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Anchors

Introduction​

An anchor is a property on the model or overlay node. It is typically used to define a signature of the model node, and exposes one or more properties of the node. You should think of an anchor as the "representative" of the model.

Anchors for sharing models​

Octo defines a set of base models - App, Account, Region, etc. These models are intentionally generic. Frequently, individual implementation libraries will re-create these base models to add more properties to better represent the infrastructure being encapsulated.

E.g. a Region model, in octo-aws-cdk, is re-created as AwsRegion containing the name of the AWS region - us-east-1. If this model was to be shared outside the library, other libraries become dependent on this exact definition. This makes it difficult for this model to be updated.

Instead, models are expected to be hidden as an internal implementation detail.
On model's behalf, its anchors are exposed.

Other libraries can now consume models using the base libraries having this anchor.

This ensures that library 2 receives a Region model having the specific properties defined in the anchor, such as,

  • Having an AWS based model.
  • Having the name of the AWS region.

This is better than sharing the exact models whose definition is likely to evolve over time.

info

Anchor exposes portion of a model with a unique signature to be ingested by other libraries or components, and is a substitute for the model for sharing purposes.

Anchors for overlays​

Anchors are also the only components that overlays interact with. Rather than ingesting the models (for reasons stated above), overlays are created with the anchors of the models. Follow the link to learn more about Overlays.

Anchor-Model Relationship​

An anchor can only be applied on a model or an overlay.

The relationship is defined by the @Anchor decorator. Here a new example anchor AwsRegionAnchor is being defined for the AwsRegion model, i.e. the anchor will be automatically added as a property on the AwsRegion model.

@Anchor('<namespace>')
export class AwsRegionAnchor extends AAnchor<{ awsRegionId: string; type: 'AWS' }, AwsRegion> {
  constructor(
    anchorId: string,
    properties: { awsRegionId: string; type: 'AWS' },
    parent: AwsRegion,
  ) {
    super(anchorId, properties, parent);
  }
}

It exposes certain properties on the anchor, which makes up the unique signature of the anchor. Other libraries, components, or overlays can consume this anchor and will get access to the properties, while ensuring that the anchor is attached to a Region model.

Summary​

In this article we discussed Anchors, who can have them, and what purpose they serve. Anchors make models shareable, and forms the basis of sharing information between different parts of the system.