Unable to follow links from POST-only resources

I’ve got a situation where I’ve got a resource that doesn’t really exist, but that I still want to follow links from.

I’m building a login/register flow, where the client will:

• Perform a POST to /authentication with the username.
• Receive a response containing one of two links - identified as either rels/authentication/authenticate or rels/authentication/register - depending on whether the username exists.
• Perform a POST to the link that was returned with the appropriate details.

So what I’ve done is:

• Create a new Ketting Client.)
• Perform a GET to / to get the home document
• Follow the link with the rel rels/authentication and perform a POST to this with the username - this is /authentication
• Follow the link with the rel rels/authentication/authenticate and perform a `POST to this with the username and password.

The result is that Ketting is then trying to do a GET to /authentication, which returns an error because the server can’t handle that, and then it errors out.

The code is effectively like this: (Restructured a little bit for brevity)

        const startAuthResource = await apiRoot.follow("rels/authentication");
        const response = await startAuthResource.post({ data: { username } });
        const authResource = await startAuthResource.follow("rels/authentication/authenticate"); // <-- Problem occurs here

[evert]

Yeahh… this is an issue.

The problem is that, generally, when following links, we follow links from a resource to another resource. When we get a POST response, the body of the response is not necessarily the representation of that startAuthResource resource.

There’s two paths to make better, and I also have a workaround:

Approach 1: The response to the POST response should be considered to be the representation of that resource. The obvious header for this is Content-Location:

https://tools.ietf.org/html/rfc7231#section-3.1.4.2

If this makes the most sense for your case, we could add this feature to Ketting.

–

Approach 2: Follow links from State objects.

I had the idea to add a .follow() function to State objects. This would allow you to follow links from any body, not just things that are ‘resources’:

 const startAuthResource = await apiRoot.follow("rels/authentication");
 const response = await startAuthResource.post({ data: { username } });
 const authResource = response.follow("rels/authentication/authenticate"); // <-- We're following from 'response'

Workaround:

const startAuthResource = await apiRoot.follow("rels/authentication");
const response = await startAuthResource.post({ data: { username } });
const authResource = startAuthResource.go(response.links.get("rels/authentication/authenticate").href);

Sidenote is that it would also be nice if you could pass a Link type to .go() so that last snippet would look like:

const startAuthResource = await apiRoot.follow("rels/authentication");
const response = await startAuthResource.post({ data: { username } });
const authResource = startAuthResource.go(response.links.get("rels/authentication/authenticate"));

[evert]

Open a few more tickets for this. I think we’ll probably have a version 7 this month. Thanks for the report!