Resolver Architecture¶

flowchart LR
  %% ── USER & ENTRY ─────────────────────
  User(("Client")) --> RP["Proxy<br/>:8012"] --> Main["Main<br/>cmd/main.go"] --> IS["Metrics<br/>:8013"]

  %% ── CORE MODULES ─────────────────────
  subgraph Mods["Core Modules"]
    Handler["Handler"]:::core
    Hosts["Hosts"]:::core
    Thr["Throttle"]:::core
    Oper["Operator Comm"]:::core
    Obs["Observability"]:::core
  end
  Main -- uses --> Handler & Hosts & Thr & Oper & Obs

  %% Request flow (compact arrows)
  Handler --> Hosts
  Handler --> Thr
  Thr --> Handler
  Handler --> Obs
  Handler -.-> Sentry["Sentry"]

  %% Operator comm
  Handler -.-> Oper
  Oper -.-> OpSvc["Operator Svc"]

  %% External deps
  Thr -.-> K8sAPI["K8s API"]
  Obs -.-> Prom["Prometheus"]

Request Routing¶

The resolver identifies the target service by reading a single header on each incoming request. Two things decide which header is read:

• By default, the resolver binary reads the standard HTTP Host header (resolver/cmd/main.go:51, HeaderForHost string ... default:"Host").
• The shipped Helm chart overrides that default to X-Envoy-Decorator-Operation (charts/elasti/values.yaml, elastiResolver.proxy.env.headerForHost) so that deployments fronted by Envoy, Istio, or similar proxies that rewrite the Host header keep working.

Whichever header is selected, its value is parsed into namespace and service by GetHost in hostmanager.go and used to look up the target service to forward the request to.

This has a practical consequence. Whatever routes traffic to the resolver, whether an Ingress controller, a service mesh, a Service selector, or a direct client, must arrive at the resolver with a header value that matches the target service's in-cluster FQDN format (for example, target-deployment.target.svc.cluster.local). If the header does not match, the resolver cannot tell which service the request was meant for and will not forward the request.

The ingress example in Demo setup shows one way to satisfy this: the NGINX annotation nginx.ingress.kubernetes.io/upstream-vhost rewrites the Host header to the service FQDN before the request reaches the resolver.

To use a different header (for example, a custom one set by your edge proxy), override the Helm value at install time:

--set elastiResolver.proxy.env.headerForHost=X-My-Custom-Host