simple(~500 lines) socket<->socket relay that listens on
unix/udp and enqueues packets into one or more workers which
send them to their destination. If the remote socket is tcp
then it uses TCP_CORK on one streaming tcp connection to send
its data.

install:
    $ git clone https://github.com/jackdoe/relay
    $ cd relay
    $ make
    $ # and then run ./bin/relay 
usage:
relay local_socket fallback_socket worker_socket ....

for exmple:
relay /tmp/relay.sock udp@events-fallback.example.com:112233 \
    tcp@events-0.example.com:112233 tcp@events-1.example.com:112233

listener socket can be only DGRAM udp/unix.

packet \         / worker -> listener-1
packet  |- relay - worker -> listener-2
packet /         \ worker -> listener-3


main_thread
    -> foreach listener
        -> start one worker thread
    -> listen for udp sereal packet
        -> append sereal packet to queue for each worker

worker
    main_loop:
    -> if queue is non-empty
        -> lock queue
           hijack queue (move items from queue to local storage)
           unlock queue
        -> open connection to listener
        -> while items in hijacked queue
            -> try to send top item to listener
                -> if it fails: write remaining items to disk
                   goto main_loop
                -> if it is ok: pop item from hijacked queue.
        -> close connection

                 
testing:
    $ cd relay/test
    $ ./setup.sh
    # after testing is done you can see the results
    $ ./stat.sh  | sort -n -t'-' -k 2

legend:
    po = polling
    sp = spinlock
    ga = garbage reuse
    ad = array data (data[65535] instead of *data)

    result-packet_size-relay-flags:packets_per_second ... second_best ...

config:
everything after first # is comment
example config file (lets say /etc/relay.conf):

    udp@localhost:12344 #local
    # tcp@localhost:12345 old fallback
    sock/fallback.sock #fallback
    tcp@relay-1:1111 
    tcp@relay-2:1111 
    tcp@relay-3:1111 
    tcp@relay-4:1111 

run app with: relay /etc/relay.conf

If you want to change something for example relay-4 with relay-5, you
have to edit the config file and just killall -1 relay, this will
close/reopen all connections (including fallback).