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Server Bundles

Server bundles allow you to split your server-side code into multiple output files, enabling different routes to be deployed to different servers or serverless functions.

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Overview

By default, React Router builds a single server bundle containing all routes. Server bundles let you:
  • Deploy different routes to different servers
  • Split large applications across multiple serverless functions
  • Deploy to different regions or platforms
  • Optimize cold start times for serverless deployments
  • Reduce bundle sizes for individual functions

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Configuration

Define server bundles in react-router.config.ts: 
import type { Config } from "@react-router/dev/config";

export default {
  async serverBundles({ branch }) {
    // Return a bundle ID based on the route branch
    const isAdminRoute = branch.some((route) => route.id.startsWith("admin"));
    const isApiRoute = branch.some((route) => route.id.startsWith("api"));
    
    if (isAdminRoute) return "admin";
    if (isApiRoute) return "api";
    return "main";
  },
} satisfies Config;

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Server Bundle Function

The serverBundles function is called for each route: 
type ServerBundlesFunction = (args: {
  branch: BranchRoute[];
}) => string | Promise<string>;

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branch

An array of routes from root to the current route: 
type BranchRoute = {
  id: string;      // Route ID
  path: string;    // Route path
  file: string;    // Route file path
  index?: boolean; // Is index route
};

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Examples

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Split by Route Prefix

export default {
  serverBundles({ branch }) {
    // Check route IDs in the branch
    const routeId = branch[branch.length - 1].id;
    
    if (routeId.startsWith("routes/admin")) return "admin";
    if (routeId.startsWith("routes/api")) return "api";
    if (routeId.startsWith("routes/blog")) return "blog";
    
    return "main";
  },
} satisfies Config;

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Split by Path Pattern

export default {
  serverBundles({ branch }) {
    // Check if path contains specific segments
    const hasAdminPath = branch.some((route) => 
      route.path?.includes("admin")
    );
    const hasApiPath = branch.some((route) => 
      route.path?.includes("api")
    );
    
    if (hasAdminPath) return "admin";
    if (hasApiPath) return "api";
    
    return "app";
  },
} satisfies Config;

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Split by File Location

export default {
  serverBundles({ branch }) {
    // Check route file paths
    const route = branch[branch.length - 1];
    
    if (route.file.startsWith("routes/admin/")) return "admin";
    if (route.file.startsWith("routes/marketing/")) return "marketing";
    if (route.file.startsWith("routes/app/")) return "app";
    
    return "default";
  },
} satisfies Config;

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Regional Deployment

export default {
  serverBundles({ branch }) {
    const route = branch[branch.length - 1];
    
    // Deploy different routes to different regions
    if (route.path?.startsWith("/eu")) return "eu-bundle";
    if (route.path?.startsWith("/asia")) return "asia-bundle";
    if (route.path?.startsWith("/us")) return "us-bundle";
    
    return "global-bundle";
  },
} satisfies Config;

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Build Output

With server bundles enabled, the build output structure changes:

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Without Server Bundles

build/
├── client/
│   └── assets/
└── server/
    └── index.js        # Single server bundle

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With Server Bundles

build/
├── client/
│   └── assets/
└── server/
    ├── main/           # Main bundle
    │   └── index.js
    ├── admin/          # Admin bundle
    │   └── index.js
    └── api/            # API bundle
        └── index.js

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Build Manifest

The build manifest includes server bundle information: 
type ServerBundlesBuildManifest = {
  routes: RouteManifest;
  serverBundles: {
    [serverBundleId: string]: {
      id: string;
      file: string; // Relative path to bundle
    };
  };
  routeIdToServerBundleId: Record<string, string>;
};
Access in buildEnd hook: 
export default {
  serverBundles({ branch }) {
    // ...
  },
  
  async buildEnd({ buildManifest }) {
    if (buildManifest.serverBundles) {
      console.log("Server bundles:", buildManifest.serverBundles);
      
      for (const [bundleId, bundle] of Object.entries(buildManifest.serverBundles)) {
        console.log(`Bundle ${bundleId}: ${bundle.file}`);
        
        // Deploy each bundle separately
        await deployBundle(bundleId, bundle.file);
      }
    }
  },
} satisfies Config;

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Deployment

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Deploy to Different Servers

export default {
  async buildEnd({ buildManifest, reactRouterConfig }) {
    if (!buildManifest.serverBundles) return;
    
    for (const [bundleId, bundle] of Object.entries(buildManifest.serverBundles)) {
      const bundlePath = path.join(
        reactRouterConfig.buildDirectory,
        bundle.file
      );
      
      if (bundleId === "admin") {
        // Deploy admin bundle to admin server
        await deployTo("admin.example.com", bundlePath);
      } else if (bundleId === "api") {
        // Deploy API bundle to API server
        await deployTo("api.example.com", bundlePath);
      } else {
        // Deploy main bundle to main server
        await deployTo("example.com", bundlePath);
      }
    }
  },
} satisfies Config;

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Serverless Functions

Deploy each bundle as a separate function: 
export default {
  async buildEnd({ buildManifest, reactRouterConfig }) {
    if (!buildManifest.serverBundles) return;
    
    for (const [bundleId, bundle] of Object.entries(buildManifest.serverBundles)) {
      const functionConfig = {
        name: `app-${bundleId}`,
        runtime: "nodejs20.x",
        handler: bundle.file,
        memory: bundleId === "admin" ? 512 : 256,
        timeout: bundleId === "api" ? 30 : 10,
      };
      
      await deployServerlessFunction(functionConfig);
    }
  },
} satisfies Config;

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Running Multiple Bundles Locally

For development or preview, you may need to run multiple bundles: 
import { createRequestHandler } from "@react-router/express";
import express from "express";
import * as mainBuild from "./build/server/main/index.js";
import * as adminBuild from "./build/server/admin/index.js";
import * as apiBuild from "./build/server/api/index.js";

const app = express();

app.use(express.static("build/client"));

// Route requests to appropriate bundles
app.use("/admin", createRequestHandler({ build: adminBuild }));
app.use("/api", createRequestHandler({ build: apiBuild }));
app.use("*", createRequestHandler({ build: mainBuild }));

app.listen(3000);

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Vite Environment API

With the Vite Environment API enabled, server bundles create separate environments: 
export default {
  future: {
    v8_viteEnvironmentApi: true,
  },
  
  serverBundles({ branch }) {
    // Each bundle gets its own environment
    const route = branch[branch.length - 1];
    if (route.id.startsWith("admin")) return "admin";
    return "main";
  },
} satisfies Config;
Environments are named: ssrBundle_<bundleId>

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Shared Code

Code shared between bundles is automatically deduplicated by Vite: 
// Shared utility used by multiple routes
export function formatDate(date: Date) {
  return date.toLocaleDateString();
}
If used by routes in different bundles, it’s included in each bundle but the code is identical.

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Bundle Size Optimization

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Minimize Shared Dependencies

Keep bundles small by minimizing shared dependencies: 
// ❌ Imports large library in every route
import { someUtil } from "large-library";

// ✅ Only import in routes that need it
import { someUtil } from "large-library";

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Split Heavy Routes

export default {
  serverBundles({ branch }) {
    const route = branch[branch.length - 1];
    
    // Heavy data processing routes in separate bundle
    if (route.id.includes("reports") || route.id.includes("analytics")) {
      return "heavy";
    }
    
    return "main";
  },
} satisfies Config;

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Caveats

  1. Client bundle is shared - Only server code is split, client code is always in one bundle
  2. Route discovery still works - All routes are discoverable regardless of bundle
  3. Shared code is duplicated - Dependencies are included in each bundle that uses them
  4. Cold starts may vary - Different bundles have different cold start times
  5. Deployment complexity - Multiple bundles require coordinated deployment

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Best Practices

  1. Group related routes - Keep related functionality in the same bundle
  2. Consider bundle size - Balance number of bundles vs. size of each
  3. Test locally - Ensure all bundles work together correctly
  4. Monitor performance - Track bundle sizes and cold start times
  5. Document bundle strategy - Make it clear which routes go in which bundles

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See Also

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