[jsr369-experts] Re: [servlet-spec users] Re: Trailer header implementation

From: Martin Mulholland <mmulholl_at_us.ibm.com>
Date: Wed, 3 May 2017 17:04:04 -0400

Thank you,
Martin Mulholland.
WebSphere Application Server Web Tier Architect
email: mmulholl_at_us.ibm.com

IBM RTP, PO BOX 12195, 503/C227,
3039 Cornwallis Rd, RTP, NC 27709-2195
 t/l 444-4319, external (919)-254-4319

Greg Wilkins <gregw_at_webtide.com> wrote on 05/03/2017 08:54:07 AM:

> From: Greg Wilkins <gregw_at_webtide.com>
> To: jsr369-experts_at_servlet-spec.java.net
> Date: 05/03/2017 08:54 AM
> Subject: [servlet-spec users] [jsr369-experts] Re: Trailer header
> implementation
>> On 3 May 2017 at 13:02, Mark Thomas <markt_at_apache.org> wrote:
>> Hi,
>> I've been working on trailer header implementation. No particular
>> with requests but the response implementation is identifying a lot of
>> questions.
>> The first one is triggered by Section 4.4 for RFC 7230:
>> <quote
>> When a message includes a message body encoded with the chunked
>> transfer coding and the sender desires to send metadata in the form
>> of trailer fields at the end of the message, the sender SHOULD
>> generate a Trailer header field before the message body to indicate
>> which fields will be present in the trailers. This allows the
>> recipient to prepare for receipt of that metadata before it starts
>> processing the body, which is useful if the message is being
>> and the recipient wishes to confirm an integrity check on the fly.
>> </quote>
>> 1. How is the container meant to construct the Trailer header field?
>> point of using a supplier was so the header names and values did not
>> have to be known when the response was committed. However, section 4.4
>> requires that the names are known.

> Firstly, it's only a SHOULD, so I'm guessing it wont be well
> respected. For example are we going to reject trailers not listed
> in a prior Trailers header?
> But then I guess at the time the supplier is set, the app knows it
> is adding a supplier with certain capabilities, so it should know
> what trailers may be set.

I think we should enforce the "SHOULD" at the servlet level so consider it
a must.

Also what if the application sets the trailer header using the existing
setHeader() api and then subsequently uses the new api to set the promised
trailer headers. Should we detect such a setting of the trailer header and

then ignore headers set using setTrailers if they were not indicated by
the trailer header.

>> The remaining questions are a little simpler:
>> 2. What happens if setTrailerFields() is called after the response is
>> committed?
> Good question. We can go either: If when it is set, the response
> is committed with a transport mode that cannot support trailers then
> an illegalStateException can be thrown; alternately we can just
> never call the supplier to get the trailers.
> Jetty currently does the later, but I can see reasons for the former.

It is just the trailer header which must be sent before the response is
committed so in theory we could accept this after commit if the
trailer header was providedand sent before commit. This complicates things

but does not seem a unreasonable expectation from an app.

>> 3. What happens if setTrailerFields() is called and trailer fields are
>> not supported (HTTP 0.9, HTTP 1.0, some proxy protocols, etc.)?
> Same as above.

Difficulty is if the trailer fields include some valid headers and some
invalid headers so I vote for just ignoring the invalid ones.

>> 4. What happens if setTrailerFields() is called multiple times?
> In Jetty, each call replaces the previous. So you can null out the
> supplier or give a new one.
> However, I can see that there might be competing concerns that want
> to add multiple trailers. Perhaps we need addTrailerFields and
> support multiple suppliers (a bit over the top), or perhaps just ISE
> if it has already been set?

I would go with replacing the previous, also if we replace a previous this
could definitely be done after commit, just to replace previous values.

>> 5. What happens if setTrailerFields() is called with a null Supplier?

> Jetty currently nulls out the supplier.


>> My current thinking:
>> 1) Do we ignore the 'SHOULD' or do we change the API? I'm leaning
>> towards changing the API on the basis that if we are going to add
>> trailer support we should do it as correctly as possible.
> +0 as it feels a bit over complicated. i guess we could do
> setTrailer(String name, Supplier<String>)
> To set a supplier for each field and then we'd know how to set the
> Trailers header.
> But to be symmetric, would we'd need to police the Trailers header
> on incoming requests? symmetry also pushes us towards
> String getTrailer(String name)
> rather than the map.

As I said before I think we enforce the SHOULD. However I don't see
a need to be symmetric, we have no control over what clients send in
and we do the best we can with what we get. If the app really wants
the header field it can ignore any trailer headers but we should still
be willing to provide them.

>> 2) Throw an IllegalStateException
>> But. It also depends on 1). If we don't have to set the Trailers
>> header it may still be possible to send the trailer fields
> +1
>> 3) Throw an IllegalStateException
> +1
>> 4) Use the most recent Supplier
> +1
>> 5) Allow it and protect against any possible NPE in the container
> +1
>> Of these, 1) is the most serious to address. After that, 2) & 3) since
>> the application has no way to tell if this is going to happen. Do we
>> need a boolean areTrailerFieldsSupported() method?
>> Mark

> --
> Greg Wilkins <gregw@webtide.com> CTO http://webtide.com