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SCS Image Metadata Standard


Many clouds offer standard Operating System images for their users' convenience. To make them really useful, they should contain metadata (properties) to allow users to understand what they can expect using these images.

The specification is targeting images that are managed by the service provider, provided for public consumption. The spec or parts of it however might turn out to be useful whenever someone manages images for somebody else's consumption.


We categorize the image properties into a few buckets

  • Technical requirements and features
  • Image handling aspects
  • Licensing/Maintenance/Support aspects


We suggest plain OS images to be named "Distribution Version", e.g. "openSUSE Leap 15.3" or "Ubuntu 20.04" or "CentOS 8", "Windows Server 2012R2". We do not normally recommend to add more detailed patch levels into the name.

Special variants that include specific non-standard features should be named "Distribution Version Feature1 Feature2".

There are several policies possible to provide updated images to include the latest bug- and security fixes. This is outlined in update policy description below.

Technical requirements and features

This is dependent on whether we deal with VM images or container images.

For VM images (OpenStack), we recommend to use the properties as described in the OpenStack Image documentation.

The following properties are considered mandatory:

  • architecture, hypervisor_type
  • min_disk_size (in GiB), min_ram (in MiB)
  • os_version, os_distro
  • hw_rng_model, hw_disk_bus (scsi recommended, and then setting hw_scsi_model is also recommended)

Note: Disk sizes tend to be measured in GB = 10^9 and not GiB = 2^30 in the disk industry, but OpenStack uses GiB.

The following properties are recommended (if the features are supported):

  • os_secure_boot, hw_firmware_type
  • hw_watchdog_action, hw_mem_encryption, hw_pmu, hw_video_ram, hw_vif_multiqueue_enabled

The trait:XXX=required property can be used to indicate that certain virtual hardware features XXX are required.

Image handling

Image updating

It is recommended that provider managed images are regularly updated. This means that users referencing an image by name will always get the latest image for the operating system indicated by that name (which includes a version number, but not the patch level).

Technically, the thus updated image is a new image and will thus carry a new UUID. It is recommended that the old image gets renamed (e.g. build date or patch level attached) and hidden (os_hidden=true), but remains accessible via its (unchanged) UUID for some time.

The update handling by the provider is described via the properties replace_frequency, uuid_validity, provided_until, and hotfix_hours.

The replace_frequency, provided_until, and hotfix_hours fields reference to the image as referenced by its name.

yearlythe image will get replaced at least once per year
quarterlythe image will get replaced at least once per quarter
monthlythe image will get replaced at least once per month
weeklythe image will get replaced at least once per week
dailythe image will get replaced at least once per day
critical_bugthe image will get replaced for critical issues only
neverthe image referenced by name will never change (until the date provided_until)

Note the at least wording: Providers can replace images more often. The frequency is starting from the first release; so an image published on 2021-04-14 with an update frequency of monthly, should be replaced no later than 2021-05-14. Due to weekends etc., up to 3 days later is not considered a violation of this policy. So a valid sequence from an image with monthly update frequency might be 2021-04-14, 2021-05-14, 2021-06-15, 2021-07-14, 2021-07-27 (hotfix), 2021-08-13 ...

Promises to update the registered public images typically depend on upstream image providers (Linux distributors, OS vendors) keeping their promises to build and provide updated images. Failures from upstream are not a reason to claim the cloud provider to be in violation of his promises. However, if the provider observes massive upstream failures (which can e.g. cause increased security risks), we advise the provider to inform the users.

We recommend updating images at least monthly.

The hotfix_hours field indicates how providers deal with critical security issues that affect the images; it is an optional field that contains a numerical value, indicating how quickly (in hours) a new image is provided after the latter of the points in time that the issue becomes public and a tested fix is available as maintenance update from the upstream distribution. A value of 0 indicates a best-effort approach without firm SLAs; the field not being present indicates no commitment. A value of 48 would indicate that the provider commits to a new image within 48hrs. A critical issue is defined as a security vulnerability with a CVSS score of 9.0 or higher that affects a package that is included in the image.

The provided_until field is supposed to contain a date in YYYY-MM-DD format that indicates until when an image under this name will be provided and (according to the replace_frequency) updated at least. (Providers are free to provide updates for longer or leave the non-updated image visible for longer.) If this field is set to none, no promises are made, if it is set to notice, updates will be provided until a deprecation notice is published. (The values are the same as for below uuid_validity, except that forever and last-N don't make any sense.)

The uuid_validity field indicates how long the public image will be referencable by its UUID.

noneUUID will only be valid as long as the content does not change
last-NThe last N images for newer replacement will remain accessible via UUID
YYYY-MM-DDUUID will be valid until at least the date YYYY-MM-DD
noticeUUID will remain valid until a deprecation notice will be published
foreverUUID will remain valid for as long as the cloud operates

Note that the old images must be hidden from the image catalogue or renamed (or both) to avoid failing referencing by name. Note that last-N may be limited by the provided_until date.

The three properties uuid_validity, provided_until and replace_frequency are mandatory; the field hotfix_hours is optional.

All dates are in UTC.


Providing an image with name OPSYS MAJ.MIN with replace_frequency=monthly, provided_until=2022-09-30, uuid_validity=2022-12-31, hotfix_hours=0 means that we will have a new image with this name at least once per month (starting from the initial release) until the end of September 2022. Old images will be hidden and/or renamed, but remain accessible via their UUID until at least the end of 2022 (in Universal Time). The provider makes an effort to replace images upon critical security issues out of order.

Image Origin

  • Mandatory: image_source needs to be a URL to point to a place from which the image can be downloaded. (Note: This may be set to the string "private" to indicate that the image can not be freely downloaded.)

  • Mandatory: image_description needs to be a URL (or text) with release notes and other human-readable data about the image.

  • Recommended tag: managed_by_VENDOR

Note that for most images that come straight from an upstream source, image_description should point to an upstream web page where these images are described. If download links are available as well on that page, image_source can point to the same page, otherwise a more direct link to the image should be used, e.g. directly linking the .qcow2 or .img file. If providers have their own image building machinery or do some post-processing on top of upstream images, they should point to the place where they document and offer these images.

Image build info

  • Mandatory: image_build_date needs to be YYYY-MM-DD or YYYY-MM-DD hh:mm[:ss] (time in UTC, 24hrs clock). All publicly released and generally recommended patches before this date must be included in the image build. If the cutoff date is earlier, this cutoff date needs to be set instead, even if the actual build happens significantly after the cutoff date. If not all patches can be included for a good reason, then the patchlevel field (see below) must be used to describe the patch status.

  • Mandatory: image_original_user is the default login user for the operating system which can connect to the image via the injected SSH key or provided password. (This can be set to none if no default username exists for the operating system.)

  • Optional: patchlevel can be set to an operating specific patch level that describes the patch status — typically we would expect the image_build_date to be sufficient.

  • Recommended: os_hash_algo and os_hash_value: The sha256 or sha512 hash for the image file. (This references the image file in the format it is stored in, we recommend raw over qcow2 for systems that use ceph.) Note that these values are typically generated automatically upon image registration.

  • Recommended tag: os:OPERATINGSYSTEM

Licensing / Maintenance subscription / Support

Some images require a license; in many cases the cloud providers include the license cost by a per-use (e.g. hourly) fee. However, it is also possible sometimes that customers use their own license agreements with the OS vendor with a bring-your-own-license (BYOL) program. These properties may be attached to the image. Note that free Linux images might not use any of these properties, except maybe maintained_until. Note that Windows images would typically require license_included, subscription_included. A boolean property that is not present is considered to be false.

  • Optional: license_included (boolean) indicates whether the flavor fee includes the licenses required to use this image. This field is mandatory for images that contain software that requires commercial licenses.
  • Optional: license_required (boolean) indicates whether a customer must bring its own license to be license compliant. This can not be true at the same time as the previous setting. This field is mandatory IF customers need to bring their own license to use the image.
  • Optional: subscription_included (boolean) indicates that the image contains already a maintenance subscription which typically gives access to bug fixes, security fixes and (minor) function updates. If a subscription is included, the CSP should have prepared the image to also receive the provided maintenance updates from the vendor (optionally via a mirror).
  • Optional: subscription_required (boolean) indicates that the customer requires a maintenance subscription from the OS vendor in order to receive fixes (which is often also a prerequisite to be eligible for support).
  • Optional: maintained_until: YYYY-MM-DD promises maintenance from the OS vendor until at least this date (in UTC).
  • Optional: l1_support_contact contains a URI that provides customer support contact for issues with this image. Note that this field must only be set if the service provider does provide support for this image included in the image/flavor pricing (but it might be provided by a contracted 3rd party, e.g. the OS vendor).

Conformance Tests

The script retrieves the image list from a configured cloud and checks each image for the completeness and consistency of mandatory properties.