26 Mar
2026
26 Mar
'26
2:14 p.m.
Hi Florian,
Since we're taking a tangent into namespace versioning, I've changed
the subject to not distract from the XEP-0377 feedback thread.
On Thu, 26 Mar 2026 at 13:53, Florian Schmaus <flo(a)geekplace.eu> wrote:
Hi Matthew,
On 26/03/2026 14.19, Matthew Wild wrote:
This is news to me. Is this documented anywhere? My understanding of
community consensus is that introduction of new elements and
attributes constitutes a breaking change. Maybe I read the consensus
wrong, or attitudes have shifted (was this ever documented beyond the
text I quoted in XEP-0053 which doesn't exactly define "breaking
change"?).
If I implement strict schema validation in my implementation, and then
the schema for the namespace changes, this is a "breaking change" for
me. Unless we document somewhere that XMPP implementations should be
forgiving of unknown elements/attributes (even in namespaces they
believe they understand)?
On Thu, 26 Mar 2026 at 13:02, Stephen Paul Weber
<singpolyma(a)singpolyma.net> wrote:
Handling unknown child elements in unknown namespaces is certainly a
requirement for XMPP implementations. But for a specific implemented
namespace defined in a XEP, it is absolutely valid for implementations
to reject elements which were not defined in the namespace's schema.
We even have error conditions explicitly defined for use in such
cases.
I am personally not one of the people who feel most strongly about
this, and there have been times in the past where I personally would
have preferred to sneak in some elements to established namespaces (if
the disruption would be lower than that caused by a namespace change).
However it is a position that we have taken as a community since the
namespace versioning was introduced, that generally schemas do not
change after publication (I can imagine exceptions for very young
unimplemented experimental XEPs). This is because some applications do
choose the strict validation option (sometimes this is for technical
reasons, because it ensures they can accurately translate XML to
custom data structures).
That many implementations will happily ignore extra elements just adds
to the problems around this particular change, given the nature of
these elements being user opt-ins (if any clients are sending them
today, they are 100% being ignored). It's actually a perfect example
of why namespace versioning was introduced.
I am sorry, but this is not how I perceive the situation.
I think we both agree that implementations are supposed to ignore
unknown elements. However, if I understood you correctly, you limit this
to unknown elements in an different namespace from the parent element.
I, on the other hand, believe this is also true for unknown elements in
the same namespace.
In general, we want it to be fine for recipients to ignore unknown
elements (and probably attributes). This allows us to evolve a protocol
as long as possible without namespace bumps.
Generally, adding previously undefined elements is considered to be a
breaking change that requires a new namespace, especially in a
protocol that is already deployed.
Adding new elements seems like the least breaking change? Unless they're
marked as manadatory, any implementation needs to be able to handle unknown
child elements in any position they don't expect anyway. Obviously, this only works if the new element (or
attribute) can be
safely ignored by the recipient without breaking functionality. For
example, hints that the recipient could process, but doesn't have to.
Noted. Though in the case of XEP-0377 which sparked this discussion,
it clearly does not fall into this category - the XEP introduced
various requirements determined by the presence or absence of the
added elements, which existing implementations of the XEP would
automatically violate.
I am not sure why we should artificially limit
ourselves here and create
an extra burden due to additional namespace bumps if we don't have to?
If everyone agrees that it's fine, I'm fine with that! As I noted in
my previous mail, I don't feel strongly about this and none of my
implementations generally reject unknown elements.
And that's me saying this. The person who also
argues for strict
verification because it eventually increases the robustness of the
overall ecosystem. For example, rejecting "foo" as an the value of an
attribute declared to be xs:int, as it is clearly a violation of the
specification.
I don't really understand how you can be in favour of strict and
relaxed processing at the same time. But I don't necessarily need to
understand :)
It seems maybe we just have different ideas of what "strict
verification" is. I think validating xs:int contents is just normal
expected behaviour, I wouldn't call that "strict".
Regards,
Matthew