Non-blocking socket writes in Java versus blocking socket writes

Question

Why would someone prefer blocking writes over non-blocking writes? My understanding is that you would only want blocking write if you want to make sure the other side got the TCP packet once the write method returned, but I am not even sure that's possible. You would have to flush and flush would have to flush the underlying operating system write socket buffer. So is there any disadvantage of non-blocking socket writes? Does having a large underlying write socket buffer a bad idea in terms of performance? My understanding is that the smaller the underlying socket write buffer the more likely you will hit slow/buggy client and have to drop/queue packets in the application level while the underlying socket buffer is full and isWritable() is returning false.

Answer 1

My understanding is that you would only want blocking write if you want to make sure the other side got the TCP packet once the write method returned

Your understanding is incorrect. It doesn't ensure that.

Blocking writes block until all the data has been transferred to the socket send buffer, from where it is transferred asynchronously to the network. If the reader is slow, his socket receive buffer will fill up, which will eventually cause your socket send buffer to fill up, which will cause a blocking write to block, blocking the whole thread. Non-blocking I/O gives you a way to detect and handle that situation.

Answer 2

The problem with non blocking writes is that you may not have anything useful to do if the write is incomplete. You can end up with loops like

// non-blocking write
while(bb.remaining() > 0) sc.write(bb);

OR

// blocking write
sc.write(bb);

The first can burn CPU and the second might be more desirable.

The big problem is reads. Once you decide whether you want blocking or non-blocking reads, your writes have to be the same. Unfortunately there is no way to make them different. If you want non-blocking reads, you have to have non-blocking writes.