SQR-059: RSP Notebook container tag conventions

  • Adam Thornton

Latest Revision: 2021-06-15

Tag formats for RSP Notebook Images

The Rubin Observatory Sciplat-Lab containers are identified via their tags. These tags have conventional formats understood by Cachemachine and used to order the images and construct their display names. For certain types of tags, it is also possible to transform the tag data into a semantic-version-compatible format, which therefore makes specifying version constraints much easier.

Tag Types

There are six categories of acceptible tags:

  • Alias
  • Release
  • Weekly
  • Daily
  • Release Candidate
  • Experimental

Anything that does not match the format of one of these tags will be categorized as:

  • Unknown

Tag Fragments

In general, tags can be postfixed with an optional Cycle number or a rest set of one-or-more underscore-separated fields. The Cycle is currently unique to T&S RSP instances, and specifies the version of XML used in this image. It will have the form c<digits>.<digits> where the first group of digits is the cycle number and the second group of digits is the build iteration for that cycle, e.g. c0019.001.

The rest fragment will most often encode a build datestamp, but can be anything (but see the discussion in “Semantic Versions”).

For image types for which semantic versions are supported, both of these fields will end up packed into the build semver field. If both are present in a single tag, the cycle must precede rest.

Note on “Semantic Version”

Certain tag types (“Release”, “Weekly”, “Daily”, and “Release Candidate”) can be used to construct version numbers that are syntactically equivalent to a semantic version [major].[minor].[patch] with, perhaps, prerelease and build fields appended.

These are not actually semantic versions, in that the compatibility guarantees of true semantic versions are not present. The version numbers are constructed purely to aid sortability and version choosing, because there are excellent extant tools to manipulate semantic versions.

Within a given release type for a given image, semantic versions (if supported for that type) can be used to compare two versions of the Lab container: a higher number is more recent. Images are comparable only within a tag type. That is, the semantic versions from (e.g.) a weekly and a release image are not comparable.

The cycle and rest fields end up within the build field of a semantic version. The information in cycle is already semver-compatible, and rest is transformed as follows: underscores become dots, and any nonalphanumeric characters remaining are simply dropped.

Alias Tags

Alias tags are exactly those tags we do not expect to be stable. There are currently two items in this category: the recommended tag, and latest, which is defined by Docker as special and is the tag-of-last resort: if a docker image is specified without a tag, its tag is implicitly latest. Our builds include latest_weekly, latest_daily, and latest_release tags, but Data Management does not make use of them in the spawner options form.

The “recommended” tag is a singleton: it is always an alias pointing to the image that, at any moment in time, we believe most users should be using. Conventionally, this will be a recent weekly build, and conventionally it has been recommended. It can be any arbitrary string, and this will be useful, for example, to allow different recommended images in different environments (e.g. recommended_idf versus recommended_summit).

Alias tags are passed in to the tag parsing machinery from the outside as a list of strings. Only a tag that is an exact string match to one of those strings will be categorized as an alias tag. If it is set, the “recommended” tag will always be prepended to the alias tag list. If the alias tag list does not include latest, even though that is by its nature an alias tag, the latest image will not be prepulled.

Tag Format

An alias tag can be an arbitrary string (set in the cachemachine configuration); conventionally the recommended tag has been recommended.

Display Name

The display name for an alias tag is constructed by replacing any underscores with spaces and then titlecasing the result. Thus recommended has the display name Recommended, and if the tag perfectly_cromulent were an alias tag, it would have the display name Perfectly Cromulent. This may be modified according to tag resolution to include a list of other names this image is known by, for instance, Recommended (Weekly 2021_20).

Semantic Version

The “recommended” tag does not itself have a semantic version; however the underlying image to which it is a pointer almost certainly does.

Notes on alias tags

An alias tag requires some special handling of the tag within cachemachine to determine what the image digest corresponding to the recommended image is, and construct a mapping of its display name to the actual image–this information is used both in the spawner options form to tell the user what they’re getting with and in the Lab UI to remind the user what they’re using.

Most users will use the “recommended” tag most of the time, and the tag is generally applied to the most recent weekly build, as soon as it has been possible to certify a weekly build by subjecting it to our QA process.

Any other defined alias tags will appear in the list between “recommended” and the first of the non-alias images.


Release images are the (roughly twice a year) official stack releases. They are intended to be kept available indefinitely, and for the RSP machinery to, at any point in time, be able to run the current release and the two prior to it. (If older releases are required, it may take some work, up to and including a separately-constructed RSP instance) to make them runnable.

Tag Format

Release tags are now of the form r[major]_[minor]_[patch], e.g. r21_0_1. Prior to Release 18, they were not underscore-separated, e.g. r170. The first two digits are the major version, and the last one is the minor version. In this form, the patch version is always 0. Cycle and rest are permitted, so, for instance, all of r_21_0_1_c0019.001, r_21_0_1_20210703, and r_21_0_1_c0019.001_20210703 are allowed.

Display Name

The display name for a release is of the form Release r[major].[minor].patch; thus r21_0_1 has the display name Release r21.0.1. Additional components (cycle and extra) are permitted and will be appended in the following form: r21_0_1_c0020.002_20210703 becomes Release r21.0.1 (SAL Cycle 0020, Build 002) [20210703].

Semantic Version

The semantic version of a release tag is, actually, [major].[minor].[patch]. r21_0_1 has version 21.0.1.

Cycle and build version will be added as described above. Thus: r21_0_1_c0020.002_20210703 would have the semantic version 21.0.1+c0020.002.20210703.

Notes on “release” category

These tags differ from other categories by not having an underscore between the type and the release identifier. There is no reason for this other than historic convention.


Weekly images are the bread-and-butter workhorse images. Most users will use the latest weekly that has been blessed as “recommended”. There are three noteworthy things about the weekly images. First, they are the feedstock for “recommended”; second, it is always a particular weekly image that is chosen as the basis for a release image; and third, we make claim that the weekly image is going to be fit-for-purpose and therefore not utterly broken.

Tag Format

Weekly tags are of the form w_[year]_[week], e.g. w_2021_19. They may have additional cycle and rest components; w_2021_19_c0019.001 is an acceptable weekly tag, for instance, as is w_2021_19_20210513 or indeed w_2021_19_c0019.001_20210513.

Display Name

The display name is Weekly [year]_[week]; w_2021_19 has the display name Weekly 2021_19. As with releases and release candidates, additional components are formatted and appended. Thus w_2021_19_c0019.001 would have the display name Weekly 2021_19 (SAL Cycle 0019, Build 001).

Semantic Version

A weekly’s semantic version is [year].[week].0. w_2021_19 has the version 2021.19.0. Any additional components are used as the semver build string (with underscores replaced by periods), so w_2021_19_c0019.001 would become 2021.19.0+c0019.001.


Daily images are, as the name implies, produced every night. They are not guaranteed to work. They are generally used only by users needing bleeding-edge features that haven’t made it into a weekly yet.

Tag Format

Daily tags are of the format d_[year]_[month]_[day]; as with weekly builds, additional underscore-separated components may exist.

Display Name

A Daily display name is Daily [year]_[month]_[day], so d_2021_05_11 becomes Daily 2021_05_11. Additional components are handled as for weeklies.

Semantic Version

The version for a daily image is [year].[month].[day]. d_2021_05_11 is simply 2021.05.11. Additional components go into the build string, as for other image types.

Release Candidate

A release candidate follows the same rules as a release, except that it will have one and only one additional component, rc[number], which is an incrementing sequence number.

Tag Format

The tag format is exactly that of a release format, with an additional underscore-separated component, rc[number]. Cycle and rest are permitted.

Display Name

The display name resembles a Release version, except that it begins with “Release Candidate”; the additional component will be appended with a dash (to match the semantic version string). r22_0_0_rc1 will have the display name Release Candidate r22.0.0-rc1.

Semantic Version

The primary components of the version are the same as release: major, minor, patch (in general, patch will be 0 because it will be a prerelease). rc[number] will be used as the prerelease (rather than the build) field. Thus, r22_0_0_rc1 will have the version 22.0.0-rc1, and r22_0_0_rc1_c0020.003_20210609 would have the version 22.0.0-rc1+c0020.003.20210609.


Experimental tags are used mostly by people working on the Lab machinery itself (which is to say, mostly the author of this technote at this point). They start with exp_ and that’s really all you can say about them (but see below).

Tag Format

The experimental tag starts with exp_. In practice (and largely as an artifact of the build process), it often looks like exp_[some-other-tag]_[descriptor], e.g. exp_w_2021_13_nosudo.

Display Name

My preference is to try the strategy hinted at above: the first word of the display name is “Experimental”, and then the rest of the tag following exp_ is fed through the display name parsing process again; much of the time this will result in a sane display name string. For instance exp_w_2021_13_nosudo would yield Experimental Weekly 2021_13 [nosudo]. If that re-parse fails, just use the string following exp_ as the name. For instance exp_ajt_test would give the display name Experimental ajt_test.

Semantic Version

Experimentals will not have a semantic version string. The only way to sort them is lexigraphically by tag, and no temporal information is implied.

Unknown Images

Any image whose tag is not parseable according to any of the above categories falls into an unknown type. Fundamentally these are handled rather like experimentals. There is no display name separate from the tag string, and there is no semantic version. They have no sort order other than lexigraphic.