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

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sdkennedy2 wants to merge 11 commits intosdkennedy2/apps-vite-backend-buildfrom

sdkennedy2/apps-vite-dev-server

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sdkennedy2 commented Mar 17, 2026 •

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Motivation

High Code Apps backend functions need a local development experience. Currently, backend functions are only built at production build time — there's no way to invoke them during local development with hot reload.

Changes

Adds a Vite dev server middleware that intercepts requests to backend function endpoints, bundles the target function on-the-fly using Vite's build API, executes it, and returns the result. This gives developers a local dev loop for backend functions without needing a full production build.

Key pieces:

dev-server.ts — Express-style middleware that handles /execute-action requests. On each invocation it bundles the requested function using Vite's in-memory build, evaluates the bundle, and returns the result. Includes long-poll support for streaming responses.
build-config.ts — Extracted shared Vite build config (getBaseBackendBuildConfig) used by both the production multi-entry build and the dev single-function build, eliminating duplication of config options (resolve conditions, output format, virtual module plugin, etc.).
virtual-entry.ts — Added generateDevVirtualEntryContent for the dev build's virtual entry point, alongside the existing production entry generator.
Dev server validates requests and returns proper HTTP status codes (400 for bad requests, 404 for unknown functions, 403 for missing credentials, 500 for build/runtime errors).
Uses doRequest from @dd/core/helpers/request for outbound calls (consistency with codebase conventions, retry support).

QA Instructions

Run yarn dev in a project with backend functions
Trigger a backend function invocation from the frontend
Verify the function executes and returns results correctly
Verify hot reload — change the function source and re-invoke without restarting the dev server

Blast Radius

Only affects the Vite plugin for apps with backend functions during local development
No changes to production build output
Gated by the apps plugin — only active when apps configuration is present

sdkennedy2 mentioned this pull request

[APPS] Inject backend functions as host build entry points (Phase 1: Rollup/Vite) #289

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Collaborator Author

sdkennedy2 commented Mar 17, 2026 •

edited

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sdkennedy2 mentioned this pull request

[APPS] Add e2e test and frontend/ archive prefix for apps plugin #290

Merged

sdkennedy2 changed the title ~~[APPS] Add vite dev server middleware for backend functions using host bundler~~ [APPS] Add vite dev server middleware for backend functions

sdkennedy2 requested a review from sarenji

March 17, 2026 14:49

sdkennedy2 marked this pull request as ready for review

March 17, 2026 14:50

sdkennedy2 requested a review from yoannmoinet as a code owner

March 17, 2026 14:50

sdkennedy2 force-pushed the sdkennedy2/apps-vite-dev-server branch from a1630ef to 4f85ed9 Compare

March 17, 2026 18:02

sdkennedy2 force-pushed the sdkennedy2/apps-backend-functions-upload-v2 branch from 264286b to 2f9f167 Compare

March 17, 2026 18:02

sdkennedy2 commented

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sdkennedy2 force-pushed the sdkennedy2/apps-vite-dev-server branch from 7e20d87 to e465412 Compare

March 18, 2026 15:53

sdkennedy2 force-pushed the sdkennedy2/apps-backend-functions-upload-v2 branch from 2f9f167 to d682d52 Compare

March 18, 2026 15:53

Base automatically changed from sdkennedy2/apps-backend-functions-upload-v2 to master

March 19, 2026 15:43

sarenji reviewed

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sarenji left a comment

Some initial thoughts on this

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packages/plugins/apps/src/backend/vite/dev-server.test.ts Outdated Show resolved Hide resolved

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sdkennedy2 changed the base branch from master to graphite-base/291

March 20, 2026 20:38

sdkennedy2 force-pushed the sdkennedy2/apps-vite-dev-server branch from 95203d8 to a0932f3 Compare

March 20, 2026 20:38

sdkennedy2 mentioned this pull request

[APPS] Use separate vite.build() for backend functions, drop Rollup backend support #292

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sdkennedy2 changed the base branch from graphite-base/291 to sdkennedy2/apps-vite-backend-build

March 20, 2026 20:52

sarenji reviewed

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yoannmoinet reviewed

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yoannmoinet left a comment

You should check in the other parts of the code for hints on how to use what's available to you in the repo.

packages/plugins/apps/src/backend/vite/dev-server.test.ts Outdated Show resolved Hide resolved

packages/plugins/apps/src/backend/vite/dev-server.test.ts Outdated Show resolved Hide resolved

packages/plugins/apps/src/backend/vite/dev-server.test.ts Outdated

+              describe('Dev Server Middleware', () => {
+                  beforeEach(() => {
+                      jest.clearAllMocks();

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yoannmoinet Mar 23, 2026

nit: this is unnecessary as it is part of the Jest configuration.

packages/plugins/apps/src/backend/vite/dev-server.test.ts Outdated Show resolved Hide resolved

packages/plugins/apps/src/backend/vite/dev-server.test.ts Outdated

Comment on lines +154 to +155

		// Wait for async handler to complete.
		await new Promise((resolve) => setTimeout(resolve, 100));

Member

yoannmoinet Mar 23, 2026

Same, and for all the followings as well.
You should not be doing this.

packages/plugins/apps/src/backend/vite/dev-server.ts Outdated Show resolved Hide resolved

packages/plugins/apps/src/backend/vite/dev-server.ts Outdated

+                      ],
+                  });
+                  const output = (Array.isArray(result) ? result[0] : result) as RollupOutput;

Member

yoannmoinet Mar 23, 2026

Why do you need the as RollupOutput part here?
This should be typed from viteBuild, which should not have a basic Function type.

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packages/plugins/apps/src/backend/vite/dev-server.ts Outdated Show resolved Hide resolved

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yoannmoinet reviewed

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sdkennedy2 force-pushed the sdkennedy2/apps-vite-dev-server branch 2 times, most recently from 807bbdf to c0be60c Compare

March 26, 2026 20:55

datadog-prod-us1-6 bot commented Mar 26, 2026 •

edited by datadog-prod-us1-4 bot

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✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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

🔗 Commit SHA: 81b50d1 | Docs | Datadog PR Page | Was this helpful? React with 👍/👎 or give us feedback!}

sdkennedy2 force-pushed the sdkennedy2/apps-vite-dev-server branch 5 times, most recently from 146a8f6 to 9c96053 Compare

March 30, 2026 15:09

sdkennedy2 force-pushed the sdkennedy2/apps-vite-backend-build branch from d2b2a1b to 03c5f9c Compare

March 30, 2026 15:09

sdkennedy2 and others added 6 commits

March 30, 2026 14:52


          [APPS] Add vite dev server middleware for backend functions

ad0cfdd

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


          Dependency-inject vite.build into dev server middleware instead of dy…

4725dff

…namic import


          Fix TypeScript errors in dev-server tests, remove unnecessary type casts

a45c8f1


          Use getMockLogger helper, simplify type casts in dev-server tests

d492110


          Address PR feedback: use doRequest, fix test patterns, fix virtual-en…

e9cec68

…try bug

- Replace raw fetch calls with doRequest from @dd/core/helpers/request
  for retry support and consistency with codebase conventions.
- Replace flaky setTimeout waits in tests with deterministic res.done promise.
- Replace as-any casts with as-unknown-as-Type for proper type narrowing.
- Fix generateDevVirtualEntryContent calling undefined generateMainBody
  by inlining the main body generation (matching production entry pattern).

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


          Extract shared vite.build config for production and dev backend builds

0de8724

Move common build options (configFile, logLevel, minify, target,
treeshake, preserveEntrySignatures, onwarn, resolve, output format,
and virtual module plugin) into a shared getBaseBackendBuildConfig()
helper. Each caller now only specifies what differs: production writes
multi-entry output to disk, dev builds a single function in-memory.

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

sdkennedy2 force-pushed the sdkennedy2/apps-vite-dev-server branch from 6c22cb8 to 0de8724 Compare

March 30, 2026 18:53

sdkennedy2 added 2 commits

March 30, 2026 15:29


          Move vite-specific code to src/vite/, use shared build config, simpli…

273604c

…fy auth passing


          Extract shared validateAndBundle, simplify error handling to always 500

61b0c46

yoannmoinet reviewed

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yoannmoinet left a comment

I'm really not confident about the tests on the dev-server.

packages/plugins/apps/src/vite/dev-server.test.ts Outdated Show resolved Hide resolved

packages/plugins/apps/src/vite/dev-server.test.ts Outdated

+                          middleware(req, res, jest.fn());
+                          await res.done;
+                          expect(res.statusCode).toBe(500);

Member

yoannmoinet Mar 31, 2026

Is it 500 or 400 that we're expecting for a bad request?

packages/plugins/apps/src/vite/dev-server.test.ts Outdated

+                          middleware(req, res, jest.fn());
+                          await res.done;
+                          expect(res.statusCode).toBe(500);

Member

yoannmoinet Mar 31, 2026

500?

packages/plugins/apps/src/vite/dev-server.test.ts Outdated

+                          middleware(req, res, jest.fn());
+                          await res.done;
+                          expect(res.statusCode).toBe(500);

Member

yoannmoinet Mar 31, 2026

500?

packages/plugins/apps/src/vite/dev-server.test.ts

+                          expect(res.statusCode).toBe(500);
+                          const body = JSON.parse(res.getBody());
+                          expect(body.success).toBe(false);
+                          expect(body.error).toContain('HTTP 403');

Member

yoannmoinet Mar 31, 2026

403 now?

These tests are not re-assuring to be honest.

Collaborator Author

sdkennedy2 Apr 1, 2026 •

edited

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It would be better to return a 403 here so the developer gets a clear signal that their credentials are wrong rather than a generic 500. However, doRequest throws a plain Error with the status code embedded in the message string ("HTTP 403 Forbidden") rather than a structured error with a .status property. To forward upstream status codes cleanly, we'd need to refactor doRequest to throw a typed error class — which would be a broader change across the codebase. Would you prefer we tackle that refactor as part of this PR or handle it as a follow-up?

Collaborator Author

sdkennedy2 Apr 1, 2026

From a perspective of unblocking onboarding design partners I'd advocate we leave this as is.

Member

yoannmoinet Apr 1, 2026

Makes sense to tackle this later.
I guess the error in question is this one?

build-plugins/packages/core/src/helpers/request.ts

Line 126 in f3ed032

bail(new Error(errorMessage));

packages/plugins/apps/src/vite/dev-server.ts

Comment on lines +186 to +188

		const maxRetries = 10;

		for (let attempt = 0; attempt < maxRetries; attempt++) {

Member

yoannmoinet Mar 31, 2026

Isn't this all covered by doRequest already?
I don't understand what this does that doRequest doesn't support yet.

Collaborator Author

sdkennedy2 Apr 1, 2026

These serve different purposes. doRequest handles HTTP-level retries — transient network failures, 5xx responses, etc. The poll loop here is application-level: the long-polling endpoint returns HTTP 200 with { done: false } when its server-side wait window (~30s) expires before the query finishes. We re-poll with a fresh request until done: true. doRequest can't express this because the HTTP request itself succeeds — it's the response payload that says "not ready yet, ask again."

Additionally, doRequest doesn't expose enough structure in its error handling to support this pattern — it throws a plain Error with the status code baked into the message string rather than a typed error object, so there's no way to programmatically distinguish between "server said not ready" vs "server returned an error" without string parsing. The poll loop gives us clean control over both cases. I'll add a comment to make the distinction clearer in the code.

Member

yoannmoinet Apr 1, 2026

Sounds good, maybe a comment explaining this would be helpful.

Collaborator Author

sdkennedy2 Apr 1, 2026

Good call 👍. I added a comment.

packages/plugins/apps/src/vite/dev-server.ts Outdated

+                          if (!fullAuth) {
+                              sendError(
+                                  res,
+,

Member

yoannmoinet Mar 31, 2026

403 seems more appropriate here.

sdkennedy2 added 3 commits

April 1, 2026 12:36


          Return proper HTTP status codes (400/404) for client errors

98706eb


          Add comment explaining long-poll vs doRequest retry distinction

658b8db


          Fix auth error status code: 503 -> 403 for missing credentials

81b50d1

yoannmoinet approved these changes

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yoannmoinet left a comment

LGTM

sdkennedy2 mentioned this pull request

[APPS] Support importing .backend.ts files as RPC proxies from frontend code #306

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