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From joshua portway <joshli...@stain.org>
Subject [mina] controlling packets
Date Mon, 23 Jan 2006 12:04:39 GMT
Hi,
	I have a protocol which communicates via UDP with a hardware device  
which requires each message to bit split into separate network  
packets, each of which is preceded by a header which contains  
information about which message the packet is part of, what packet  
number it is, the total length of data in the message, and the  
position of the data in this packet in the message etc.

	I'm not sure of the best way to implement this in Mina. I've  
implemented a custom ProtocolCodecFactory, loosely based on the Mina  
DemuxingProtocolCodecFactory, along with a MessageEncoder base class.  
Specific MessageEncoder subclasses write a series of ByteBuffers  
which make up th econtent of the message to a method in the baseclass  
and then call a "flush" method which divides the data up into  
packets, prepends the message header to the packet and sends it to  
the ProtocolEncoderOutput.

	The problem I'm getting is that if the message I'm sending has data  
contained in a big ByteBuffer (which most of my messages do), then I  
need to divide the buffer into specific packet sized chunks to send.  
Since Mina ByteBuffers don't have a "split" method (presumably  
because they get re-used) I have been setting the position and limit  
of the input buffer to point to the chunk I want to send, and then  
passing it off to the ProtocolEncoderOutput. The problem is that  
because the ProtocolEncoderOutput runs on another thread, I can't  
just change the buffer position and limit for the next chunk and   
send it because I have to wait for the ProtocolEncoderOutput to  
finish before I change the ByteBuffer positions. The only other  
alternative would be to copy each packet into a new buffer which  
seems quite wasteful. Latency and throughput are VERY important to my  
app, because it's encoding video streams and pushing them to the  
network in real time, so I'd rather not be doing a lot of copying of  
buffers.

	My current home-made network layer doesn't copy ByteBuffers, but  
creates view buffers onto the source buffer using ByteBuffer.split,  
and passes the header for a packet and the view buffer for the data  
to the channels gathering write(ByteBuffer[]) method. Am I doing  
something wrong in the way I'm using Mina ? Is there a way to do this  
without copying buffers ?

	Ideally, I think there should be a "packetizing" API in Mina to  
allow easy control of how a message is packetized, and to allow each  
packet to be wrapped in a protocol-specific packet header. As far as  
I can tell no such thing exists, and the closes I've found is the  
"mergeAll" method of ProtocolEncoderOutput, but again- this seems to  
be implemented by copying all the input ByteBuffers into a single  
output ByteBuffer, which can't be the most optimal way of doing it,  
surely ? Or am I being naiive ? does the GatheringByteChannel.write 
(ByteBuffer[]) method just copy all it's input buffers into a single  
output buffer anyway ?

	thanks for any help, sorry about the long length of the email :)

	josh

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