Websockets. Send a response and then immediately disconnect the user

In a naive way, your Lambda can invoke postToConnection then deleteConnection. However, this doesn't guarantee the message would be delivered to the client before disconnecting it.

Ideally, the client should disconnect when the message arrives. If you still want to disconnect the client from the server side, you'll need to implement ACK between the client and the server. If the client needs to send a ACK message after the receiving the response. Then your Lambda can safely disconnect the client.