Invoking a Lambda to message connected websockets - node.js

Question

Is it possible to setup a stand-alone WebSocket service with lambdas that can be invoked from lambdas in separate services?
I've got an existing system that does things and then attempts to broadcast an update to connected clients by invoking a lambda in a websocket service like so:
const lambda = new Lambda({
region: 'us-east-1',
endpoint: 'https://lambda.us-east-1.amazonaws.com'
});
lambda.invoke({
FunctionName: `dev-functionName`,
Payload: JSON.stringify({payload, clientGroup}),
InvocationType: 'Event'
});
This triggers the correct lambda, which then
gets the connection IDs from a Dynamo table
sets domainName to {api-id}.execute-api.us-east-1.amazonaws.com
attempts to message connections them like so:
const ws = create(`https://${domainName}/dev/#connections/${ConnectionId}`);
// Also tried
//const ws = create(`https://${domainName}/dev`);
//const ws = create(`${domainName}/dev`);
//const ws = create(`${domainName}`);
const params:any = {
Data: JSON.stringify(payload),
ConnectionId
};
try{
return ws.postToConnection(params).promise();
} catch (err) {
if(err.statusCode == 410){
await removeConn(ConnectionId); // Delete connection from Dynamo
} else {
throw err;
}
}
The create function just returns:
return new AWS.ApiGatewayManagementApi({
apiVersion: '2018-11-29',
endpoint: domainName
});
CloudWatch logs suggest that all functions are triggering and completing successfully with no errors. It also shows that connections are being retrieved from Dynamo. However the clients are not receiving any messages.
When running the projects locally and using localhost urls, everything works as expected. What am I doing wrong here?

Answer 1

First, the correct domain for the endpoint is
const endpoint = `${domainName}/${"dev"}`;
Second, to see errors in CloudWatch, you need to await the postToConnection promise
Third, the external services were calling a REST endpoint in the WebSocket service. This meant that there are 2 entries added to API Gateway. The REST APIs need the following permissions for the WebSocket API:
Action:
- "execute-api:Invoke"
- "execute-api:ManageConnections"