review comments

Author: kirillgroshkov

.changeset/big-drinks-invite.md

@@ -0,0 +1,8 @@
+---
+"@react-router/dev": patch
+---
+
+Introduce a `prerender.unstable_concurrency` option, to support running the prerendering concurrently, potentially speeding up the build.
+
+RFC https://github.com/remix-run/react-router/discussions/14080
+fixes https://github.com/remix-run/react-router/issues/14383 

packages/react-router-dev/config/config.ts

@@ -149,13 +149,18 @@ export type ReactRouterConfig = {
   /**
    * An array of URLs to prerender to HTML files at build time.  Can also be a
    * function returning an array to dynamically generate URLs.
+   *
+   * `unstable_concurrency` defaults to 1, which means "no concurrency" - fully serial execution.
+   * Setting it to a value more than 1 enables concurrent prerendering.
+   * Setting it to a value higher than one can increase the speed of the build,
+   * but may consume more resources, and send more concurrent requests to the
+   * server/CMS.
    */
-  prerender?:
-    | boolean
-    | Array<string>
-    | ((args: {
-        getStaticPaths: () => string[];
-      }) => Array<string> | Promise<Array<string>>);
+  prerender?: PrerenderPaths
+    | {
+        paths: PrerenderPaths;
+        unstable_concurrency?: number;
+      };
   /**
    * An array of React Router plugin config presets to ease integration with
    * other platforms and tools.
@@ -200,6 +205,10 @@ export type ReactRouterConfig = {
   ssr?: boolean;
 };
 
+export type PrerenderPaths = boolean
+  | Array<string>
+  | ((args: { getStaticPaths: () => string[] }) => Array<string> | Promise<Array<string>>);
+
 export type ResolvedReactRouterConfig = Readonly<{
   /**
    * The absolute path to the application source directory.
@@ -226,16 +235,6 @@ export type ResolvedReactRouterConfig = Readonly<{
    * function returning an array to dynamically generate URLs.
    */
   prerender: ReactRouterConfig["prerender"];
-  /**
-   * Defaults to 1, which means "no concurrency" - fully serial execution.
-   * Setting it to a value more than 1 enables concurrent prerendering.
-   * For example, prerenderConcurrency of 2 would run prerender with the
-   * concurrency of 2 (two pages at a time).
-   * Setting it to a value higher than one can increase the speed of the build,
-   * but may consume more resources, and send more concurrent requests to the
-   * server/CMS.
-   */
-  prerenderConcurrency?: number;
   /**
    * Control the "Lazy Route Discovery" behavior
    *

packages/react-router-dev/vite/plugin.ts

@@ -79,6 +79,7 @@ import {
   createConfigLoader,
   resolveEntryFiles,
   configRouteToBranchRoute,
+  type PrerenderPaths,
 } from "../config/config";
 import { getOptimizeDepsEntries } from "./optimize-deps-entries";
 import { decorateComponentExportsWithProps } from "./with-props";
@@ -2659,7 +2660,7 @@ async function handlePrerender(
   }
 
   let buildRoutes = createPrerenderRoutes(build.routes);
-  const { prerenderConcurrency = 1 } = reactRouterConfig
+  const concurrency = getPrerenderConcurrency(reactRouterConfig.prerender);
   await pMap(build.prerender, async path => {
     // Ensure we have a leading slash for matching
     let matches = matchRoutes(buildRoutes, `/${path}/`.replace(/^\/\/+/, "/"));
@@ -2733,7 +2734,7 @@ async function handlePrerender(
           : undefined,
       );
     }
-  }, { concurrency: prerenderConcurrency });
+  }, { concurrency });
 }
 
 function getStaticPrerenderPaths(routes: DataRouteObject[]) {
@@ -2921,6 +2922,10 @@ export async function getPrerenderPaths(
   let prerenderPaths: string[] = [];
   if (prerender != null && prerender !== false) {
     let prerenderRoutes = createPrerenderRoutes(routes);
+    if (typeof prerender === "object") {
+      prerender = (prerender as { paths: PrerenderPaths }).paths
+    }
+
     if (prerender === true) {
       let { paths, paramRoutes } = getStaticPrerenderPaths(prerenderRoutes);
       if (logWarning && !ssr && paramRoutes.length > 0) {
@@ -2947,6 +2952,17 @@ export async function getPrerenderPaths(
   return prerenderPaths;
 }
 
+const DEFAULT_PRERENDER_CONCURRENCY = 1
+
+function getPrerenderConcurrency(
+  prerender: ResolvedReactRouterConfig["prerender"],
+): number {
+  if (typeof prerender === "object") {
+    return (prerender as { unstable_concurrency?: number }).unstable_concurrency || DEFAULT_PRERENDER_CONCURRENCY
+  }
+  return DEFAULT_PRERENDER_CONCURRENCY
+}
+
 // Note: Duplicated from react-router/lib/server-runtime
 function groupRoutesByParentId(manifest: GenericRouteManifest) {
   let routes: Record<string, Omit<RouteManifestEntry, "file">[]> = {};