Thank you everyone for the feedback. This is really helpful.
I updated the PR to use "??" instead of "?:" as the fallback separator. I
also added support for a custom literal fallback if the user doesn't want
the default "-". Thus, this for instance:
'%<{x-primary-id}cqh??{x-secondary-id}cqh??"missing-id">'
On Tue, Mar 31, 2026 at 11:37 AM Serris Santos <[email protected]> wrote:
> I like the idea. I think this adds a lot of flexibility that can be useful
> for logging.
>
> I agree that "?:" looks like a ternary operator. It also makes it seem like
> there is only one fallback but the code allows multiple. +1 on "??"
>
> And +1 on allowing hard coded fallbacks. Since this feature introduces
> fallbacks, I think it is a good opportunity to also allow users to
> customize our default fallback ("-")
>
> Best,
> Serris Santos
>
>
> On Mon, Mar 30, 2026 at 7:52 AM Chris McFarlen <[email protected]> wrote:
>
> > This seems good. I think `?:` looks too much like a ternary operator. I
> > recommend `??` as there is some precedence for that to mean "or this".
> >
> > Chris
> >
> >
> > Sent with Proton Mail secure email.
> >
> > On Tuesday, March 24th, 2026 at 9:48 AM, Brian Neradt <
> > [email protected]> wrote:
> >
> > > Hi [email protected],
> > >
> > > I wanted to invite feedback on a small logging feature I’m working on
> for
> > > ATS by request from our ops team. ATS currently supports transactional
> > > logging of specific header field values with expressions such as:
> > >
> > > %<{x-primary-id}cqh>
> > >
> > > `cqh` is the ATS log formatting field that prints the value of a
> > particular
> > > client request header field, in this case x-primary-id. Today, if that
> > > header is not present, ATS logs the normal missing value, `-`. The
> > proposed
> > > feature allows a fallback chain so users can log an alternate header
> when
> > > the primary one is absent.
> > >
> > > Example:
> > >
> > > %<{x-primary-id}cqh?:{x-secondary-id}cqh>
> > >
> > > Note the `?:`, the proposed operator which signals the chain. This is
> > > evaluated left to right:
> > >
> > >
> > > - If x-primary-id exists, log that
> > > - Otherwise if x-secondary-id exists, log that
> > > - Otherwise log -
> > >
> > >
> > > A few notes about the current behavior:
> > >
> > > - This alternate chain feature is limited to header field log
> formats
> > > for now (cqh, pqh, ssh, etc.).
> > > - The chain can contain an arbitrary set of alternate field names
> > (i.e.,
> > > not just one alternate, but two, three, etc.).
> > > - An existing header field with an empty value is treated as
> present.
> > >
> > >
> > > Slices remain per-candidate, so this is valid:
> > >
> > > %<{x-primary-id}cqh[0:8]?:{x-secondary-id}cqh[0:16]>
> > >
> > > The goal is to make log configuration more flexible for deployments
> that
> > > need to accommodate multiple header conventions for the same type of
> data
> > > without duplicating formatting fields and requiring post-processing of
> > > transaction logs.
> > >
> > > For reference, here is a PR that implements this feature:
> > >
> > > https://github.com/apache/trafficserver/pull/13018
> > >
> > > Please let me know if you have feedback or concerns. If you have ideas
> > > other than the `?:` operator, for instance, I'm definitely open to
> trying
> > > something else.
> > >
> > > Thanks!
> > > Brian Neradt
> > >
> > > --
> > > "Come to Me, all who are weary and heavy-laden, and I will
> > > give you rest. Take My yoke upon you and learn from Me, for
> > > I am gentle and humble in heart, and you will find rest for
> > > your souls. For My yoke is easy and My burden is light."
> > >
> > > ~ Matthew 11:28-30
> > >
> >
>
--
"Come to Me, all who are weary and heavy-laden, and I will
give you rest. Take My yoke upon you and learn from Me, for
I am gentle and humble in heart, and you will find rest for
your souls. For My yoke is easy and My burden is light."
~ Matthew 11:28-30
