The cache-config relation

The cache-config interface connects the Ingress Configurator charm (requirer) to the Content Cache charm (provider). It configures the Content Cache charm with backend origins, healthcheck parameters, and caching rules so that HAProxy can route traffic through Content Cache.

Databag fields

The Ingress Configurator charm writes the following fields to its application databag:

Field	Type	Required	Description
backends	JSON array of strings	Yes	List of backend URLs (e.g. ["http://10.0.0.1:8080"]) that content-cache should proxy.
backend_hostname	string	No	Hostname content-cache presents (SNI and HTTP Host header) when connecting to the origin backend over HTTPS. Required when backend-protocol is https. Sourced from the cache-backend-hostname config; unrelated to and never derived from hostname. content-cache passes this to nginx as proxy_ssl_name and proxy_set_header Host.
fail_timeout	string	Yes	Time after which a backend is marked unavailable following a failure (e.g. "30s").
healthcheck_interval	string	Yes	Healthcheck interval in milliseconds (e.g. "10000" for 10 s).
healthcheck_path	string	Yes	URL path used for healthchecks (e.g. "/").
healthcheck_valid_status	JSON array of integers	Yes	HTTP status codes that indicate a healthy backend (e.g. [200]).
healthcheck_ssl_verify	JSON boolean	Yes	Whether nginx should verify the backend TLS certificate during healthchecks.
proxy_cache_valid	JSON array of strings	Yes	Cache validity rules in nginx format (e.g. ["200 1h"]). An empty array emits no proxy_cache_valid directives, so caching defers to the Cache-Control and Expires headers of the backend.

backend_hostname is sourced from the cache-backend-hostname charm config, which is normally left unset for HTTP backends – content-cache passes the incoming Host header through unchanged in that case. Setting cache-backend-hostname changes the Host header the backend receives on every request, so only set it when the backend application is aware it’s being fronted by a cache (e.g. its own hostname allowlist or CORS configuration accepts this value).

The Content Cache charm writes the following field to its unit databag:

Field	Type	Description
cache-backend	string	URL on which Content Cache is reachable (e.g. "https://10.1.0.5:30000"). Ingress Configurator uses this to redirect HAProxy traffic through Content Cache.