rkt/Documentation/image-fetching-behavior.md

Image Fetching Behavior

When fetching, rkt will try to avoid unnecessary network transfers: if an updated image is already in the local store there's no need to download it again.

This behavior can be controlled with the --pull-policy flag.

General Behavior

The following table describes the meaning of the --pull-policy flag.

This flag accepts one of three options:

Option	Description
new	Default behavior in run and prepare Check the store, and if the image is missing fetch from remote
update	Default behavior in fetch Attempt to fetch from remote, but if the remote image matches something in our store don't pull it
never	Only check the store, and don't fetch from remote.

Details

Here we detail the actions taken by rkt when fetching from store and remote for each type of image argument.

Fetch from	Image argument	Detailed behavior
store	file://	Use the specified file
store	http(s)://	Check for the URL in the local store. If found, use the corresponding image.
store	docker://	Check for the URL in the local store. If found, use the corresponding image.
store	image name	Check local store. If found, use that image. If there's a file in the current directory named like the image name, use that file instead.
remote	file://	Use the specified file
remote	http(s)://	Search in the store if the URL is available. If it's available and the saved Cache-Control maxage > 0 determine if the image should be downloaded. If it's not expired use the image. Otherwise download (sending if available the saved ETag). If the download returns a 304 Not Modified use the image already saved in the local store.
remote	docker://	Fetch using docker2aci.
remote	image name	Execute discovery logic. If discovery is successful use the discovered URL doing the above remote http(s):// image case. If there's a file in the current directory named like the image name, use that file instead.