dynamic routing with subset selectors

[bcollard]

The goal is to use dynamic values in subset selectors For example, a header may contain the label's key/value that will be used to route the request to a given pods subset

[jannes]

in terms of declaring this, a subsetHeaders field could be added to the subset field. e.g:

routes:
  - matchers:
    - headers:
      - name: subset-selector
      prefix: /example
    name: subset-route-dynamic
    routeAction:
      single:
        subset:
          subsetHeaders:
            - subset-selector
        upstream:
          name: example-upstream
          namespace: gloo-system

[chrisgaun]

We are looking into this to see if this is in the Envoy API

[jannes]

It seems to me that we can just use envoy's header_to_metadata filter to rewrite the specified header to a key/value pair in the envoy.lb namespace.

Apparently envoy will automatically select a subset without having to specify a metadata_match in the route, which is not very clear from the docs, but judging from https://github.com/envoyproxy/envoy/pull/3254 this is the case. Also: https://github.com/envoyproxy/envoy/blob/main/docs/root/configuration/http/http_filters/header_to_metadata_filter.rst.

So whereas for the subset->values config something like this is generated:

{
   "match": ....,
   "route": {
      "cluster": ....,
      "metadata_match": {
         "filter_metadata": {
            "envoy.lb": {
               "example-key": "example-value"
            }
         }
      }
   },
   ....
}

for the subset->subsetHeader we could generate instead:

{
   "match": ....,
   "route": {
      "cluster": ....
   },
   "typed_per_filter_config": [
      {
         "name": "envoy.filters.http.header_to_metadata",
         "config": {
            "request_rules": [
               {
                  "header": "example-header",
                  "on_header_present": {
                     "metadata_namespace": "envoy.lb",
                     "key": "example-key",
                     "type": "STRING"
                  },
                  "remove": false
               }
            ]
         }
      }
   ],
   ....
}

subset->values and subset->subsetHeader would need to be mutually exclusive

What do you think?

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