Application graph architecture documentation

[brooke-hamilton]

Description

Architecture document for the application graph subsystem to docs/architecture/application-graph.md, in support of the current graph work.

The document covers:

• End-to-end request flow from rad app graph through the CLI SDK, UCP proxy, and Applications RP
• How resource data gets into storage during deployment (async PUT lifecycle, ApplyDeploymentOutput, output resources)
• Storage backend comparison (apiserver, postgresql, inmemory)
• Server-side graph computation: resource-type discovery, application and environment resource listing, and the computeGraph() algorithm with BFS expansion
• Connection resolution (connectionsResolver, routesPathResolver, findSourceResource) and the three resolution strategies
• Data model (ApplicationGraphResponse, ApplicationGraphResource, ApplicationGraphConnection, ApplicationGraphOutputResource)
• CLI display modes (text and JSON output)
• Full API wire format
• Notable design details (no persistent graph store, partial results over errors, bidirectional connections, transitive inclusion)

Also updates:

• docs/architecture/README.md — adds the new page to the index and reading order
• .github/copilot-instructions.md — adds an Architecture Documentation section pointing agents to this index

Type of change

• This pull request is a minor refactor, code cleanup, test improvement, or other maintenance task and doesn't change the functionality of Radius (issue link optional).

Contributor checklist

• An overview of proposed schema changes is included in a linked GitHub issue.
  • Yes
  • Not applicable
• A design document PR is created in the design-notes repository, if new APIs are being introduced.
  • Yes
  • Not applicable
• The design document has been reviewed and approved by Radius maintainers/approvers.
  • Yes
  • Not applicable
• A PR for the samples repository is created, if existing samples are affected by the changes in this PR.
  • Yes
  • Not applicable
• A PR for the documentation repository is created, if the changes in this PR affect the documentation or any user facing updates are made.
  • Yes
  • Not applicable
• A PR for the recipes repository is created, if existing recipes are affected by the changes in this PR.
  • Yes
  • Not applicable

[Copilot]

Pull request overview

Adds a new living architecture document for the Application Graph subsystem and wires it into the repo’s architecture-doc index and Copilot guidance.

Changes:

• Add docs/architecture/application-graph.md describing request flow, storage inputs, graph computation, and API/CLI output formats.
• Update docs/architecture/README.md to include the new document in the index and reading order.
• Update .github/copilot-instructions.md to point contributors/agents to the architecture-doc index.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
docs/architecture/README.md	Adds the Application Graph doc to the architecture index and recommended reading order.
docs/architecture/application-graph.md	New end-to-end architecture write-up for how the application graph is computed and surfaced.
.github/copilot-instructions.md	Adds guidance to consult the architecture-doc index before answering subsystem questions/creating diagrams.