Is websocket connection reliable?

Question

I always thought that web socket guarantees delivery of data as it is built on top of TCP which is reliable. I was reading the Java web socket API documentation on this link https://docs.oracle.com/javaee/7/api/javax/websocket/RemoteEndpoint.html
It says that

There is no guarantee of the successful delivery of a web socket message to the peer, but if the action of sending a message causes an error known to the container, the API throws it.

Shouldn't TCP gurantee the message delivery?

Answer 1

Reliable != Guaranteed. Reliable means you'll be notified if there was a failure (or success) - if the end users unplugs their lan cable, even tcp can't 'guarantee delivery.'

Answer 2

"Guarantee" is a bit misleading. TCP will make a best attempt to send ordered data sent by one TCP stack to be received in-order by another TCP stack. If this cannot be done, the TCP stacks will engage in an error situation. This is no different than a money-back guarantee you get from a store; failure mode means you get a replacement and try again... eventually if too many failures and get your money back (e.g., connection broken).

Also... a TCP "guarantee" does not mean the data is "guaranteed" to be handled correctly at the application levels of both stacks. This is the responsibility of application protocols. After its work is done, TCP hands it off to the application layers above it which implement things like pub/sub, http, rpc, tuple spaces, email, ftp, transactions, databases, etc, etc...