NLog：如果填充字段，则记录到单个文件

Question

I have a set of Windows Services - 7 in all. These services track different aspects of different workflows.

In them, I set a field called scanid:

public static void SetScanid(this ILogger logger, int? scanid)
{
    GlobalDiagnosticsContext.Set("scanid", scanid);
}

in the nlog.config file, I use it to populate a portion of the logs for quick find/seek:

<variable name="messageLayout" value="[${longdate}] -- [SID:${gdc:item=scanid}] ... " />

At the start of any given process, I set the SCANID and at the end I clear it.

Now, in 7 different log files, I have the SID as a searchable field.

Could I instead: create a target that logs to a given file ONLY when that field is populated?

Say, something like

<target xsi:type ="File"
    name="sidTarget"
    fileName="Logs\Process-${gdc:item=scanid}.log"
    when=" ??? scanid is not null or empty ??? "
    ... />

Or is there something in the rules to cover that?

Answer 1

You could configure that in the <rules> section.

eg

<logger name="*" writeTo="sidTarget">
  <filters>
    <when condition="length('${gdc:item=scanid}') == 0" action="Ignore" />
  </filters>
</logger> 

See filtering log messages and <when> docs

Answer 2

Alternative solution could be to use the NLog FallbackGroup target. It allows you to group multiple targets so they become a single target. If the first target in the group fails, then it will retry on the next target in the group (Using a different filename).

See also https://github.com/nlog/NLog/wiki/FallbackGroup-target