Can you provide an example of using conditional pattern matching in Grok

Example: Conditional Grok Patterns

Suppose you have logs of two types: `apache_access` and `apache_error`. You want to apply different Grok patterns based on these types.

logstash
filter {
  if [type] == "apache_access" {
    grok {
      patterns_dir => "/path/to/patterns"
      match => { "message" => "%{COMBINEDAPACHELOG}" }
    }
  } else if [type] == "apache_error" {
    if "ModSecurity" in [message] {
      grok {
        patterns_dir => "/path/to/patterns"
        match => { "message" => "%{HTTPERRORDATE:date} \[%{HTTPMODULE:module}:%{LOGLEVEL:loglevel}\] \[pid %{POSINT:pid}\] \[client %{IP:IP}\] ModSecurity: %{GREEDYDATA:message}" }
      }
    } else {
      grok {
        patterns_dir => "/path/to/patterns"
        match => { "message" => "%{HTTPERRORDATE:date} \[%{HTTPMODULE:module}:%{LOGLEVEL:loglevel}\] \[pid %{POSINT:pid}\] %{WORD:errorid}: %{GREEDYDATA:message}" }
      }
    }
  }
}

In this example:
- For logs of type `apache_access`, a specific Grok pattern (`COMBINEDAPACHELOG`) is applied.
- For logs of type `apache_error`, the presence of "ModSecurity" in the message determines which Grok pattern to use.

Handling Non-Matching Patterns

If a Grok pattern fails to match, Logstash adds a `_grokparsefailure` tag. You can handle this by using conditionals to apply alternative actions:

logstash
filter {
  grok {
    match => { "message" => "%{COMMONAPACHELOG}" }
  }
  
  if "_grokparsefailure" in [tags] {
    # Try an alternative pattern or process the event differently
    grok {
      match => { "message" => "%{SYSLOGBASE}" }
    }
    mutate {
      remove_tag => ["_grokparsefailure"]
    }
  }
}

In this case, if the first Grok pattern fails, Logstash attempts to match another pattern (`SYSLOGBASE`) and removes the `_grokparsefailure` tag if successful.