docs: Add Argo CD components architecture doc

docs/developer-guide/architecture/components.md

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+# Component Architecture
+
+Argo CD is designed with a component based architecture. The goal is
+to separate the responsibility in different deployable units in order
+to have the following benefits:
+
+- **Modularity**: Provides great level of flexibility. Components
+  interact with each other via an interface. This means that as long
+  as the interface contract is respected, a given component can be
+  replaced without requiring the rest of the system to adapt. It is
+  also possible to run the system without certain components if a
+  specific group of functionality isn't desired.
+- **Single responsibility**: Helps to determine where the different
+  types of functionality should be implemented which drives for
+  better system cohesiveness.
+- **Reusability**: Clearly defined interfaces helps in functionality
+  discoverability which benefits reusability of services.
+
+The default Argo CD installation is composed by different components
+and different Kubernetes controllers. The controllers aren't
+categorized as components as they have proprietary interfaces (CRDs)
+and therefore, miss the modular nature. There are more resources
+created while installing Argo CD (ConfigMaps, Services, etc), but for
+simplicity we are covering just the ones directly related with the
+componentized architecture.
+
+## Dependencies
+
+The diagram below has represented all dependencies between the
+different components used by the default Argo CD installation:
+
+![Components Diagram](../../assets/argocd-components.png)
+
+There are 4 logical layers represented in the diagram:
+
+- **UI**: This is the presentation layer. Users interact with Argo CD
+  mainly by components from this layer.
+- **Application**: The capabilities required to support the components
+  from the UI layer.
+- **Core**: The main Argo CD gitops functionality is implemented by
+  components and Kubernetes controllers from the Core layer.
+- **Infra**: Represent the tools that Argo CD depends on as part of
+  its infrastructure.
+
+The logical layers also help making the diagram easier to follow as
+dependencies are represented in a top-down relationship. This means
+that components from the top layers will be allowed to depend on any
+component from any of the bottom layers. However components from the
+bottom layers will never depend on any ones from upper layers.
+
+## Responsibility
+
+Below you can refer to a brief description of Argo CD components and
+its main responsibilities.
+
+### Webapp
+
+Argo CD ships with a powerful web interface that allows managing
+applications deployed in a given Kubernetes cluster.
+
+### CLI
+
+Argo CD provides a CLI that can be used by users to interact with Argo
+CD API. The CLI can also be used for automation and scripting.
+
+### API Server
+
+Defines the proprietary API exposed by Argo CD that powers the Webapp
+and the CLI functionalities.
+
+### Application Controller
+
+The Application Controller is responsible for reconciling the
+Application resource in Kubernetes syncronizing the desired
+application state (provided in Git) with the live state (in
+Kubernetes). The Application Controller is also responsible for
+reconciling the Project resource.
+
+### ApplicationSet Controller
+
+The ApplicationSet Controller is responsible for reconciling the
+ApplicationSet resource.
+
+### Repo Server
+
+Repo Server plays an important role in Argo CD architecture as it is
+responsible for interacting with the Git repository to generate the
+desired state for all Kubernetes resources that belongs to a given
+application.
+
+### Redis
+
+Redis is used by Argo CD to provide a cache layer reducing requests
+sent to the Kube API as well as to the Git provider. It also supports
+a few UI operations.
+
+### Kube API
+
+Argo CD controllers will connect to the Kubernetes API in order to run
+the reconciliation loop.
+
+### Git
+
+As a gitops tool Argo CD requires that the desired state of the
+Kubernetes resources to be provided in a Git repository.
+
+We use "git" here to stand in for an actual git repo, a Helm repo,
+or an OCI artifact repo. Argo CD supports all those options.
+
+### Dex
+
+Argo CD relies on Dex to provide authentication with external OIDC
+providers. However other tools can be used instead of Dex. Check the
+[user management
+documentation](../../operator-manual/user-management/index.md) for
+more details.
+
+

mkdocs.yml

@@ -170,6 +170,7 @@ nav:
   - developer-guide/index.md
   - Architecture:
     - developer-guide/architecture/authz-authn.md
+    - developer-guide/architecture/components.md
   - Code Contribution Guide: developer-guide/code-contributions.md
   - Toolchain Guide: developer-guide/toolchain-guide.md
   - developer-guide/contributors-quickstart.md