Systems

Internal building blocks of the framework. Each "system" cuts across multiple files and layers — the App Router rendering pipeline, the build orchestrator, the dev server, the route server. They are organized here because they don't map cleanly to a single workspace package: most live inside packages/next/src/, but each has a distinct responsibility worth documenting on its own.

What's covered

System	Lives in
App Router	packages/next/src/server/app-render/, packages/next/src/client/components/
Pages Router	packages/next/src/server/render.tsx, packages/next/src/pages
Server runtime	packages/next/src/server/{base-server,next-server,lib/router-server}.ts
Build pipeline	packages/next/src/build/
Dev server	packages/next/src/server/dev/
Image optimization	packages/next/src/server/image-optimizer.ts, packages/next/src/client/image-component.tsx
Server actions	packages/next/src/server/app-render/action-handler.ts
Middleware and edge	packages/next/src/server/web/
Caching	packages/next/src/server/lib/incremental-cache/, packages/next/src/server/use-cache/

How they fit together

graph TD
    Build[Build pipeline] --> Manifests[Manifests<br/>.next/*]

    DevServer[Dev server] --> RouterServer
    Start[Production start] --> RouterServer

    RouterServer[Server runtime<br/>router-server, base-server, next-server] --> Match{Match}
    Match -- app/ --> AppRouter[App Router<br/>app-render.tsx]
    Match -- pages/ --> PagesRouter[Pages Router<br/>render.tsx]
    Match -- middleware --> Edge[Middleware/edge]
    Match -- /_next/image --> Image[Image optimization]

    AppRouter --> ServerActions[Server actions<br/>action-handler.ts]
    AppRouter --> Cache[Caching<br/>incremental-cache, use-cache]
    PagesRouter --> Cache
    Edge --> EdgeRuntime[Edge runtime<br/>@edge-runtime/vm]

    Manifests -.read by.-> RouterServer
    Manifests -.read by.-> AppRouter
    Manifests -.read by.-> PagesRouter

The Build pipeline produces the manifests in .next/ that the runtime systems read at request time. The Server runtime is the entry point for both dev and production, dispatching to one of the two routers (or to middleware / image optimization).

For the runtime classes themselves — BaseServer, NextServer, NextDevServer — see server runtime.