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Proposal for new command "kustomize localize" (#4590)

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* Propose kustomize localize

Write mini KEP proposal for new command kustomize localize.

* Incomplete commit

* Update proposal based on Katrina's feedback

Changes are summarized in comments in PR

* Address feedback in comments
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Anna Song
2022-05-15 10:28:17 -07:00
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# Localize Command
**Authors**:
- annasong20
**Reviewers**:
- natasha41575
- KnVerey
**Status**: implementable
## Summary
The `kustomize localize` command creates a “localized” copy, of both the target kustomization and files target
references, in which the kustomization files contain, instead of remote references, local paths to their downloaded
locations. The command is part of an effort to enable `kustomize build` to run without network access.
## Motivation
monopole originally proposed the command `kustomize localize`
in [this issue](https://github.com/kubernetes-sigs/kustomize/issues/3980).
Users run `kustomize build` in many environments with limited network access. For example, CI/CD pipelines often only
have access to the internal network. Server-side applications like Config Sync are concerned with the security
vulnerabilities of git, which `kustomize build` uses to fetch remote files.
These use cases would benefit from a kustomize solution that downloads all remote files that a `kustomize build` target
references, into a copy of target that references the downloaded files instead. Admins could upload the localized copy
to an internal repo so that pipelines and applications can run `kustomize build` on the copy without a network
dependency.
The proposed command nearly achieves the solution by downloading all remote files directly referenced by the target or
by a recursively referenced kustomization file. The command also downloads remote exec binaries of referenced KRM
functions, which are the only potential source of remote files other than kustomizations. The only remote files that
this proposal does not cover and that `kustomize build` still needs to run are remote images and custom fields in KRM
functions. Downloaded images would live only in local caches and thus, are not worth localizing. Kustomize cannot
currently identify custom fields, though this may change with one of the proposed solutions
in [issue #4154](https://github.com/kubernetes-sigs/kustomize/issues/4154).
The proposed command has the added benefit of increasing user confidence in the integrity of their kustomization builds.
Locally downloaded files, unlike urls, give users full control of file content. At the same time, the command does this
without modifying the original kustomization so that users can always run the command on the original again to fetch
upstream changes.
**Goals:**
1. This command should localize
* all remote files that a kustomization file directly references
* remote exec binaries of referenced KRM functions
This command achieves this goal if, in the absence of remote images and custom fields in KRM
functions, `kustomize build` can run on the localized copy without network access.
**Non-goals:**
1. This command should not localize remote images or custom fields in KRM functions.
2. This command should not copy files that the target kustomization does not reference.
3. This command should not serve as a package manager.
## Proposal
The command takes the following form:
<pre>
<b>kustomize localize</b> <ins>target</ins> <ins>newDir</ins> [-s <ins>scope</ins>] [-n]
</pre>
where the arguments are:
* `target`: a directory with a top-level kustomization file that kustomize will localize; can be a path to a local
directory or a url to a remote directory
* `newDir`: optional destination directory of the localized copy of `target`; if not specified, the destination is a
directory named `localized-{target}` in the same directory as `scope`
and the flags are:
* `-s`, `--scope`
`scope`: optional root directory, files outside which kustomize is not allowed to copy and localize; if not specified,
takes on value of `target`
* `-n`, `--no-verify`: do not verify that the outputs of `kustomize build` for `target` and `newDir` are the same after
localization
The command creates a copy of the `target` kustomization and the local files that `target` references at `newDir`. We
define the "files that `target` references" as:
* kustomization files that `target` directly or transitively references
* configuration files that referenced kustomization files reference
* exec binaries of referenced KRM functions
Here, configuration file means a non-kustomization yaml file. The command cannot run on `target`s that need
the `--load-restrictor LoadRestrictionsNone` flag on `kustomize build`. The command only copies referenced files that
reside inside `scope`.
The command localizes the copy of `target` at `newDir` by downloading all remote files that `target`
references. Users do not have executable permission for downloaded exec binaries that KRM functions reference and for
each such exec binary, the command will print a warning message to that effect.
The command creates a new `localized-files` directory, next to the file that referenced the downloaded files, to hold
said files. Inside `localized-files`, the downloads are located on path:
<pre>
<ins>domain</ins> / <ins>organization</ins> / <ins>repo</ins> / <ins>version</ins> / <ins>path/to/file/in/repo</ins>
</pre>
where `version` corresponds to the `ref` query string parameter in the url, though ideally `version` ia a stable tag as
opposed to a branch.
The command replaces remote references in `newDir` with the local paths of the downloaded files. To help ensure
that `newDir` is a clean copy, the command additionally overwrites absolute path references into `target` to point
to `newDir`.
As a convenience to the user, in the absence of the `--no-verify` flag, the command automatically tries to
run `kustomize build`, without any flags, on `target` and the localized `newDir` to compare their outputs. The command
indicates success if the outputs match and throws an error with the diff summary otherwise. This check, however, is not
useful for certain `target`s, including those that need flags to build. In these cases, the command prints next steps
that users can follow to check the output themselves. For example, for `target`s that reference KRM functions with a
remote exec binary, the command suggests the user:
1. add executable permissions for the downloaded exec binaries in `newDir` **that the user trusts**
2. run `kustomize build` with flags `--enable-alpha-plugins --enable-exec` and self-verify the outputs
**Error cases**:
* `target` does not have a top-level kustomization file
* `kustomize build` needs `--load-restrictor LoadRestrictionsNone` to run on `target`
* `newDir` already exists
* `scope` does not contain `target`
* `target` references a local path that traverses outside of `scope`
* remote reference does not have a `version`
* `localized-files` directory already exists
* kustomization file is malformed
* cycle of kustomization file references exists
* `kustomize build` produces different output for `target` and `newDir` in the absence of `--no-verify`
Depending on feedback, we may add an `--overwrite` flag in the future to allow users to update an existing `newDir` by
running the command again.
**Warning cases**:
* KRM function references remote exec binary, in which case the downloaded exec binary is not executable
* KRM function has container image that the user might not have locally
### User Stories
#### Story 1
My company’s CI/CD pipeline currently pulls an `example` directory from our internal package management site. I want the
CI/CD pipeline to additionally run `kustomize build example/overlay`. My setup looks like this:
```shell
└── example
├── overlay
│ └── kustomization.yaml
└── base
└── kustomization.yaml
```
```shell
# example/overlay/kustomization.yaml
resources:
- ../base
- https://raw.githubusercontent.com/kubernetes-sigs/kustomize/v1.0.6/examples/helloWorld/deployment.yaml
```
```shell
# example/base/kustomization.yaml
resources:
- github.com/kubernetes-sigs/kustomize/examples/multibases?ref=v1.0.6
- https://raw.githubusercontent.com/kubernetes-sigs/kustomize/v1.0.6/examples/helloWorld/configMap.yaml
```
I get an error from `kustomize build` in the pipeline because my configurations have remote references, but my CI/CD
pipeline does not have external network access.
Fortunately, I remember that I can run `kustomize localize` on `example/overlay` on my local machine. I can then upload
the localized directory to my company’s internal package management site for the CI/CD pipeline to pull and build
instead. I run
`kustomize localize example/overlay -s example`, where my `target` is `example/overlay`, I accept the default location
and name of `newDir`, and I expand my `scope` to `example` because `example/overlay` references `example/base`. I get
the following output:
```shell
$ kustomize localize example/overlay -s example
SUCCESS: example/overlay, localized-overlay produce same kustomize build output
```
```shell
├── example # old kustomization directory
│ ├── overlay
│ │ └── kustomization.yaml
│ └── base
│ └── kustomization.yaml
└── localized-overlay # the new, localized kustomization directory
├── kustomization.yaml
├── base
│ ├── kustomization.yaml
│ └── localized-files
│ └── github.com
│ └── kubernetes-sigs
│ └── kustomize
│ └── v1.0.6
│ └── examples
│ ├── helloWorld
│ │ └── configMap.yaml
│ └── multibases
│ ├── base
│ │ └── pod.yaml
│ ├── dev
│ ├── kustomization.yaml
│ ├── production
│ └── staging
└── overlay
├── kustomization.yaml
└── localized-files
└── github.com
└── kubernetes-sigs
└── kustomize
└── v1.0.6
└── examples
└── helloWorld
└── deployment.yaml
```
```shell
# localized-overlay/overlay/kustomization.yaml
resources:
- ../base
- ./localized-files/github.com/kubernetes-sigs/kustomize/examples/helloWorld/deployment.yaml
```
```shell
# localized-overlay/base/kustomization.yaml
resources:
- ./localized-files/github.com/kubernetes-sigs/kustomize/examples/multibases
- ./localized-files/github.com/kubernetes-sigs/kustomize/examples/helloWorld/configMap.yaml
```
Now, I upload `localized-overlay` from my local setup to my company’s internal package management site. I change the
commands in my CI/CD pipeline to pull `localized-overlay` before running `kustomize build localized-overlay`, and the
command executes successfully!
### Risks and Mitigations
One could argue that while uploading the localized `newDir` to a repository, a user could accidentally leak Secrets that
were originally remote in a more private repo. This event is not very likely, as the servers that the user intends to
consume the localized kustomizations often only have access to internal, private networks and repos. Nonetheless, users
should have enough context in the case of Secrets to make the right decision. Users should have a basic understanding of
the files that their `target` kustomizations reference and of the files that they plan to upload to repos. Secret
configurations are also not too difficult to identify.
Exec binaries that KRM functions reference are a different story. `kustomize localize` downloads remote exec binaries
that, if malicious, are capable of almost anything during subsequent `kustomize build` calls. The command mitigates this
risk by leaving these downloaded exec binaries without executable permissions and warning the user, as mentioned in
**Proposal**. `kustomize build` can only run the exec binary after the user deems the binary safe and changes its
permissions.
Still another risk may be that if a user's kustomization tree is large, `kustomize localize` may be copying files from
unexpected locations. The command mitigates this risk with the `scope` flag. If not set, `kustomize localize` only
copies files in `target`. Otherwise, the user specifies `scope`and understands that `kustomize localize` only copies
files in `scope`. The qualification that the command can only localize `target`s that follow load restrictions helps
mitigate this risk as well.
### Dependencies
N/A
### Scalability
Large kustomization trees slows the performance of `kustomize localize`. These trees can have large local subtrees, have
large remote subtrees, be deeply nested, or be wide, with each overlay referencing multiple bases. Regardless of the
cause, large kustomization trees inevitably takes longer to copy and download. Parts of the kustomize code are not
thread-safe, which precludes parallel execution.
On a separate note, a chain of remote kustomization directories in which the current kustomization file references the
next remote kustomization directories could create a `newDir` with deeply nested `localized-files`. This directory
structure would impede users’ navigation of `newDir`. However, this scenario should be unlikely as most kustomizations
only consist of a few layers.
The creation of the `localized-files` directory local to the referencing kustomization file additionally prevents the
different layers of kustomization files from sharing the same copy of the remote files. Following the same logic,
different potential `target` directories cannot share copies either.
## Drawbacks
Users whose layered kustomizations form a complex directory tree structure may have a hard time finding an
appropriate `scope`. However, many kustomizations exist in repositories, allowing the user to easily choose the repo
root as a valid `scope`. The warning messages that `kustomize localize` outputs for reference paths that extend
beyond `scope` should also help.
## Alternatives
* Instead of downloading into `newDir`, `kustomize localize` could download all remote files into a directory specified
by some global environment variable (like in Golang), which would preclude deeply nested directories and allow
different kustomization files to share configurations. On top of that, if `kustomize build` had the added
functionality to check for previous downloads of remote references at said global location, `kustomize localize` would
not need to overwrite the remote references in `target` to the local downloads. As a result, `kustomize localize`
would need to neither write to `target` nor copy `target` into `newDir`. The user would not need to specify a `scope`
either. <br></br>
Despite its advantages, the alternative design violates the self-contained nature of each kustomize layer. Users would
be unable to upload a fully localized kustomization directory in version control. Furthermore, this alternative
complicates the existing kustomize workflow by requiring the setup of global environment variables.
<br></br>
* The command could, instead of making a copy, modify `target` directly. However, users would not have an easy way to
undo the command, which is undesirable.
<br></br>
* Instead of requiring the user to specify a second argument `scope`, the command could by definition limit its copying
to `target`. However, in the case of **Story 1**, the command would force the user to set `target` to `example` in
order to include `example/base` in the localization of `example/overlay`. The user would then have to create a
kustomization file at `example` that points to `example/overlay` under the `resources` field. The creation of the
kustomization file solely for this purpose is messy and more work for the user.
## Rollout Plan
This command will have at least alpha and GA releases. Depending on user feedback, we may add a beta.
### Alpha
This release will not support
* load restrictions check
* KRM functions
* absolute paths
* `target` kustomization with reference cycles
* any verification in the form of `--no-verification` flag or automatically running `kustomize build` at the end of
localization
The entire command will be new in the alpha release, and so will not require an alpha flag. The command will not be
available in `kubectl kustomize` either as kubectl only has `kustomize build` builtin.
### Beta/GA
This release should have all features documented in this proposal. Though, we may make changes based on user feedback.