How to host a high performing Netty App in a fluctuating network environment?

[ljohnston]

Hi George

You can't tell directly if a network is congested or not. As Mike has noted
TCP uses various algorithms to decide if a network is congested or not but
they basically come down to whether a series of packets sent at a given time
are acknowledged within a certain time window. If they are then there's no
congestion, if they're not then there's congestion.

You don't state whether you are streaming over TCP or UDP. It's important
because with TCP it is possible to infer whether there is congestion. With
UDP it depends on whether your protocol has some form of congestion
avoidance. If it doesn't then you'll send UDP packets onto the network at
the top speed the server can manage only for them to be dropped at the first
congested router. Without some form of congestion avoidance you can't detect
this unless the client tells you it's not receiving anything.

Assuming you are using TCP, and that you are using NIO sockets, then there
is a way you can potentially detect congestion on a single connection. NIO
sockets have a writeBufferhighWaterMark and a writeBufferLowWaterMark.
Normally Netty forwards your data to the network stack send buffer
immediately. If Netty is unable to do so, because the send buffer is full,
then these two parameters specify how much data Netty will queue before
setting Channel.isWritable to false.

Once the network stack send buffer drains, and Netty is able to reduce it's
queue size below writeBufferLowWaterMark, Netty will raise an event stating
that the channel is again writable.

By checking if the channel is writable after is call to Channel.write, and
capturing the channelInterestChanged event, you can probably infer whether a
channel is managing to process your traffic in a timely manner. If it isn't,
and you see the same behaviour on other channels, then you can probably
infer network congestion.

You can tweak the network stack send buffer and Netty
writeBufferHighWaterMark sizes to find the levels which allow you to detect
congestion in reasonable time.

One final thought, I assume the server you will redirect the client to is on
a completely different network which doesn't share common links with the
congested network. Otherwise the redirect might not help.

Cheers
Lee

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