How Docker calculates the hash of each layer? Is it deterministic?

Question

I tried to find this information around the Docker official docs, but had no success.

Which pieces of information does Docker take into account when calculating the hash

Answer 1

Thanks @thaJeztah. Answer is in https://gist.github.com/aaronlehmann/b42a2eaf633fc949f93b#id-definitions-and-calculations

1. layer.DiffID: ID for an individual layer

   Calculation: DiffID = SHA256hex(uncompressed layer tar data)

2. layer.ChainID: ID for a layer and its parents. This ID uniquely identifies a filesystem composed of a set of layers.

   Calculation:

   • For bottom layer: ChainID(layer0) = DiffID(layer0)
   • For other layers: ChainID(layerN) = SHA256hex(ChainID(layerN-1) + " " + DiffID(layerN))

3. image.ID: ID for an image. Since the image configuration references the layers the image uses, this ID incorporates the filesystem data and the rest of the image configuration.

   Calculation: SHA256hex(imageConfigJSON)