Conditional rate limiting. about skipper HOT 1 OPEN

I was looking at Common Expression Language (CEL) recently also used by Kubernetes for various rules.

We may think about a new filter that would set header value based on the result of CEL expression and then use it in the clusterClientRatelimit (which applies rate limit when header is not empty) like so:

*
-> customFilterSettingHeaderXHeaderInCaseOfConditions()

-> setRequestHeaderExpr("Ratelimit-A", `request.Header["X-Header"] != "" ? request.Header["X-AnotherHeader"] : null`)
-> clusterClientRatelimit("A", 30, "10s", "Ratelimit-A")

-> setRequestHeaderExpr("Ratelimit-B", `request.Header["X-Header"] == "" ? request.Header["X-AnotherHeader"] + request.Header["X-True-Ip"] : null`)
-> clusterClientRatelimit("B", 10, "100s", "Ratelimit-B")

-> "http://192.3.123.1:909"

For brevity we may put Header (and other request fields) directly into CEL environment (i.e. omit request. prefix) and use shorter expressions like Header["X-Header"] && URL.Path == "/foo" && URL.Query.Has("q") ? "/foo" + Header["X-AnotherHeader"] : null

I also investigated whether it is possible to wire stdlib http.Request and other types into CEL and it seems possible, see google/cel-go#875

from skipper.