/mercurius-gateway-subscriptions-repro

Primary LanguageJavaScript

Issue

GraphQL subscriptions are not terminated correctly when there is gateway in the chain. Works fine when client connects directly to a backend service.

Steps:

  1. run npm i to install dependencies
  2. run npm run start:service to start a test service providing GraphQL schema - port 3002
  3. run npm run start:gateway to start gateway service that federates our service - port 3001
  4. run npm start to start frontend - port 3000

You can switch connecting to gateway or service directly by adjusting BACKEND_PORT constant in src/index.js

Note: also provided start:<service>:dev command to run nodemon for easier development.

To reproduce:

To better see the issue, add logging to node_modules/mercurius/lib/subscription-connection.js in handleGQLComplete function on line 336, for example console.log('=== handleGQLComplete for id: ', id) and restart both service and gateway.

  1. When you start the app the client is configured to connect through the gateway
  • click multiple times on the increment button to increment ID which is used as a variable for graphql subscription
  • service.js should log from subscribe callback with new ID each time you clicked
  • observe gateway logs to see that each time ID was incremented, handleGQLComplete was called
  • observe service logs to see that handleGQLComplete was not called once
  • now click 'Add notification' button to see the filter handler invoked X times you incremented the ID, but according to gateway subscriptions with old variables were closed?
  • now shutdown gateway and observe service logs to see that handleGQLComplete was additionally logged X times the ID was incremented, but these should have been closed during the incrementing when gateway closed them?
  1. Now switch BACKEND_PORT as per instructions so the client connects directly to service and restart service
  • first step is the same, increment ID multiple times
  • observe service logs to see subscribe callback called each time you clicked
  • you will also see each time ID was incremented, previous subscription was closed
  • if you click 'Add notification' you will see filter callback invoked only once as previous subscriptions were closed

Conclusion

It seems that gateway doesnt close subscriptions to(/in?) federated services even though they were closed by gateway itself.