ApolloServer: "Could not connect to websocket endpoint ws://localhost:4000/subscriptions. Please check if the endpoint url is correct." - docker

Question

I can't implement any subscriptions because it suddenly disconnects from it when I try to listen to some endpoint with GraphQL Playground:
{"error": "Could not connect to websocket endpoint wss://localhost:4000/graphql. Please check if the endpoint url is correct."}
I'm using ApolloServer alone, no express or anything else. It is containerized with Docker using node14 image, the port is properly fowarded, queries and mutations works properly.
This is the configuration snippet:
const server = new ApolloServer({
typeDefs: mergedTypeDefs,
resolvers: mergedResolvers,
playground: {
subscriptionEndpoint: 'ws://localhost:4000/graphql'
},
subscriptions: {
keepAlive: 9000,
onConnect: (connParams, webSocket, context) => {
console.log('CLIENT CONNECTED');
},
onDisconnect: (webSocket, context) => {
console.log('CLIENT DISCONNECTED')
}
},
context: {
models
}
});
I tried everything, from using 'wss' instead of 'ws' to change the path. I checked for typos and didn't find one. What bothers me is that the paths are the same so It should at least try to notify me by the onConnect or onDisconnect but it doesn't.
This is the message through Chrome's dev tools:
WebSocket connection to 'wss://localhost:4000/graphql' failed: WebSocket is closed before the connection is established.
I tested subscriptions with a 'tutorial' project outside the container and it works fine.
Sometimes, the only function of subscriptions that works is onDisconnect but after 2-5 seconds after receiving the error message on PlayGround, and Still it doesn't gives me any insight on the problem.
Any help or suggestion is appreciated.

Answer 1

wss will definitely not work locally without certificate, so use ws as a protocol.
If it works without Docker, then everything should be fine code-wise.
You should also make sure that you've mapped ports correctly, i.e. exposed port 4000 https://docs.docker.com/engine/reference/commandline/run/#publish-or-expose-port--p---expose