[APPS] Use separate vite.build() for backend functions, drop Rollup backend support by sdkennedy2 · Pull Request #292 · DataDog/build-plugins

sdkennedy2 · 2026-03-20T20:38:52Z

Motivation

Backend functions were previously built by injecting virtual modules into the host bundler's build via resolveId/load hooks, with separate Rollup and Vite plugin implementations that duplicated logic. This approach was fragile — it coupled backend function bundling to the host build's configuration (plugins, aliases, output settings) and required maintaining two codepaths (Rollup + Vite).

This PR switches to a standalone vite.build() call that runs after the host build completes, giving full control over the backend build configuration. It also drops Rollup backend support entirely, simplifying to vite-only.

Changes

Replaced the "inject into host build" approach with a dedicated vite.build() for backend functions:

// In closeBundle hook (runs after host build finishes):
const result = await vite.build({
    configFile: false,       // Don't inherit user's vite plugins
    build: {
        write: true,
        outDir: tempDir,     // Isolated output directory
        rollupOptions: {
            input: virtualEntries,
            output: { format: 'es', entryFileNames: '[name].js' },
            treeshake: false,  // Preserve action-catalog bridges intact
        },
    },
    plugins: [/* virtual module resolver */],
});

Why a separate build? Backend functions need different build settings than the frontend — no minification, ES module format, preserved entry signatures, and no interference from the user's vite plugins. Running a standalone vite.build() with configFile: false provides this isolation cleanly.

Key changes:

Deleted backend/rollup.ts — Rollup backend support removed
Simplified backend/index.ts — removed resolveId/load hooks, now just delegates to the vite plugin
New buildBackendFunctions() in backend/vite/index.ts — runs a separate vite.build() via closeBundle hook, writes output to a temp directory, populates backendOutputs map for the upload plugin
Refactored virtual-entry.ts — extracted generateMainBody() helper for reuse
Updated shared.ts — isActionCatalogInstalled() now accepts a fromDir parameter for correct resolution when the plugin is linked, fixed regex escaping in action-catalog bridge
Restricted backendSupportedBundlers from ['rollup', 'vite'] to ['vite']

QA Instructions

Scaffold or use an existing high code app with backend functions
Link the local vite plugin and run yarn dev in build-plugins
Run a production build (npx vite build) and verify backend function .js files are generated and included in the upload archive
Confirm the build works without the user's vite.config.ts plugins interfering with the backend build

Blast Radius

Only affects vite production builds when apps.backendDir is configured
Rollup users with backend functions will now see a warning that backend functions are not supported for their bundler (previously attempted but was untested)
No impact on webpack, esbuild, or rspack users

Documentation

…ackend support Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sdkennedy2 · 2026-03-20T20:39:08Z

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yoannmoinet · 2026-03-23T18:40:07Z

packages/plugins/apps/src/backend/vite/index.ts

+ * Produces one standalone JS file per function in a temp directory.
+ */
+async function buildBackendFunctions(
+    vite: any,


Can't this be typed correctly?

My thought process initially was trying to avoid importing vite since I would need to add it as a dependency in our app's package.json.

The more I think about it we could add vite as a dev dependency if we are just importing the type though. One annoying thing is vite doesn't actually export very good types it only exports declarations for specific pieces of code.

So I think I'll need to do the following:

import type { build } from 'vite'; ... async function buildBackendFunctions( viteBuild: typeof build, functions: BackendFunction[], backendOutputs: Map<string, string>, buildRoot: string, log: Logger, ): Promise<void> {

Let me know if you are ok adding vite as dev dependency in the package.json

devDeps is ok I think, since it won't impact the user's experience.
And we're already depending on rollup for the same reason.
If the typeof solution works, let's go with it.

yoannmoinet · 2026-03-23T18:46:12Z

packages/plugins/apps/src/backend/index.ts

    functions: BackendFunction[],
    backendOutputs: Map<string, string>,
    log: Logger,
+    pluginContext?: BackendPluginContext,


Why is this optional?

Good callout. It is not. I'm just going to inline the two properties within BackendPluginContext as parameters passed into getBackendPlugin as well. It seems arbitrary to have these two separated.

yoannmoinet · 2026-03-23T18:47:46Z

packages/plugins/apps/src/backend/shared.ts

-            const actionPath = actionId.replace(/^com\\\\.datadoghq\\\\./, '');
+            const actionPath = actionId.replace(/^com\\.datadoghq\\./, '');


Isn't this tested?

Just to clarify, you are calling out that the behavior captured within the string should also be tested?

I was wondering why did you have to change this.
And wondering if the output of it was actually tested in order to catch this kind of mistake before merge.

There was testing done, but looking at the test app it looks like it tested the more basic case of returning static data. It must have been a previous iteration that tested the action catalog support.

Since we don't have this functionality enabled for customers yet, I don't think this was an issue. We are working towards a point where we can do a true end to end test of developing an app and publishing. At the point we enable this for customers we should feel good about it.

I do think its a good callout that we should make sure we have get tests around the integration between the high code apps logic in different repros and between different libraries though.

yoannmoinet · 2026-03-23T18:49:32Z

packages/plugins/apps/src/backend/virtual-entry.ts

+export function generateVirtualEntryContent(
+    functionName: string,
+    entryPath: string,
+    projectRoot?: string,


Why is this optional?

Good callout I'll remove it.

yoannmoinet · 2026-03-23T18:51:57Z

packages/plugins/apps/src/index.ts

+    // Backend build plugin — builds backend functions via a separate vite.build().
+    // Only supported for vite.
+    const backendSupportedBundlers = ['vite'];
    if (hasBackend && backendSupportedBundlers.includes(context.bundler.name)) {


You don't need this part backendSupportedBundlers.includes(context.bundler.name) since you control this from the other side by giving the { vite: ... } part.

Good call 👍

…-entry tests

…ckend temp dir

datadog-datadog-prod-us1 · 2026-03-26T20:59:01Z

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

_{This comment will be updated automatically if new data arrives.

🔗 Commit SHA: 90d9b77 | Docs | Datadog PR Page | Was this helpful? React with 👍/👎 or give us feedback!}

sdkennedy2 · 2026-03-26T21:10:02Z

packages/plugins/apps/src/vite/build-backend-functions.ts

+ * Build all backend functions using a separate vite.build() call.
+ * Produces one standalone JS file per function in a temp directory.
+ */
+export async function buildBackendFunctions(


I put this here because the logic is largely coupled to vite versus the code in the backend folder.

[APPS] Use separate vite.build() for backend functions, drop Rollup b…

c291d6d

…ackend support Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sdkennedy2 requested a review from yoannmoinet as a code owner March 20, 2026 20:38

sdkennedy2 mentioned this pull request Mar 20, 2026

[APPS] Add vite dev server middleware for backend functions #291

Open

yoannmoinet reviewed Mar 23, 2026

View reviewed changes

sdkennedy2 added 3 commits March 25, 2026 17:50

Address PR feedback: type vite.build, add shared.test.ts, fix virtual…

66b0712

…-entry tests

Refactor getBackendPlugin/getVitePlugin to use options object pattern

9996da7

Move vite plugin to src/vite/, inline virtual-entry body, clean up ba…

c137106

…ckend temp dir

Remove duplicate backend/vite/index.ts (moved to src/vite/)

d85a3c7

sdkennedy2 commented Mar 26, 2026

View reviewed changes

Revert accidental yarn dev changes to published package.json exports

90d9b77

		const actionPath = actionId.replace(/^com\\\\.datadoghq\\\\./, '');
		const actionPath = actionId.replace(/^com\\.datadoghq\\./, '');

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sdkennedy2 commented Mar 20, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Documentation

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sdkennedy2 commented Mar 20, 2026 •

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