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App Container Image Discovery

An App Container Image name has a URL-like structure, for example However, there is no scheme on this name, so it cannot be directly resolved to an App Container Image URL. Furthermore, attributes other than the name may be required to unambiguously identify an image (version, OS and architecture). These attributes are expressed in the labels field of the Image Manifest. App Container Image Discovery prescribes a discovery process to retrieve an image based on the name and these attributes.

Image Discovery is inspired by Go's remote import paths.

There are three URL types:

  • Image URLs
  • Signature URLs
  • Public key URLs

Discovery Templates

Image Discovery uses one or more templates to render Image and Signature URLs (while the Public keys URLs aren't templates).

To discriminate between the image and its signature, the templates must contain {ext} and its values MUST be either aci (for the image) or aci.asc (for the signature).

Discovery URL

Image Discovery locates the templates using HTTPS+HTML meta tags retrieved from a discovery URL.

The template for the discovery URL is:


For example, if the client is looking for it will request:

then inspect the HTML returned for meta tags that have the following format:

<meta name="ac-discovery" content="prefix-match url-tmpl">
<meta name="ac-discovery-pubkeys" content="prefix-match url">
  • ac-discovery MUST contain a URL template that can be rendered to retrieve the ACI or associated signature
  • ac-discovery-pubkeys SHOULD contain a URL that provides a set of public keys that can be used to verify the signature of the ACI

Some examples for different schemes and URLs:

<meta name="ac-discovery" content="{os}/{arch}/{name}-{version}.{ext}">
<meta name="ac-discovery" content=" hdfs://{name}-{version}-{os}-{arch}.{ext}">
<meta name="ac-discovery-pubkeys" content="">

When evaluating ac-discovery tags, the client MUST first ensure that the prefix of the AC Name being discovered matches the prefix-match, and if so it MUST perform a simple template substitution to determine the URL at which the resource can be retrieved - the effective equivalent of:

curl $(echo "$urltmpl" | sed -e "s/{name}/$name/" -e "s/{version}/$version/" -e "s/{os}/$os/" -e "s/{arch}/$arch/" -e "s/{ext}/$ext/")

where name, version, os, and arch are set to their respective values for the image, and ext is either aci or aci.asc for retrieving an App Container Image or signature respectively.

Note that multiple ac-discovery tags MAY be returned for a given prefix-match (for example, with different scheme names representing different transport mechanisms). In this case, the client implementation MAY choose which to use at its own discretion. Public discovery implementations SHOULD always provide at least one HTTPS URL template.

In our example above, using the HTTPS URL template, the client would attempt to retrieve the following URLs:


If the first attempt at fetching the initial discovery URL returns a 4xx status code or does not contain any ac-discovery meta tags then attempt the parent path in the name. For example if the user has and we first try but do not discover a meta tag then try and then try

All HTTP redirects MUST be followed when the discovery URL returns a 3xx status code.

Discovery URLs that require interpolation are RFC6570 URI templates.


Implementations of the spec are responsible for enforcing any signature validation rules set in place by the operator. For example, in a testing environment, signature validation might be disabled, in which case the implementation would omit the signature retrieval.

Implementations MUST ensure that the initial name and labels used for discovery matches the name and labels in the Image Manifest.

A label is considered to match if it meets one of two criteria:

  • It is present in the discovery labels and present in the Image Manifest with the same value.
  • It is absent from the discovery labels and present in Image Manifest, with any value.


Authentication during the discovery process is optional. If an attempt at fetching any resource (the initial discovery URL, an App Container Image, or signature) returns a 401 Unauthorized, implementations should enact the authentication policy set by the operator. For example, some implementations might only perform HTTP basic authentication over HTTPS connections.