Aspire

Microsoft's code-first, multi-language toolchain for building, running, and deploying distributed applications. The repo is the .NET Aspire monorepo: the AppHost SDK, the new aspire CLI, the observability dashboard, service-discovery infrastructure, project templates, integrations, and a VS Code extension. Version in eng/Versions.props is 13.6.0, built against the .NET 10 SDK (global.json → 10.0.400).

Core Premise

You describe how services, frontends, containers, databases, caches, and their connections fit together in code — a single "app host" program — and the same definition runs the whole app locally and carries into deployment. The canonical language is C#, but the README shows the identical definition in TypeScript, and the Aspire.Hosting.CodeGeneration.* projects emit Go, Java, Python, and Rust SDKs too.

How it works

• App model. The DistributedApplicationBuilder fluent API composes resources (AddRedis, AddProject, AddNodeApp). Resources are thin named entities — IResource is just Name + an Annotations collection — and capability interfaces (IResourceWithEndpoints, IResourceWithConnectionString, …) are derived from annotations rather than a class hierarchy.
• Local orchestration. At run time the app host translates the model into Kubernetes-style custom resources (Container, Executable, Service, Endpoint, …) and drives the Developer Control Plane (DCP) — a separate Go binary (microsoft/dcp) that exposes a Kubernetes API — via the standard k8s client.
• Observability by default. The dashboard hosts an in-memory OTLP server; the app host injects OTEL_SERVICE_NAME, OTEL_RESOURCE_ATTRIBUTES, and OTEL_EXPORTER_OTLP_ENDPOINT into every resource, so traces/metrics/logs "just work" locally (documented in docs/open-telemetry-architecture.md).
• Deployment. Publishing is a pipeline of steps (DistributedApplicationPipeline / PipelineStep). The manifest JSON (src/Schema/aspire-8.0.json) is the interchange format with Azure Developer CLI, and there are Docker Compose and Kubernetes/Helm publishers.
• Polyglot via code generation. The Aspire Type System ("ATS") marks C# capabilities with [AspireExport] / [AspireDto] / [AspireValue] attributes, scans assemblies, and generates guest SDKs. Handle types proxy back to the .NET runtime over JSON-RPC on a Unix-domain socket ("Backchannel"), so the C# runtime stays the single engine.
• CLI. Aspire.Cli is a large System.CommandLine + Spectre.Console CLI with a TUI, scaffolding, acquisition, secrets, migrations, and — notably for agent workflows — a whole Agents/ subsystem that installs Aspire "skills" bundles and integrates with Claude Code, OpenCode, Copilot CLI, VS Code, and Playwright.