Add support for Unix domain sockets

[pgiblock]

It would be nice if this logger would support Unix Domain Sockets as the target instead of only (host, port) inet addresses. UnixDomainSocketAddress was added in Java 16. I had hoped InetSocketAddress, as accepted by FluencyBuilderForFluentd.build() could trivially be changed to SocketAddress as that the common base class, but Fluency is written to assume a TCP socket.

This feature would be particularly useful for container environments such as Docker because some not very nice tricks are required for a container to connect to a host socket. Note that Unix domain sockets are supported by Fluentd and Fluent-Bit.

[pgiblock]

For anyone else running into this problem, there is a workaround that I find cleaner than running with host networking or having to address the host by address/hostname, bind on the interface, and implement firewall rules. It requires running a little forwarder to "proxy" the data from docker-lived networking to the host-lived domain socket:

docker run -h fluent -v /var/run/fluent-bin.sock:/sock fluent/fluent-bit:1.9 \
    -i forward -o forward -p unix_path=/sock

You'll likely need to place this on the right network(s), but docker containers can now forward to the hostname "fluent" and records will appear on the actual host.

[talwgx]

I wanted to second @pgiblock 's request - I also needed a Unix Socket and implemented one quickly:

https://gist.github.com/talwgx/418aecd95358941ba9ea905f88f25fcd

But it will be much better ofc it this was baked into this really lovely library! @komamitsu

[komamitsu]

That looks a good workaround. The difficult point to take care of this issue is we need to build Fluency code with JDK 8 to create jar files to avoid https://www.morling.dev/blog/bytebuffer-and-the-dreaded-nosuchmethoderror/ and UnixDomainSocketAddress was introduced in JDK 16. Let me think for a while...