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Key Kargo Concepts

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We're currently reorganizing and updating the documentation. During this process, you may encounter sections that are incomplete, repetitive, or fragmented. Please bear with us as we work to make improvements.

The Basics​

What is a Project​

A project is a collection of related Kargo resources that describe one or more delivery pipelines and is the basic unit of organization and tenancy in Kargo.

RBAC rules are also defined at the project level and project administrators may use projects to define policies, such as whether a stage is eligible for automatic promotions of new freight.

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For technical information on the corresponding Project resource type, refer to Working with Projects.

What is a Stage?​

When you hear the term “environment”, what you envision will depend significantly on your perspective. A developer, for example, may think of an "environment" as a specific instance of an application they work on, while a DevOps engineer, may think of an "environment" as a particular segment of the infrastructure they maintain.

To eliminate confusion, Kargo avoids the term "environment" altogether in favor of stage. The important feature of a stage is that its name ("test" or "prod," for instance) denotes an application instance's purpose and not necessarily its location. This blog post discusses the rationale behind this choice.

Stages are Kargo's most important concept. They can be linked together in a directed acyclic graph to describe a delivery pipeline. Typically, such a pipeline may feature a "test" or "dev" stage as its starting point, with one or more "prod" stages at the end.

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For technical details of the corresponding Stage resource type, refer to Working with Stages.

What is Freight?​

Freight is Kargo's second most important concept. A single "piece of freight" is a set of references to one or more versioned artifacts, which may include one or more:

  • Container images (from image repositories)

  • Kubernetes manifests (from Git repositories)

  • Helm charts (from chart repositories)

Freight can therefore be thought of as a sort of meta-artifact. Freight is what Kargo seeks to progress from one stage to another. For detailed guidance on working with Freight, refer to this guide.

What is a Warehouse?​

A warehouse is a source of freight. A warehouse subscribes to one or more:

  • Container image repositories

  • Git repositories

  • Helm charts repositories

Anytime something new is discovered in any repository to which a warehouse subscribes, the warehouse produces a new piece of freight.

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For technical details of the corresponding Warehouse resource type, refer to Working with Warehouses.

What is a Promotion?​

A promotion is a request to move a piece of freight into a specified stage.

Corresponding Resource Types​

Each of Kargo's fundamental concepts maps directly onto a custom Kubernetes resource type.

Freight Resources​

Each piece of Kargo freight is represented by a Kubernetes resource of type Freight. Freight resources are immutable except for their alias field and status subresource (mutable only by the Kargo controller).

A single Freight resource references one or more versioned artifacts, such as:

  • Container images (from image repositories)

  • Kubernetes manifests (from Git repositories)

  • Helm charts (from chart repositories)

A Freight resource's metadata.name field is a SHA1 hash of a canonical representation of the artifacts referenced by the Freight resource. (This is enforced by an admission webhook.) The metadata.name field is therefore a "fingerprint", deterministically derived from the Freight's contents.

To provide a human-readable identifier for a Freight resource, a Freight resource has an alias field. This alias is a human-readable string that is unique within the Project to which the Freight belongs. Kargo automatically generates unique aliases for all Freight resources, but users may update them to be more meaningful.

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Assigning meaningful and recognizable aliases to important pieces of Freight traversing your pipeline(s) can make it easier to track their progress from one Stage to another.

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For more information on aliases, refer to the aliases and updating aliases sections of the "Working with Freight" how-to guide.

A Freight resource's status field records a list of Stage resources in which the Freight has been verified and a separate list of Stage resources for which the Freight has been manually approved.

Freight resources look similar to the following:

apiVersion: kargo.akuity.io/v1alpha1
kind: Freight
metadata:
  name: 47b33c0c92b54439e5eb7fb80ecc83f8626fe390
  namespace: kargo-demo
  labels:
    kargo.akuity.io/alias: fruitful-ferret
alias: fruitful-ferret
images:
- digest: sha256:b2487a28589657b318e0d63110056e11564e73b9fd3ec4c4afba5542f9d07d46
  repoURL: public.ecr.aws/nginx/nginx
  tag: 1.27.0
commits:
- repoURL: https://github.com/example/kargo-demo.git
  id: 1234abc
warehouse: my-warehouse
status:
  verifiedIn:
    test: {}
  approvedFor:
    prod: {}

Promotion Resources​

Each Kargo promotion is represented by a Kubernetes resource of type Promotion.

A Promotion resource's two most important fields are its spec.freight and spec.stage fields, which respectively identify a piece of Freight and a target Stage to which that Freight should be promoted.

Promotions are, in some cases, created automatically by Kargo. In other cases, they are created manually by users. In either case, a Promotion resource resembles the following:

apiVersion: kargo.akuity.io/v1alpha1
kind: Promotion
metadata:
  name: 47b33c0c92b54439e5eb7fb80ecc83f8626fe390-to-test
  namespace: kargo-demo
spec:
  stage: test
  freight: 47b33c0c92b54439e5eb7fb80ecc83f8626fe390
info

The name in a Promotion's metadata.name field is inconsequential. Only the spec matters.

When a Promotion has concluded -- whether successfully or unsuccessfully -- the Promotion's status field is updated to reflect the outcome. For example:

status:
  phase: Succeeded

Role-Based Access Control​

As with all resource types in Kubernetes, permissions to perform various actions on resources of different types are governed by RBAC.

For all Kargo resource types, Kubernetes RBAC functions exactly as one would expect, with one notable exception.

Often, it is necessary to grant a user permission to create Promotion resources that reference certain Stage resources, but not others. To address this, Kargo utilizes an admission control webhook that conducts access reviews to determine if a user creating a Promotion resource has the virtual promote verb for the Stage referenced by the Promotion resource.

info

This blog post is an excellent primer on virtual verbs in Kubernetes RBAC.

The following Role resource describes permissions to create Promotion references that reference the uat Stage only:

apiVersion: rbac.authorization.k8s.io/v1
kind: Role
metadata:
  name: uat-promoter
  namespace: kargo-demo
rules:
- apiGroups:
  - kargo.akuity.io
  resources:
  - promotions
  verbs:
  - create
  - delete
  - get
  - list
  - patch
  - update
  - watch
- apiGroups:
  - kargo.akuity.io
  resources:
  - stages
  resourceNames:
  - uat
  verbs:
  - promote

To grant a fictional user alice, in the QA department, the ability to promote to uat only, create a corresponding RoleBinding:

apiVersion: rbac.authorization.k8s.io/v1
kind: RoleBinding
metadata:
  name: alice-uat-promoter
  namespace: kargo-demo
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: Role
  name: uat-promoter
subjects:
- kind: User
  name: alice