Local dev server is not sending realtime notification


I’m developing a widget that shows a 2D line chart in a Cockpit application. To set the local enviroment up I run npm start:

> dac-cockpit@1.0.0 start
> c8ycli -p 8000 server -u https://dac-testing.cumulocity.com

Proxying requests to remote instance https://dac-testing.cumulocity.com

http://localhost:8000/apps/dac-cockpit/ 11 % - building - 9/10 modules

http://localhost:8000/apps/dac-cockpit/ 98 % - after emitting - angular-compiler

Hash: d52f23fcde3b8471d442
Version: webpack 4.46.0
Time: 46711ms
Built at: 05/03/2022 9:22:36 AM
chunk {app} app.js (app) 1.03 MiB <{bootstrap}> ={vendors~app}= ={vendors~app~branding}= [rendered]
chunk {bootstrap} bootstrap.js (bootstrap) 35.4 KiB >{app}< >{branding}< >{vendors~app}< >{vendors~app~branding}< >{vendors~branding}< [entry] [rendered]
chunk {branding} branding.js (branding) 2.06 MiB <{bootstrap}> ={vendors~app~branding}= ={vendors~branding}= [rendered]
chunk {vendors~app} vendors~app.js (vendors~app) 20.4 MiB <{bootstrap}> ={app}= ={vendors~app~branding}= [rendered] split chunk (cache group: vendors) (name: vendors~app)
chunk {vendors~app~branding} vendors~app~branding.js (vendors~app~branding) 11.3 KiB <{bootstrap}> ={app}= ={branding}= ={vendors~app}= ={vendors~branding}= [rendered] split chunk (cache group: vendors) (name: vendors~app~branding)
chunk {vendors~branding} vendors~branding.js (vendors~branding) 205 KiB <{bootstrap}> ={branding}= ={vendors~app~branding}= [rendered] split chunk (cache group: vendors) (name: vendors~branding)

http://localhost:8000/apps/dac-cockpit/ done - finished in 46 s - 692.508 ms

When I hit http://localhost:8000/apps/dac-cockpit/ the widget is not updated in real time (i.e. the line is not moving) and I get the following error in the console of dev tools:

client.umd.js?3927:948 WebSocket connection to 'ws://localhost:8000/notification/realtime' failed: Invalid frame header

I tried also on Firefox without any luck:

The connection to ws://localhost:8000/notification/realtime was interrupted while the page was loading.

This problem occurs only locally. When I deploy the application on my production tenant the widget is updated in real time.


  • OS: Fedora 35
  • Node v16.3.0
  • Chrome 101.0.4951.41
  • Firefox 99.0.1


there was an issue within the Cumulocity WebSDK (to be precise in the @c8y/cli package) that caused problems with websockets during local development.
The issue has been addressed in fix releases of the WebSDK above and



This topic was automatically closed 90 days after the last reply. New replies are no longer allowed.