Roundup Tracker - Issues


Author rouilj
Recipients marcus.priesch, rouilj
Date 2021-11-30.18:25:36
Message-id <>
In-reply-to <>
Hi Marcus:

In message <>,
Marcus Priesch writes:
>I am currently developing a web-frontend which uses the REST API
>(via vuex-rest-api) 


>Currently the ETag Header is only sent when i GET a resource.

Yup this is by design as I didn't find any RFC that discussed ETag
generation for anything but GET and HEAD.

>Now, when the user changes parts of the ressource and i update it using
>a PUT request (encapsulated in a POST) i get back the changed parts,
>but no ETag Header.

Correct. The ETag header maps the URL to the full representation of
the URL not a partial representation. ICU PUT responses are not
supposed to be cached by intermediate caches or the browser but ...

>The same happens when the user creates a new ressource with a POST
>request i get back the id and URL, but also no ETag Header.

Same answer as above.

>Therefore i always need to issue an additional GET request on the same
>ressource afterwards to have the ETag information for further editing
>(PUT'ing) the ressource.

That appears to be the case in REST. (IIRC you can issue a HEAD and
get back the ETag without having to transfer the bytes in the
representation. It is the same amount of work as a GET for the server
but does save transmission costs.)

>Which is 1) ressource consuming and 2) not convenient ;)


>So the Question is: 
>is the current behaviour by design ? 

I am going to say yes.

>or can we change it so that the Etag Header gets sent automatically
>when issuing PUT, POST or PATCH.

would seem to indicate not as the response doesn't include a the full
representation like a GET.

While does
indicate that the resource should be returned on a put/patch it is
possible to have a partial representation which should not have an
ETag header. (We supply a partial representation.)

Since the partial representation is not a valid representation of the
whole (https://.../issue/1) URL, it must not be cached (using the etag
header) at the very least.

The PATCH request has the same partial representation problem IIRC.

For a POST we don't return any representation of the newly created
object, so an ETag header isn't allowed.

So adding the header would not appear to be correct. I wonder if
adding an @etag to the metadata part of the response would work? So for
POST you get:

    "data": {
        "link": "https://.../demo/rest/data/issue/2280",
        "id": "2280",
        "@etag": "xyzzy....rstq"

for PUT:

      "data": {
          "attribute": {
              "title": "This is now my title"
          "type": "issue",
          "link": "https://.../demo/rest/data/issue/23",
          "id": "23",
	  "@etag": "xyzzy....rstq"

and similarly for PATCH:

      "data": {
          "attribute": {
              "nosy": [
          "type": "issue",
          "link": "https://.../rest/data/issue/23",
          "id": "23",
          "@etag": "xyzzy....rstq"

where the @etag value is what you would get if you were to make a
GET/HEAD request. I am not sure how easy this is to implement though.

While I was looking into this, it turns out that the ETag must be
different for different content-encodings.  We do
explicitly set the vary header to include accept-encoding, but
apparently, that is not enough.

I am not sure how that affects the use of ETag and intermediate
caches. I suspect the easiest way to handle this is to take the ETag
and append the content-encoding. So

  ETag: 584f82231079e349031bbb853747df1c-gzip
  Content-Encoding: gzip

  ETag: 584f82231079e349031bbb853747df1c-br
  Content-Encoding: br

  ETag: 584f82231079e349031bbb853747df1c-zstd
  Content-Encoding: zstd

and then strip the suffix when comparing it server side. So as a
client you don't care about the ETag value, it still remains opaque.
But an intermediate cache will have three different copies that they
can return depending on what the accept-header is.
Date User Action Args
2021-11-30 18:25:37rouiljsetrecipients: + rouilj, marcus.priesch
2021-11-30 18:25:37rouiljlinkissue2551173 messages
2021-11-30 18:25:36rouiljcreate