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Update the document and focus only on how images are tagged
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With this commit the scope of the document is reduced and it is only
considered how images are finally tagged into the registry. How are
they consumed is out of the scope of the document.
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@davidcassany
davidcassany committed Oct 1, 2018
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233 changes: 122 additions & 111 deletions 2018/001-containers-tagging-strategy.md
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Expand Up @@ -45,68 +45,69 @@ points to a unique image.

## Problem description

Kubernetes pulls the images referenced in the Kubernetes manifest. Kubernetes
checks if those images are available locally and if not pulls them from the
registry. It means that if an image reference is overwritten with the same
reference Kubernetes will not pull it again and then images of a running
cluster start to diverge from the images available inside the registry.

The above situation could be specially tricky when, for instance, new nodes are
added to a cluster that is running silently with outdated images. This is
particularly tricky because it can easily end up in a situation where the
cluster is running multiple versions of the same image. Let's imagine the
cluster was running some outdated mariadb image, then when a new node is added
and this node pulls the images from the registry (including the mariadb one)
it will pull and run different versions of the images without noticing it.
Running a cluster with multiple mariadb versions without control or being
enforced by the administrator is clearly dangerous and painful to debug if it
causes some issue at some point.

The tagging strategy of the delivered images must be aligned with the build and
update workflow. Otherwise it is pretty simple to end up in similar situations
as exposed above.
With the current workflow, where each container image build is pushed to a
registry, some strategy to identify versions and releases from the registry
client perspective is not as abvious as it should.

It is important to note that container image updates happen due to a source
change (KIWI descriptor file or Dockerfile update) or due to a former package
update. Thus any update of a package required during the image build,
triggers a new image release to the registry. According to that images
publised in the registry are expected to be updated often.

Having updates on images without any visible change into the image references
can be confusing from the registry client perspective. In this context pooling
tags for concrete namespace will result always into the same list, being
immutable across updates.

```
GET /v2/<name>/tags/list
```

However downloaded binaries of an image will not be immutable. It should
be easy from the registry client perspective to track image updates and not
just endup with different image binaries and wihout any option to reference
an old image without having to use the image digest.

Another situation where the tags of the image should be used with care is when
the image delivers certain concrete application and the image is tagged
according to this application version. This practice is helpful in
order to create readable and meaningfull image references.

For instance, a reference like `<registry>/opensuse/mariadb:10.2` clearly
suggests the image is providing `mariadb` v10.2.

While this may look like a simple and safe way of tagging the image it could
easly lead to some confusing inconsistences when the image updates are pushed
to the registry in an automated fashion as it is the case with the Open
Build Service. In this case the tag 10.2 would be part of the image sources
and the match within the package version and tag would happen manually. This
clearly turns to be problematic in rolling release distributions where doing
this match manually is not really possible. But also could be problematic
if the exacte package version as tag is being used, as even in the scope of a
non rolling distribution the package version could be silently increased.


## Current situation

Before SUSE container registry images have been and are being delivered wrapped
in RPMs. This way the update and delivery worklfow is the same as any other
packages. However within the containers ecosystem this presents some issues:

* Containers ecosystems based on Kubernetes are not designed to interact with
RPM repositories but container registries.

* Installing, updating and uninstalling RPMs can be costly. In CaaSP it
requires a transactional update.

* Using RPMS requires additional services to handle images upload into docker
or crio daemons. See container-feeder tool or sle2docker. While systems like
Kubernetes are already prepared to pull and load images from a registry.

* Big downloads, RPMs do not benefit from layers reuse. RPMs contain all
container layers even only unique layers are finally loaded into the daemon.

In this context stable tags for references are being used. For CaaSP images are
tagged with a fixed tag (usually the version of the main containerized
application) defined in the kiwi file description and not modified during
updates, however the RPM includes a metadata file used by the container-feeder.
At loading time container-feeder re-tags the image with the '<tag>-<release>'
and 'latest' tags, this way, even using stable tags in manifests the images
are locally tagged with multiple tags, including the specific release tight to
an RPM and image build.

## Proposed change

Introduce an app or service to track updates in the registry, something similar
as what zypper does for RPMs and repositories. This is a key point and the
proposed strategy here assumes there is going to be something like that
available.

The proposed change mimics the tagging strategy exposed above when using images
wrapped in RPMs, but using the SUSE container registry and OBS instead.

`kiwi` and `skopeo` tools can build images with multiple tags (since versions
9.15.3 and 1.30.0 respectively). Thus the main idea is to keep using stable
tags to facilitate image references in manifests but also tag images in
Tags are hardcoded in image sources, thus any image rebuild ends up with the
same tag. Currently
[replace_using_package_version](https://github.com/openSUSE/obs-service-replace_using_package_version)
is an OBS service that can be used to replace a needle from sources with the
version of a given package.

This is helpful to tie and chain a tag to certain package version, however
it does not cover the case were the image is being updated due to another
package, in this case, any rebuild would be tagged the same.

## Proposed changes

Tags from the registry perspective and client perspective are cheap and simple.
Having an image tagged multiple time with different refences causes no issues
and can be helpful to easily provide some additional context.

The main idea is to keep using stable tags to facilitate image references
in systems where no dynamic references are possible but also tag images in the
registry with additional dynamic tags, based on included packages versions or
image release numbers. This way we could have in the registry multiple images
with a meaningful tags pointing to a specific version, but also a stable tag
Expand All @@ -117,9 +118,9 @@ Imagine there is a mariadb image tagged as below in the registry (emulating

```
REPOSITORY TAG IMAGE ID
caasp/mariadb 10.2 1ade07d13d13
caasp/mariadb latest 1ade07d13d13
caasp/mariadb 10.2.15-3.2 1ade07d13d13
opensuse/mariadb 10.2 1ade07d13d13
opensuse/mariadb latest 1ade07d13d13
opensuse/mariadb 10.2.15-3.2 1ade07d13d13
```

Note that the build ID is being used as the release number. Build ID are
Expand All @@ -131,10 +132,10 @@ mariadb release happens adding the new image could look like that:

```
REPOSITORY TAG IMAGE ID
caasp/mariadb 10.2 70b5d81549ec
caasp/mariadb latest 70b5d81549ec
caasp/mariadb 10.2.15-3.2 1ade07d13d13
caasp/mariadb 10.2.15-3.3 70b5d81549ec
opensuse/mariadb 10.2 70b5d81549ec
opensuse/mariadb latest 70b5d81549ec
opensuse/mariadb 10.2.15-3.2 1ade07d13d13
opensuse/mariadb 10.2.15-3.3 70b5d81549ec
```

Note that old image would be still accessible with the concrete version tight
Expand All @@ -144,50 +145,60 @@ result in:

```
REPOSITORY TAG IMAGE ID
caasp/mariadb 10.2 70b5d81549ec
caasp/mariadb latest 284549eacf84
caasp/mariadb 10.2.15-3.2 1ade07d13d13
caasp/mariadb 10.2.15-3.3 70b5d81549ec
caasp/mariadb 10.3 284549eacf84
caasp/mariadb 10.3.1-3.4 284549eacf84
opensuse/mariadb 10.2 70b5d81549ec
opensuse/mariadb latest 284549eacf84
opensuse/mariadb 10.2.15-3.2 1ade07d13d13
opensuse/mariadb 10.2.15-3.3 70b5d81549ec
opensuse/mariadb 10.3 284549eacf84
opensuse/mariadb 10.3.1-3.4 284549eacf84
```

Aging all three image versions are accessible and there is a stable reference
to get the latest. However this time, since the mariadb version changed, there
is also version tag to get the latest release of an specific version.

To get to this sort of tagging strategy some modifications are required in our
workflow and toolchain.

* Need correct versions of KIWI and skopeo in relevant streams (mostly SLE 15)
With it the stable tags could be already defined ('latest' and '10.3' of the
above example)

* Need OBS to add/modify tags at build time using the KIWI command call to
append the build ID and/or release numbers. Currently the tag can be modified
at call time in KIWI with the `--set-container-tag`. Probably this can be
used to append release numbers to a tag.

* Need OBS to push to the registry the image with all the tags included within
the built binary.

* SUSE requires some tool to track changes in the registry and be capable to
detect when updates are available.

Tagging an image with the main package version it can be achieved by using the
`obs-service-replace_using_package_version` service. This is currently being
used in Head project in IBS & OBS.

One of the advantages of using this strategy is that it does not necessarily
require a Kubernetes manifest update for each image update. The manifest could
be set to some stable mariadb version (e.g. 10.2) and still be updated if the
image requires a rebuild due some security fixes in the SLE base image or even
in mariadb itself. In addition, Kubernetes manifests can be configured with
the [imagePullPolicy="Always"](https://kubernetes.io/docs/concepts/containers/)
to make sure Kubelet always pulls de image from the registry regardless if it
was already pulled or not. This way Kubelet is always asking for the latest
image for a given tag.

Finally, using this approach, will be simple to clearly identify a local image
running in a cluster as all supported images will always be in the registry
tagged at least with a complete version and release numbers.
For that to happen it required for the build system to support three features:

* Support for multiple tags

Multiple tags are possible in `kiwi` since v9.15.3 (only if `skopeo`
>= 1.30 is also available present). The image tarballs in that case support
multiple tags, but this is not reflected to the registry when pushed,
only the main tag is included.
However the Build Service has an alternative to include additional tags into
the pushed images. Including a syntax like

```
<!-- OBS-AddTag: <name>:<tag> -->
```
into the XML kiwi file will make OBS to push the image with the provided
additional tag.

* Support to include build ID within a tag

Currently there is no way to achieve that. However a couple of quick
(and potentially hacky) options could be considered to fill the gap:

1. Modify the kiwi build recipe in OBS to retag the image at build time.
Use `--set-container-tag` command line flag to modify the tag in sources
and append the build ID. Requires an update of the build recipe.
However this would only modify the given tag, not append a new one.

2. Adding some kind of post hook to the kiwi build to include the buildID
into the build image. This could be done relatively easy in current
skopeo versions by calling:

```
skopeo copy docker-archive:<image-tarball>:<name>:<tag>
docker-archive:<image-tarball>:<name>:<tag>
--additional-tag <name>:<tag_including_buildID>
```
However this requires recopying the image or even uncompress, copy and
recompress again if the build was using compressed builds.
3. Extend `OBS-AddTag: <name>:<tag>` syntax to enable the buildID inclusion.
* Support to tag according to the version of a package, or part of it.
It can be achieved using the `replace_using_package_version` OBS service.
To fill the gap only some way to include the buildID into the already
defined tags is missing.

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