search

devcontainers / spec

Development Containers: Use a container as a full-featured development environment.
https://containers.dev
Creative Commons Attribution 4.0 International
3.62k stars 234 forks source link

Devcontainers/spec rebuilding reference #337

Closed 049940 closed 1 year ago

049940 commented 1 year ago

Dev Container Features reference

Development Container Features are self-contained, shareable units of installation code and development container configuration. The name comes from the idea that referencing one of them allows you to quickly and easily add more tooling, runtime, or library "Features" into your development container for you or your collaborators to use.

Note: While Features may be installed on top of any base image, the implementation of a Feature might restrict it to a subset of possible base images.

For example, some Features may be authored to work with a certain linux distro (e.g. debian-based images that use the apt package manager).

Feature metadata is captured by a devcontainer-feature.json file in the root folder of the Feature.

Tip: This section covers details on the Features specification. If you are looking for summarized information on creating your own Features, see the template and core Features repositories.

Folder structure

A Feature is a self contained entity in a folder with at least a devcontainer-feature.json and install.sh entrypoint script. Additional files are permitted and are packaged along side the required files.

+-- feature
|    +-- devcontainer-feature.json
|    +-- install.sh
|    +-- (other files)

devcontainer-feature.json properties

The devcontainer-feature.json file defines metadata about a given Feature.

All properties are optional except for id, version, and name.

Property Type Description
id string Required: Identifier of the Feature. Must be unique in the context of the repository where the Feature exists and must match the name of the directory where the devcontainer-feature.json resides. ID should be provided lowercase.
version string Required: The semantic version of the Feature (e.g: 1.0.0).
name string Required: A "human-friendly" display name for the Feature.
description string Description of the Feature.
documentationURL string Url that points to the documentation of the Feature.
licenseURL string Url that points to the license of the Feature.
keywords array List of strings relevant to a user that would search for this Feature.
options object A map of options that will be passed as environment variables to the execution of the script.
containerEnv object A set of name value pairs that sets or overrides environment variables.
privileged boolean Sets privileged mode for the container (required by things like docker-in-docker) when the Feature is used.
init boolean Adds the tiny init process to the container (--init) when the Feature is used.
capAdd array Adds container capabilities when the Feature is used.
securityOpt array Sets container security options like updating the seccomp profile when the Feature is used.
entrypoint string Set if the Feature requires an "entrypoint" script that should fire at container start up.
customizations object Product specific properties, each namespace under customizations is treated as a separate set of properties. For each of this sets the object is parsed, values are replaced while arrays are set as a union.
dependsOn object An object (*) of Feature dependencies that must be satisified before this Feature is installed. Elements follow the same semantics of the features object in devcontainer.json. [See Installation Order* for further information](#installation-order).
installsAfter array Array of ID's of Features (omitting a version tag) that should execute before this one. Allows control for Feature authors on soft dependencies between different Features. See Installation Order for further information.
legacyIds array Array of old IDs used to publish this Feature. The property is useful for renaming a currently published Feature within a single namespace.
deprecated boolean Indicates that the Feature is deprecated, and will not receive any further updates/support. This property is intended to be used by the supporting tools for highlighting Feature deprecation.
mounts object Defaults to unset. Cross-orchestrator way to add additional mounts to a container. Each value is an object that accepts the same values as the Docker CLI --mount flag. The Pre-defined devcontainerId variable may be referenced in the value. For example:
"mounts": [{ "source": "dind-var-lib-docker", "target": "/var/lib/docker", "type": "volume" }]

(**) The ID must refer to either a Feature (1) published to an OCI registry, (2) a Feature Tgz URI, or (3) a Feature in the local file tree. Deprecated Feature identifiers (i.e GitHub Release) are not supported and the presence of this property may be considered a fatal error or ignored. For local Features (ie: during development), you may also depend on other local Features by providing a relative path to the Feature, relative to folder containing the active devcontainer.json. This behavior of Features within this property again mirror the features object in devcontainer.json.

Lifecycle Hooks

The following lifecycle hooks may be declared as properties of devcontainer-feature.json.

Property Type
onCreateCommand string, array, object
updateContentCommand string, array, object
postCreateCommand string, array, object
postStartCommand string, array, object
postAttachCommand string, array, object

Behavior

Each property mirrors the behavior of the matching property in devcontainer.json, including the behavior that commands are executed from the context of the project workspace folder.

For each lifecycle hook (in Feature installation order), each command contributed by a Feature is executed in sequence (blocking the next command from executing). Commands provided by Features are always executed before any user-provided lifecycle commands (i.e: in the devcontainer.json).

If a Feature provides a given command with the object syntax, all commands within that group are executed in parallel, but still blocking commands from subsequent Features and/or the devcontainer.json.

NOTE: These properties are stored within image metadata.

The options property

The options property is contains a map of option IDs and their related configuration settings. The ID becomes the name of the environment variable in all caps. See option resolution for more details. For example:

{
  "options": {
    "optionIdGoesHere": {
      "type": "string",
      "description": "Description of the option",
      "proposals": ["value1", "value2"],
      "default": "value1"
    }
  }
}
Property Type Description
optionId string ID of the option that is converted into an all-caps environment variable with the selected value in it.
optionId.type string Type of the option. Valid types are currently: boolean, string
optionId.proposals array A list of suggested string values. Free-form values are allowed. Omit when using optionId.enum.
optionId.enum array A strict list of allowed string values. Free-form values are not allowed. Omit when using optionId.proposals.
optionId.default string or boolean Default value for the option.
optionId.description string Description for the option.

User environment variables

Feature scripts run as the root user and sometimes need to know which user account the dev container will be used with.

_REMOTE_USER and _CONTAINER_USER environment variables are passsed to the Features scripts with _CONTAINER_USER being the container's user and _REMOTE_USER being the configured remoteUser. If no remoteUser is configured, _REMOTE_USER is set to the same value as _CONTAINER_USER.

Additionally, the home folders of the two users are passed to the Feature scripts as _REMOTE_USER_HOME and _CONTAINER_USER_HOME environment variables.

The container user can be set with containerUser in the devcontainer.json and image metadata, user in the docker-compose.yml, USER in the Dockerfile, and can be passed down from the base image.

Dev Container ID

An identifier will be referred to as ${devcontainerId} in the devcontainer.json and the Feature metadata and that will be replaced with the dev container's id. It should only be used in parts of the configuration and metadata that is not used for building the image because that would otherwise prevent pre-building the image at a time when the dev container's id is not known yet. Excluding boolean, numbers and enum properties the properties supporting ${devcontainerId} in the Feature metadata are: entrypoint, mounts, customizations.

Implementations can choose how to compute this identifier. They must ensure that it is unique among other dev containers on the same Docker host and that it is stable across rebuilds of dev containers. The identifier must only contain alphanumeric characters. We describe a way to do this below.

Label-based Implementation

The following assumes that a dev container can be identified among other dev containers on the same Docker host by a set of labels on the container. Implementations may choose to follow this approach.

The identifier is derived from the set of container labels uniquely identifying the dev container. It is up to the implementation to choose these labels. E.g., if the dev container is based on a local folder the label could be named devcontainer.local_folder and have the local folder's path as its value.

E.g., the ghcr.io/devcontainers/features/docker-in-docker Feature could use the dev container id with:

{
    "id": "docker-in-docker",
    "version": "1.0.4",
    // ...
    "mounts": [
        {
            "source": "dind-var-lib-docker-${devcontainerId}",
            "target": "/var/lib/docker",
            "type": "volume"
        }
    ]
}

Label-based Computation

JavaScript implementation taking an object with the labels as argument and returning a string as the result:

const crypto = require('crypto');
function uniqueIdForLabels(idLabels) {
        const stringInput = JSON.stringify(idLabels, Object.keys(idLabels).sort()); // sort properties
        const bufferInput = Buffer.from(stringInput, 'utf-8');
        const hash = crypto.createHash('sha256')
                .update(bufferInput)
                .digest();
        const uniqueId = BigInt(`0x${hash.toString('hex')}`)
                .toString(32)
                .padStart(52, '0');
        return uniqueId;
}

devcontainer.json properties

Features are referenced in a user's devcontainer.json under the top level features object.

A user can specify an arbitrary number of Features. At build time, these Features will be installed in an order defined by a combination of the installation order rules and implementation.

A single Feature is provided as a key/value pair, where the key is the Feature identifier, and the value is an object containing "options" (or empty for "default"). Each key in the Feature object must be unique.

These options are sourced as environment variables at build-time, as specified in Option Resolution.

Below is a valid features object provided as an example.

"features": {
  "ghcr.io/user/repo/go": {},
  "ghcr.io/user/repo1/go:1": {},
  "ghcr.io/user/repo2/go:latest": {},
  "https://github.com/user/repo/releases/devcontainer-feature-go.tgz": { 
        "optionA": "value" 
  },
  "./myGoFeature": { 
        "optionA": true,
        "optionB": "hello",
        "version" : "1.0.0"
  }
}

Note: The :latest version annotation is added implicitly if omitted. To pin to a specific package version (example), append it to the end of the Feature.

An option's value can be provided as either a string or boolean, and should match what is expected by the Feature in the devcontainer-feature.json file.

As a shorthand, the value of a feature can be provided as a single string. This string is mapped to an option called version. In the example below, both examples are equivalent. 

"features": {
  "ghcr.io/owner/repo/go": "1.18"
}
"features": {
  "ghcr.io/owner/repo/go": {
    "version": "1.18"
  }
}

Referencing a feature

The id format specified dictates how a supporting tool will locate and download a given Feature. id is one of the following:

Type Description Example
<oci-registry>/<namespace>/<feature>[:<semantic-version>] Reference to feature in OCI registry(*) ghcr.io/user/repo/go
ghcr.io/user/repo/go:1
ghcr.io/user/repo/go:latest
https://<uri-to-feature-tgz> Direct HTTPS URI to a tarball. https://github.com/user/repo/releases/devcontainer-feature-go.tgz
./<path-to-feature-dir> A relative directory(**) to folder containing a devcontainer-feature.json. ./myGoFeature

Feature identifiers should be assumed to be case-insensitive and should be normalized to lowercase.

(*) OCI registry must implement the OCI Artifact Distribution Specification. Some implementors can be found here.

(**) The provided path is always relative to the folder containing the devcontainer.json. Further requirements are outlined in the Locally Referenced Addendum

Versioning

Each Feature is individually versioned according to the semver specification. The version property in the respective devcontainer-feature.json file is updated to increment the Feature's version.

Tooling that handles releasing Features will not republish Features if that exact version has already been published; however, tooling must republish major and minor versions in accordance with the semver specification.

Authoring

Features can be authored in a number of languages, the most straightforward being bash scripts. If a Feature is authored in a different language information about it should be included in the metadata so that users can make an informed choice about it.

Reference information about the application required to execute the Feature should be included in devcontainer-feature.json in the metadata section.

Applications should default to /bin/sh for Features that do not include this information.

If the Feature is included in a folder as part of the repository that contains devcontainer.json, no other steps are necessary.

Release

For information on distributing Features, see devcontainer-features-distribution.md.

Execution

Invoking install.sh

The install.sh script for each Feature should be executed as root during a container image build. This allows the script to add needed OS dependencies or settings that could not otherwise be modified. This also allows the script to switch into another user's context using the su command (e.g., su ${USERNAME} -c "command-goes-here"). In combination, this allows both root and non-root image modifications to occur even if sudo is not present in the base image for security reasons.

To ensure that the appropriate shell is used, the execute bit should be set on install.sh and the file invoked directly (e.g. chmod +x install.sh && ./install.sh).

Note: It is recommended that Feature authors write install.sh using a shell available by default in their supported distributions (e.g., bash in Debian/Ubuntu or Fedora, sh in Alpine). In the event a different shell is required (e.g., fish), install.sh can be used to boostrap by checking for the presence of the desired shell, installing it if needed, and then invoking a secondary script using the shell.

The install.sh file can similarly be used to bootstrap something written in a compiled language like Go. Given the increasing likelihood that a Feature needs to work on both x86_64 and arm64-based devices (e.g., Apple Silicon Macs), install.sh can detect the current architecture (e.g., using something like uname -m or dpkg --print-architecture), and then invoke the right executable for that architecture.

Installation order

By default, Features are installed on top of a base image in an order determined as optimal by the implementing tool.

If any of the following properties are provided in the Feature's devcontainer-feature.json, or the user's devcontainer.json, the order indicated by these propert(ies) are respected.

The dependsOn property

The optional dependsOn property indicates a set of required, "hard" dependencies for a given Feature.

The dependsOn property is declared in a Feature's devcontainer-feature.json metadata file. Elements of this property mirror the semantics of the features object in devcontainer.json. Therefore, all dependencies may provide the relevant options, or an empty object (eg: "bar:123": {}) if the Feature's default options are sufficient. Identical Features that provide different options are treated as different Features (see Feature equality for more info).

All Features indicated in the dependsOn property must be satisfied (a Feature equal to each dependency is present in the installation order) before the given Feature is set to be installed. If any of the Features indicated in the dependsOn property cannot be installed (e.g due to circular dependency, failure to resolve the Feature, etc) the entire dev container creation should fail.

The dependsOn property must be evaluated recursively. Therefore, if a Feature dependency has its own dependsOn property, that Feature's dependencies must also be satisfied before the given Feature is installed.

{
    "name": "My Feature",
    "id": "myFeature",
    "version": "1.0.0",
    "dependsOn": {
        "foo:1": {
            "flag": true
        },
        "bar:1.2.3": {},
        "baz@sha256:a4cdc44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855" {},
    }
}

In the snippet above, myfeature MUST be installed after foo, bar, and baz. If the Features provided via the dependsOn property declare their own dependencies, those must also be satisfied before the Feature is installed.

The installsAfter Property

The installsAfter property indicates a "soft dependency" that influences the installation order of Features that are already queued to be installed. The effective behavior of this property is the same as dependsOn, with the following differences:

{
    "name": "My Feature",
    "id": "myFeature",
    "version": "1.0.0",
    "installsAfter": [
        "foo",
        "bar"
    ]
}

In the snippet above, myfeature must be installed after foo and bar if the Feature is already queued to be installed. If second and third are not already queued to be installed, this dependency relationship should be ignored.

The 'overrideFeatureInstallOrder' property

The overrideFeatureInstallOrder property of devcontainer.json is an array of Feature IDs that are to be installed in descending priority order as soon as its dependencies outlined above are installed.

This property may not indicate an installation order that is inconsistent with the resolved dependency graph (see dependency algorithm). If the overrideFeatureInstallOrder property is inconsistent with the dependency graph, the implementing tool should fail the dependency resolution step.

This evaluation is performed by assigning a roundPriority to all nodes that match match the Feature identifier (version omitted) present in the property.

For example, given n Features in the overrideFeatureInstallOrder array, the orchestrating tool should assign a roundPriority of n - idx to each Feature, where idx is the zero-based index of the Feature in the array.

For example:

overrideFeatureInstallOrder = [
  "foo",
  "bar",
  "baz"
]

would result in the following roundPriority assignments:

const roundPriority = {
  "foo": 3,
  "bar": 2,
  "baz": 1
}

This property must not influence the dependency relationship as defined by the dependency graph (see dependency graph) and shall only be evaulated at the round-based sorting step (see round sort). Put another way, this property cannot "pull forward" a Feature until all of its dependencies (both soft and hard) have been installed. After a Feature's dependencies have been installed in other rounds, this property should "pull forward" each Feature as early as possible (given the order of identifiers in the array).

Similar to installsAfter, this property's members may not provide options, nor are they able to be pinned to a specific version tag or digest.

If a Feature is indicated in overrideFeatureInstallOrder but not a member of the dependency graph (it is not queued to be installed), the orchestrating tool may fail the dependency resolution step.

Definitions

Definition: Feature Equality

This specification defines two Features as equal if both Features point to the same exact contents and are executed with > the same options.

For Features published to an OCI registry, two Feature are identical if their manifest digests are equal, and the > options executed against the Feature are equal (compared value by value). Identical manifest digests implies that the tgz contents of the Feature and its entire devcontainer-feature.json are identical. If any of these conditions are not met, the Features are considered not equal.

For Features fetched by HTTPS URI, two Features are identical if the contents of the tgz are identical (hash to the > same value), and the options executed against the Feature are equal (compared value by value). If any of these conditions are not met, the Features are considered not equal.

For local Features, each Feature is considered unique and not equal to any other local Feature.

Definition: Round Stable Sort

To prevent non-deterministic behavior, the algorithm will sort each round according to the following rules:

  • Compare and sort each Feature lexiographically by their fully qualified resource name (For OCI-published Features, that means the ID without version or digest.). If the comparison is equal:
  • Compare and sort each Feature from oldest to newest tag (latest being the "most new"). If the comparision is equal:
  • Compare and sort each Feature by their options by:
    • Greatest number of user-defined options (note omitting an option will default that value to the Feature's default value and is not considered a user-defined option). If the comparison is equal:
    • Sort the provided option keys lexicographically. If the comparison is equal:
    • Sort the provided option values lexicographically. If the comparision is equal:
  • Sort Features by their canonical name (For OCI-published Features, the Feature ID resolved to the digest hash).

If there is no difference based on these comparator rules, the Features are considered equal.

Dependency installation order algorithm

An implementing tool is responsible for calculating the Feature installation order (or providing an error if no valid installation order can be resolved). The set of Features to be installed is the union of user-defined Features (those directly indicated in the user's devcontainer.json and their dependencies (those indicated by the dependsOn or installsAfter property, taking into account the user dev container's overrideFeatureInstallOrder property). The implmenting tool will perform the following steps:

(1) Build a dependency graph

From the user-defined Features, the orchestrating tool will build a dependency graph. The graph will be built by traversing the dependsOn and installsAfter properties of each Feature. The metadata for each dependency is then fetched and the node added as an edge to to the dependent Feature. For dependsOn dependencies, the dependency will be fed back into the worklist to be recursively resolved.

An accumulator is maintained with all uniquely discovered and user-provided Features, each with a reference to its dependencies. If the exact Feature (see Feature Equality) has already been added to the accumulator, it will not be added again. The accumulator will be fed into (B3) after the Feature tree has been resolved.

The graph may be stored as an adjacency list with two kinds of edges (1) dependsOn edges or "hard dependencies" and (2) installsAfter edges or "soft dependencies".

(2) Assigning round priority

Each node in the graph has an implicit, default roundPriority of 0.

To influence installation order globally while still honoring the dependency graph of built in (1), roundPriority values may be tweaks for each Feature. When each round is calculated in (3), only the Features equal to the max roundPriority of that set will be committed (the remaining will be > uncommitted and reevaulated in subsequent rounds).

The roundPriority is set to a non-zero value in the following instances:

(3) Round-based sorting

Perform a sort on the result of (1) in rounds. This sort will rearrange Features, producing a sorted list of Features to install. The sort will be performed as follows:

Start with all the elements from (2) in a worklist and an empty list installationOrder. While the worklist is not empty, iterate through each element in the worklist and check if all its dependencies (if any) are already members of installationOrder. If the check is true, add it to an intermediate list round If not, skip it. Equality is determined in Feature Equality.

Then for each intermediate round list, commit to installationOrder only those nodes who share the maximum roundPriority. Return all nodes in round with a strictly lower roundPriority to the worklist to be reprocessed in subsequent iterations. If there are multiple nodes with the same roundPriority, commit them to installationOrder with a final sort according to Round Stable Sort.

Repeat for as many rounds as necessary until worklist is empty. If there is ever a round where no elements are added to installationOrder, the algorithm should terminate and return an error. This indicates a circular dependency or other fatal error in the dependency graph. Implementations should attempt to provide the user with information about the error and possible mitigation strategies.

Notes

From an implementation point of view, installsAfter nodes may be added as a separate set of directed edges, just as dependsOn nodes are added as directed edges (see (1)). Before round-based installation and sorting (3), an orchestrating tool should remove all installsAfter directed edges that do not correspond with a Feature in the worklist that is set to be installed. In each round, a Feature can then be installed if all its requirements (both dependsOn and installsAfter dependencies) have been fulfilled in previous rounds.

An implemention should fail the dependency resolution step if the evaluation of the installsAfter property results in an inconsistent state (eg: a circular dependency).

Option Resolution

A Feature's 'options' - specified as the value of a single Feature key/value pair in the user's devcontainer.json - are passed to the Feature as environment variables.

A supporting tool will parse the options object provided by the user. If a value is provided for a Feature, it will be emitted to a file named devcontainer-features.env following the format <OPTION_NAME>=<value>.

To ensure a option that is valid as an environment variable, the follow substitutions are performed.

(str: string) => str
        .replace(/[^\w_]/g, '_')
        .replace(/^[\d_]+/g, '_')
        .toUpperCase();

This file is sourced at build-time for the Feature install.sh entrypoint script to handle.

Any options defined by a Feature's devcontainer-feature.json that are omitted in the user's devcontainer.json will be implicitly exported as its default value.

Option resolution example

Suppose a python Feature has the following options parameters declared in the devcontainer-feature.json file:

// ...
"options": {
    "version": {
        "type": "string",
        "enum": ["latest", "3.10", "3.9", "3.8", "3.7", "3.6"],
        "default": "latest",
        "description": "Select a Python version to install."
    },
    "pip": {
        "type": "boolean",
        "default": true,
        "description": "Installs pip"
    },
    "optimize": {
        "type": "boolean",
        "default": true,
        "description": "Optimize python installation"
    }
}

The user's devcontainer.json declared the python Feature like so


"features": {
    "python": {
        "version": "3.10",
        "pip": false
    }
}

The emitted environment variables will be:

VERSION="3.10"
PIP="false"
OPTIMIZE="true"

These will be sourced and visible to the install.sh entrypoint script. The following install.sh...

#!/usr/bin/env bash

echo "Version is $VERSION"
echo "Pip? $PIP"
echo "Optimize? $OPTIMIZE"

... outputs the following:

Version is 3.10
Pip? false
Optimize? true

Steps to rename a Feature

  1. Update the Feature source code folder and the id property in the devcontainer-feature.json properties to reflect the new id. Other properties (name, documentationUrl, etc.) can optionally be updated during this step.
  2. Add or update the legacyIds property to the Feature, including the previously used id.
  3. Bump the semantic version of the Feature.
  4. Rerun the devcontainer features publish command, or equivalent tool that implements the Features distribution specification.

Example: Renaming a Feature

Let's say we currently have a docker-from-docker Feature 👇

Current devcontainer-feature.json : 

{
    "id": "docker-from-docker",
    "version": "2.0.1",
    "name": "Docker (Docker-from-Docker)",
    "documentationURL": "https://github.com/devcontainers/features/tree/main/src/docker-from-docker",
    ....
}

We'd want to rename this Feature to docker-outside-of-docker. The source code folder of the Feature will be updated to docker-outside-of-docker and the updated devcontainer-feature.json will look like 👇 

{
    "id": "docker-outside-of-docker",
    "version": "2.0.2",
    "name": "Docker (Docker-outside-of-Docker)",
    "documentationURL": "https://github.com/devcontainers/features/tree/main/src/docker-outside-of-docker",
    "legacyIds": [
        "docker-from-docker"
    ]
    ....
}

Note - The semantic version of the Feature defined by the version property should be continued and should not be restarted at 1.0.0.

Implementation notes

There are several things to keep in mind for an application that implements Features:

samruddhikhandale commented 1 year ago

Looks like this was created by accident, closing. If needed, feel free to reopen with more details, thank you!