FluentD How to ignore pattern not match log not to forward to endpoint

We have a requirement where we need to forward only specific string logs to kibana endpoint/console. Currently we are getting pattern not match line where the matched string not found. How to ignore those logs not to send to forwarder and only send match logs.

<source>
  @type tail
  path session.txt
  pos_file session.txt.pos
  tag sessionlog
  <parse>
    @type regexp
    expression ^\<#\>\s+(?<time>\w+/\w+/\w+\s+[:0-9]+)\s+(?<hostname>[-0-9A-Z]+)\s+(?<message>.*Clip.*)$/
  </parse>
</source>

<match sessionlog>
  @type stdout
</match>

<#> 2019/11/16 13:56:33 ABC-Hostanme 278424 Dispatcher_1 Msg [Unit1] error emitted: '404'from session start: 2021-11-16T08:54:01
<#> 2019/11/16 13:56:33 ABC-Hostanme 278424 Dispatcher_1 Msg [Unit1] clip result: a1=0, a2=217, a3=152475, a4=148692

Result:

[warn]: #0 pattern not match: <#> 2019/11/16 13:56:33 ABC-Hostanme 278424 Dispatcher_1 Msg [Unit1] error emitted: '404'from session start: 2021-11-16T08:54:01
sessionlog: {"hostname":"DESKTOP-3JOOBVV","message":"278424 Dispatcher_1 Msg [Unit1] clip result: a1=0, a2=217, a3=152475, a4=148692"}

We want to get only matched pattern logs.

Solution

@sunshine, If the regexp parser cannot extract a match from the log, it will emit that error. So, its recommended that all log lines passing through the regexp parser can be matched by the expression. I recommend you use the grep filter before the regexp parser to avoid those "pattern not match" logs from fluentd.

I've pasted an example below but you can also use <exclude> blocks in the grep filter. See here for more info and examples: https://docs.fluentd.org/filter/grep

<source>
  @type tail
   path session.txt
   pos_file session.txt.pos
   tag sessionlog
</source>

<filter sessionlog>
  @type grep
  <regexp>
    key message
    pattern /INCLUDE_PATTERN_HERE/
  </regexp>
</filter>

<filter sessionlog>
  @type parser
  key_name message
  reserve_data true
  <parse>
    @type regexp
    expression ^\<#\>\s+(?<time>\w+/\w+/\w+\s+[:0-9]+)\s+(?<hostname>[-0-9A-Z]+)\s+(?<message>.*Clip.*)$/
  </parse>
</filter>

<match sessionlog>
  @type stdout
</match>