Overlays
Definition
An overlay is a special kind of model to represent infrastructure components that span across multiple models. You should think of an overlay as an infrastructure functionality that exists as a crossover between one or more models.
Just like resources, the base generic octo library does not define any base overlays.
Overlays are defined in an implementation of the octo library for a specific cloud provider
to achieve functionality that models cannot provide by themselves.
Take a Filesystem in AWS as an example.
The filesystem model can aptly represent an EFS filesystem in a Region.
But in order to be used by a server, it must be mounted to a Subnet.
Where should the functionality of adding a filesystem mount be defined?
Neither the Filesystem, nor the Subnet model can define this without adding a lot of unnecessary complexity to them.
Overlays exists to solve this problem. In this scenario, a new overlay can be created which takes both the Filesystem and Subnet models as inputs, and then encapsulates the logic of adding a filesystem mount to a server.
An overlay can also be created on a single model. Typically, you would do this to add additional functionalities to a model other than its base functionality.
An overlay enriches models by adding additional capabilities. Think of overlays as additional functionalities that can be easily added or removed from models. This flexibility enables model implementations to remain clean and free of unnecessary clutter.
Overlay-Anchor Relationship
Overlays don't really take models as inputs, for the reasons mentioned in Anchors. Instead, overlays takes Anchors as inputs.
In this diagram, we have an overlay between Model 1 and Model 2, and another overlay just on Model 2, using anchors present in both models.
An overlay is not fundamentally different from a Model, but rather an extension of it. Like them, it also has its own actions, can be serialized and deserialized, and is managed by Octo using the same state files.
Just like a model, and overlay is also considered an internal implementation. Overlays can also expose their own anchors as its representative.
Summary
In this section we discussed Overlays, and its relationship to Models and Anchors. We discussed how these add-on classes aid in maintaining complex infrastructure functionalities, and helps keep models pure and simplified.