5eea96...", 
            "sha256:20792593831cdc..."
          ]
}

jq recipes with Docker (part 2)

Method to count number of layers using jq:
- docker image inspect <id> | jq '.[0] | {LayerCount: .RootFS.Layers | length}'
```
{
  "LayerCount": 2
}
```

Let's practice!

Intermediate Docker