I think there’s no firewall in our test environments. However, there’s network control software at client machine.
The network control software occasionally lost the connection.
My question is, if the network connection lost, then jdbc client should return an IOException immediately?
OR blocked forever until TCP connection killed by operating system?
Well the problem is we don't know that the connection has failed until the TCP connection has been killed by the O/S.
Vladimir is proposing keep alive messages so that either it won't fail or we will know about it sooner if it does.
I don't think it's a good idea to flood the server with dummy messages for the purposes of keeping the connection alive. Besides the usual problems of filling up the send buffer, you'd also have to have it done in a separate thread as the usual sender is already blocked waiting for the command response.
The defaults for TCP keep alives are very high (I think two-hours before the first attempt) so it's a good idea to reduce them if you're dealing with long running WAN connections. Also, from personal experience it's usually residential networking devices (i.e. your WiFi router) that drop inactive NATed connections. This is less of an issue in a server-to-server environment in a data center. Either way, bumping up the TCP keep alives works in both situations.
