With this change, we add the following commands to the dist tool:
- `ingest`: verify and accept content into storage
- `active`: display active ingest processes
- `list`: list content in storage
- `path`: provide a path to a blob by digest
- `delete`: remove a piece of content from storage
We demonstrate the utility with the following shell pipeline:
```
$ ./dist fetch docker.io/library/redis latest mediatype:application/vnd.docker.distribution.manifest.v2+json | \
jq -r '.layers[] | "./dist fetch docker.io/library/redis "+.digest + "| ./dist ingest --expected-digest "+.digest+" --expected-size "+(.size | tostring) +" docker.io/library/redis@"+.digest' | xargs -I{} -P10 -n1 sh -c "{}"
```
The above fetches a manifest, pipes it to jq, which assembles a shell
pipeline to ingest each layer into the content store. Because the
transactions are keyed by their digest, concurrent downloads and
downloads of repeated content are ignored. Each process is then executed
parallel using xargs.
Put shortly, this is a parallel layer download.
In a separate shell session, could monitor the active downloads with the
following:
```
$ watch -n0.2 ./dist active
```
For now, the content is downloaded into `.content` in the current
working directory. To watch the contents of this directory, you can use
the following:
```
$ watch -n0.2 tree .content
```
This will help to understand what is going on internally.
To get access to the layers, you can use the path command:
```
$./dist path sha256:010c454d55e53059beaba4044116ea4636f8dd8181e975d893931c7e7204fffa
sha256:010c454d55e53059beaba4044116ea4636f8dd8181e975d893931c7e7204fffa /home/sjd/go/src/github.com/docker/containerd/.content/blobs/sha256/010c454d55e53059beaba4044116ea4636f8dd8181e975d893931c7e7204fffa
```
When you are done, you can clear out the content with the classic xargs
pipeline:
```
$ ./dist list -q | xargs ./dist delete
```
Note that this is mostly a POC. Things like failed downloads and
abandoned download cleanup aren't quite handled. We'll probably make
adjustments around how content store transactions are handled to address
this.
From here, we'll build out full image pull and create tooling to get
runtime bundles from the fetched content.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
With this changeset we introduce several new things. The first is the
top-level dist command. This is a toolkit that implements various
distribution primitives, such as fetching, unpacking and ingesting.
The first component to this is a simple `fetch` command. It is a
low-level command that takes a "remote", identified by a `locator`, and
an object identifier. Keyed by the locator, this tool can identify a
remote implementation to fetch the content and write it back to standard
out. By allowing this to be the unit of pluggability in fetching
content, we can have quite a bit of flexibility in how we retrieve
images.
The current `fetch` implementation provides anonymous access to docker
hub images, through the namespace `docker.io`. As an example, one can
fetch the manifest for `redis` with the following command:
```
$ ./dist fetch docker.io/library/redis latest mediatype:application/vnd.docker.distribution.manifest.v2+json
```
Note that we have provided a mediatype "hint", nudging the fetch
implementation to grab the correct endpoint. We can hash the output of
that to fetch the same content by digest:
```
$ ./dist fetch docker.io/library/redis sha256:$(./dist fetch docker.io/library/redis latest mediatype:application/vnd.docker.distribution.manifest.v2+json | shasum -a256)
```
Note that the hint is now elided, since we have affixed the content to a
particular hash.
If you are not yet entertained, let's bring `jq` and `xargs` into the
mix for maximum fun. The following incantation fetches the same manifest
and downloads all layers into the convenience of `/dev/null`:
```
$ ./dist fetch docker.io/library/redis sha256:a027a470aa2b9b41cc2539847a97b8a14794ebd0a4c7c5d64e390df6bde56c73 | jq -r '.layers[] | .digest' | xargs -n1 -P10 ./dist fetch docker.io/library/redis > /dev/null
```
This is just the beginning. We should be able to centralize
configuration around fetch to implement a number of distribution
methodologies that have been challenging or impossible up to this point.
The `locator`, mentioned earlier, is a schemaless URL that provides a
host and path that can be used to resolve the remote. By dispatching on
this common identifier, we should be able to support almost any protocol
and discovery mechanism imaginable.
When this is more solidified, we can roll these up into higher-level
operations that can be orchestrated through the `dist` tool or via GRPC.
What a time to be alive!
Signed-off-by: Stephen J Day <stephen.day@docker.com>
