We believe that a strong shared understanding and terminology around software and resources leads to a better Backstage experience.
This description originates from this RFC. Note that some of the concepts are not yet supported in Backstage.
We model software in the Backstage catalogue using these three core entities (further explained below):
Components are individual pieces of software
APIs are the boundaries between different components
Resources are physical or virtual infrastructure needed to operate a component
A component is a piece of software, for example a mobile feature, web site, backend service or data pipeline (list not exhaustive). A component can be tracked in source control, or use some existing open source or commercial software.
A component can implement APIs for other components to consume. In turn it might depend on APIs implemented by other components, or resources that are attached to it at runtime.
APIs form an important (maybe the most important) abstraction that allows large software ecosystems to scale. Thus, APIs are a first class citizen in the Backstage model and the primary way to discover existing functionality in the ecosystem.
APIs are implemented by components and form boundaries between components. They might be defined using an RPC IDL (e.g., Protobuf, GraphQL, ...), a data schema (e.g., Avro, TFRecord, ...), or as code interfaces. In any case, APIs exposed by components need to be in a known machine-readable format so we can build further tooling and analysis on top.
APIs have a visibility: they are either public (making them available for any other component to consume), restricted (only available to a whitelisted set of consumers), or private (only available within their system). As public APIs are going to be the primary way interaction between components, Backstage supports documenting, indexing and searching all APIs so we can browse them as developers.
Resources are the infrastructure a component needs to operate at runtime, like BigTable databases, Pub/Sub topics, S3 buckets or CDNs. Modelling them together with components and systems will better allow us to visualize resource footprint, and create tooling around them.
A large catalogue of components, APIs and resources can be highly granular and hard to understand as a whole. It might thus be convenient to further categorize these entities using the following (optional) concepts:
- Systems are a collection of entities that cooperate to perform some function
- Domains relate entities and systems to part of the business
With increasing complexity in software, systems form an important abstraction level to help us reason about software ecosystems. Systems are a useful concept in that they allow us to ignore the implementation details of a certain functionality for consumers, while allowing the owning team to make changes as they see fit (leading to low coupling).
A system, in this sense, is a collection of resources and components that exposes one or several public APIs. The main benefit of modelling a system is that it hides its resources and private APIs between the components for any consumers. This means that as the owner, you can evolve the implementation, in terms of components and resources, without your consumers being able to notice. Typically, a system will consist of at most a handful of components (see Domain for a grouping of systems).
For example, a playlist management system might encapsulate a backend service to update playlists, a backend service to query them, and a database to store them. It could expose an RPC API, a daily snapshots dataset, and an event stream of playlist updates.
While systems are the basic level of encapsulation for related entities, it is often useful to group a collection of systems that share terminology, domain models, metrics, KPIs, business purpose, or documentation, i.e. they form a bounded context.
For example, it would make sense if the different systems in the “Payments” domain would come with some documentation on how to accept payments for a new product or use-case, share the same entity types in their APIs, and integrate well with each other. Other domains could be “Content Ingestion”, “Ads” or “Search”.