`native-federation:build-notifications` SSE connections block new browser tabs in development mode

[nlegncn]

Repository: native-federation-core or native-federation runtime

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Summary

When running a Shell + multiple MFE applications in development mode using Native Federation,
opening 3+ browser tabs causes subsequent tabs to hang indefinitely until a previous tab is closed.
After investigation, the root cause is the persistent SSE (Server-Sent Events) connections created
by native-federation:build-notifications for each tab, which exhausts the browser's HTTP/1.1
parallel connection limit (6 connections per origin).

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Environment

"@angular-architects/native-federation": "21.0.4",
"@angular-devkit/core": "21.2.6",
"@angular/animations": "21.2.6",

• OS: Windows
• Setup: Shell + 4 Web Component MFEs (each using createApplication)

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Steps to Reproduce

1. Set up a Shell application with Native Federation
2. Configure 1 or more MFE remotes loaded as Web Components
3. Open the shell in a browser tab — tab loads successfully
4. Open a second tab — loads successfully
5. Open a third tab — hangs indefinitely
6. Close any existing tab — the hanging tab immediately loads

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Root Cause

In Chrome DevTools Network tab, filtering by native-federation:build-notifications reveals:

• Each browser tab opens a persistent SSE (EventSource) connection to
  native-federation:build-notifications
• These connections remain open (status: pending) for the entire lifetime of the tab
• Chrome allows a maximum of 6 parallel HTTP/1.1 connections per origin
• With a Shell + MFEs, each tab consumes 1-2 SSE connections
• After 3 tabs, the connection pool is exhausted
• New tabs cannot make any network requests — even the initial HTML document fetch is blocked
• Angular never bootstraps — the page remains blank
• When a tab is closed, its SSE connection is released and the blocked tab immediately loads

Key observation: This issue only occurs in development mode. In production builds,
native-federation:build-notifications does not exist, so the problem does not affect end users.

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Network Evidence

Each open tab shows these persistent connections:

[HusseinKabbout]

I am also encountering this behaviour when developing a Quarkus application using Quinoa. The issue there is not multiple tabs but the whole frontend stops working. Quarkus has its own hot reload logic and all requests to the npm dev server are being proxied by Quarkus. So reloading the frontend in the browser, new connections will be created and the old ones are still being kept alive by Quarkus. The only solution which worked for me is having uBlock origin block the SSE connection requests. Then everything works as expected.

[CaspianVahagn]

Maybe it would make sense to move from SSE to Sockets.
I suspect this is also a limiting factor for SSR dev development since no nodeJS default new EventSource(...) implementation exists.

This part here:

https://github.com/angular-architects/module-federation-plugin/blob/main/libs/native-federation-runtime/src/lib/watch-federation-build.ts

Could maybe become a websocket instead of SSE.
And maybe excluded from SSR because it will be undefined or changed to restart the host server and reload the federation for SSR.