Add support for Redis unix sockets

[brennentsmith]

Adds support for unix sockets on connecting to redis. Can result in a 50% increase in performance (https://redis.io/topics/benchmarks) and provides a touch of local ACL’s (permissions on the local file descriptor).

Added this PR to keep feature parity between redis-input and output. logstash-plugins/logstash-input-redis#64

[brennentsmith]

Just signed the CLA. Rebuild

[jsvd]

I believe this PR should follow the same strategy as the input, where either you select path or hosts. I don't see much of a reason to load balance between hosts and a unix socket. Wdyt?

[brennentsmith]

What if someone wants to run multiple redis instances on their local VM, and use different unix sockets to connect to them? (similar to the in documentation example of 127.0.0.1:6379 + 127.0.0.2:6379)

I agree that it's not exactly a common scenario, but I'd argue to leave it up to the person configuring logstash to select the options they want, rather than pull functionality.

[jstoja]

To be honest, I was about to totally disagree with this, but actually many other softwares (most common being databases and webservers) allow you listen on both. So I'd say why not.

The issue there is with this is that the unix socket is not an host, and therefore should in my opinion be used from the path parameter. Having both IPs and paths seem wrong in the same parameter seem wrong to me.

[process0]

Are there plans to merge this? Having a unix socket for output is reasonable expectation, seeing that the corresponding input plugin supports it.