serrislew commented on code in PR #13018:
URL: https://github.com/apache/trafficserver/pull/13018#discussion_r3011284050
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doc/admin-guide/logging/formatting.en.rst:
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@@ -314,8 +314,27 @@ prior to the log field's name, as so::
Format = '%<{User-agent}cqh>'
The above would insert the User Agent string from the client request headers
-into your log entry (or a blank string if no such header was present, or it did
-not contain a value).
+into your log entry (or ``-`` if no such header was present).
+
+Header fields can also be chained with a fallback operator, `?:`, when you want
+the log to use the first header that exists. For example::
+
+ Format = '%<{x-primary-id}cqh?:{x-secondary-id}cqh>'
+
+|TS| evaluates the candidates from left to right and logs the first header that
+exists. If none of the headers exist, |TS| logs ``-``. A header that exists but
+has an empty value is considered present, so |TS| logs the empty value instead
+of falling back. So in the example above, the value of x-primary-id of the
+client request is logged if it exists, otherwise the value of x-secondary-id is
+logged if it exists, otherwise ``-`` is logged if neither of the headers is
+present.
+
+Note that this fallback chain feature is only supported for header field
Review Comment:
So `%<{x-primary-id}cqh?:Foo>` wouldn't be supported? I guess you can do a
workaround to make an internal header with value "Foo" and then use that
instead? `%<{x-primary-id}cqh?:{@Foo}>`
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