Docker Caching, how does it really work?

Question

I understand that docker stores every images with layers. If I have multiple users on one development server, and everyone is running the same Dockerfile, but storing the image as, user1_myapp. And user2 is storing it as user2_myapp. Now again, they are using the same Dockerfile.

The question is, if the image is for example 100mb, are both images taking 100mb each, or are they sharing the same image and using only 100mb instead of 200mb?

Answer 1

Yes, the two images will share the same layers if you meet the prerequisites. Docker layers are reused independently of the resulting image name. The requirements to use a cached layer instead of creating a new one are:

• The build command needs to be run against the same docker host where the previous image's cache exists.
• The previous layer ID must match between the cache layer and the running build step.
• The command currently being run, or the source context if you are running a COPY or ADD, must be identical. Docker does not know if you are running a command that pulls from an external changing resource (e.g. git clone or apt-get update), which can result in a false cache hit.
• You cannot have disabled caching in your build command.

Keep in mind that layers are immutable, once created they are never changed, just replaced with different layers with new ID's when you run a different build. When you run a container, it uses a copy-on-write RW layer specific to that container, which allows multiple containers and images to point to the same image layers when they get a cache hit.

If you are having problems getting the cache to match in the two builds, e.g. importing a large file and something like the file timestamp doesn't match, consider creating an intermediate image that contains the common files. Then each project can build FROM that intermediate image.