If you have multiple applications running on a box with the logstash client, you probably want to split the config file up a bit. According to this thread, you can use a directory as the value for the config input:
/opt/logstash/bin/logstash agent --config /etc/logstash.d --log /var/log/logstash/logstash.log
and the files in that directory will be conflated in lexicographical order.
This means you need some sort of scheme to keep your inputs before your outputs, etc. I’m not too bothered about the order of the individual inputs, so I prefixed each category with a different number:
$ ls -la /etc/logstash.d total 32 drwxr-xr-x 2 logstash logstash 4096 Apr 14 13:42 . drwxr-xr-x 98 root root 4096 Apr 16 01:28 .. -rwxr-xr-x 1 logstash logstash 140 Jan 8 13:34 1_foo_inputs.conf -rwxr-xr-x 1 logstash logstash 122 Jan 8 12:55 1_bar_inputs.conf -rwxr-xr-x 1 logstash logstash 99 Jan 8 12:06 1_syslog_inputs.conf -rwxr-xr-x 1 logstash logstash 237 Jan 8 13:34 2_foo_filters.conf -rwxr-xr-x 1 logstash logstash 229 Jan 8 12:55 2_bar_filters.conf -rwxr-xr-x 1 logstash logstash 170 Apr 14 13:42 3_output.conf
ensuring that inputs will be before filters, and both will be before outputs.
This means that the logstash config for each app can be deployed with it, without needing to amend a shared file.