> ## Documentation Index > Fetch the complete documentation index at: https://backstage.spotify.com/docs/llms.txt > Use this file to discover all available pages before exploring further. # System Model > The core entity model used by the catalog — Components, APIs, Resources, Systems, Domains, Groups, Users, and how they relate to each other. We believe that a strong shared understanding and terminology around software and resources leads to a better Portal experience. ## Core Entities We model software in the Portal Software Catalog 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 ### 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 consume APIs implemented by other components, or directly depend on components or resources that are attached to it at runtime. ### API 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 Portal 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 an allowlisted set of consumers), or private (only available within their system). As public APIs are going to be the primary way interaction between components, Portal supports documenting, indexing and searching all APIs so we can browse them as developers. ### Resource 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. ## Organizational Entities ### User A user describes a person, such as an employee, a contractor, or similar. ### Group A group describes an organizational entity, such as for example a team, a business unit, or a loose collection of people in an interest group. ## Ecosystem Modeling 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 ### System 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. ### Domain 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”. ## Other ### Location A location is a marker that references other places to look for catalog data. ### Type The type field in the system has no set meaning. It is up to the user to assign their own types and use them as desired, such as for link validation or creating custom UI components. Some common pre-defined types are depicted in the [ecosystem modeling diagram](#ecosystem-modeling). ### Template A template definition describes both the parameters that are rendered in the frontend part of the scaffolding wizard, and the steps that are executed when scaffolding that component.