Does NIO coalesce reads?

[leknor]

My code currently works when I use it but I think I'm making some assumptions that may not hold true under heady load.

I have a bunch of java.nio.channels.DatagramChannel's which I've setup so I can send/receive UDP packets. I then register them with a Selector and wait for input.  That so far seems to work.

My potential problem is with the channel.read(ByteBuffer) method. If two packets have been received on a channel before I can read them will the next read(ByteBuffer) operation read them both or will I need to read the channel twice? Personally I'm hoping for the latter but increasingly I'm feelling like the former is the actuall behavior.

[Herkules]

I hvae no idea of UDP, but for TCP definately messages sent and data read have nothing in common - it is just  a bytestream and any number of bytes might be read.  From zero up to serveral messages at once, also only parts of messages.

But I assume that is the nature of TCP and for UDP I'd expect the behavior you hope for.

[leknor]

I asked the author of the Java NIO book and here is his reply:

Datagrams are chunky.

There are basically two types of network connections: streams (TCP) and datagrams (UDP).  TCP streams (as modeled by SocketChannel objects) function like a pipe where everything blends together into a continuous stream.

Datagrams (as modeled by the DatagramChannel class) are independent chunks.  No datagram has any relationship to any other.  The metaphor that best applies is letters in a mailbox. Each has a data payload but there is no notion of order or relationship.  Datagrams could be sent in the order A, B, C
but arrive at the destination in the order B, C, A.

So, the answer to your question is "no".  It's one read per datagram and they will never mix together.  One gotcha to think about is that if the ByteBuffer you're reading into is not big enough to hold the entire datagram, the excess data will be silently discarded.

... which is what I wanted to hear.  

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[gregorypierce]

If that's the case I've definitely got a bug to report in NIO for JDK1.4.1_02 as on occasion I've seen several messages come in at the same time all concatenated together using TCP or UDP. Caused me MUCH headache.

[leknor]

TCP should be coalesed.

How'd you verify that UDP is being coalesed? Are you sure it's the JVM or could it be a buggy network driver somewhere. If you need to do some packet sniffing try: http://ethereal.com/ , that's what I use for both linux and windows.