Groking and then mutating? - logstash

Question

I am running the following filter in a logstash config file:
filter {
if [type] == "logstash" {
grok {
match => {
"message" => [
"\[%{DATA:timestamp}\]\[%{DATA:severity}\]\[%{DATA:instance}\]%{DATA:mymessage}, reason:%{GREEDYDATA:reason}",
"\[%{DATA:timestamp}\]\[%{DATA:severity}\]\[%{DATA:instance}\]%{GREEDYDATA:mymessage}"
]
}
}
}
}
It kind of works:
it does identify and carve out variables "timestamp", "severity", "instance", "mymessage", and "reason"
Really what I wanted was to have text which is now %{mymessage} to be the ${message} but when I add any sort of mutate command to this grok it stops working (btw, should there be a log that tells me what is breaking? I didn't see it... ironic for a logging solution to not have verbose logging).
Here's what I tried:
filter {
if [type] == "logstash" {
grok {
match => {
"message" => [
"\[%{DATA:timestamp}\]\[%{DATA:severity}\]\[%{DATA:instance}\]%{DATA:mymessage}, reason:%{GREEDYDATA:reason}",
"\[%{DATA:timestamp}\]\[%{DATA:severity}\]\[%{DATA:instance}\]%{GREEDYDATA:mymessage}"
]
}
mutate => {
replace => [ "message", "%{mymessage}"]
remove => [ "mymessage" ]
}
}
}
}
So in summary I'd like to understand:
Are there log files I can look at to see why/where a failure is happening?
Why would my mutate commands illustated above not work?
I also thought that if I never used the mymessage variable but instead just referred to message as the variable that maybe it would automatically truncate message to just the matched pattern but that appeared to append the results instead ... what is the correct behaviour?

Answer 1

Using the overwrite option is the best solution, but I thought I'd address a couple of your questions directly anyway.
It depends on how Logstash is started. Normally you'd run it via an init script that passes the -l or --log option. /var/log/logstash would be typical.
mutate is a filter of its own, not a part of grok. You could've done like this (or used rename instead of replace + remove):
grok {
...
}
mutate {
replace => [ "message", "%{mymessage}" ]
remove => [ "mymessage" ]
}

Answer 2

I'd do it a different way. For what you're trying to do, the overwrite option might be more apt.
Something like this:
grok {
overwrite => "message"
match => [
"message" => [
"\[%{DATA:timestamp}\]\[%{DATA:severity}\]\[%{DATA:instance}\]%{DATA:message}, reason:%{GREEDYDATA:reason}",
"\[%{DATA:timestamp}\]\[%{DATA:severity}\]\[%{DATA:instance}\]%{GREEDYDATA:message}"
]
]
}
This'll replace 'message' with the 'grokked' bit.
I know that doesn't directly answer your question - about all I can say is when you start logstash, it writes to STDOUT - at least on the version I'm using - which I'm capturing and writing to a file. In here, it reports some of the errors.
There's a -l option to logstash that lets you specify a log file to use - this will usually show you what's going on in the parser, but bear in mind that if something doesn't match a rule, it won't necessarily tell you why it didn't.