examples(rhai): Building surrogate cache keys

[abernix]

Let's get an example using Rhai script that allows building a surrogate response cache key from subgraph responses. e.g.:subgraph a returns cache-key: one-way and subgraph b returns cache-key: or-another, and the response key returned to the client is surrogate-cache-key: one-way,or-another

blocked by #661

Worth noting that CAN actually be done as a Rust plugin today, but we just want to make it more simple and allow it in Rhai.

[Geal]

right now we do not store response headers anywhere and have not thought about how and where they could be stored, so I propose we put off this example until we solve that

[Geal]

this should be doable now that rhai plugins have access to subgraph response headers

[kroehre]

Looking forward to this example, I'm not seeing subgraph response headers in any of the rhai plugin response objects

[garypen]

Most responses expose response.headers. This will contain the response headers which will eventually be propagated back to the client. The Subgraph response.headers can be read to examine headers returned by a subgraph.

I'm going to guess that the existence of request.sub_headers on the SubgraphRequest is the source of confusion...

In a Request, headers are the headers which were originally supplied by the client (possibly modified during the progression of a plugin pipeline). For SubgraphRequest we also need a way to indicate the headers that we are going to send to downstream subgraphs, which is what request.sub_headers represents on the SubgraphRequest. Is that helpful?

[kroehre]

Hey thanks for the reply! For some reason response.headers is null when I print them from the subgraph_service hook callback:

fn subgraph_service(service, subgraph) {
  const response_callback = Fn("process_response");
  service.map_response(response_callback);
}

fn process_response(response) {
  print(`${response.headers}`);
}

In our case the goal is to propagate a few specific headers from subgraph responses.

[garypen]

Three options:

• the subgraph isn't returning any headers
• the headers aren't being propagated from the router subgraph handling
• the rhai language binding has a bug which means it isn't picking up the headers properly.

I'll work through them and see if I can figure out what is happening.

[garypen]

And the winner is:

• the headers aren't being propagated from the router subgraph handling

I've raised #1094 and will look into this.

[garypen]

@kroehre I've just merged the fix for #1094. This is now available on main and will be in the next release of the router.

[kroehre]

@garypen nice work! thank you