proxy-mirror

proxy-mirror

Sends a fire-and-forget clone of the incoming request to a shadow upstream. Useful for comparing a new backend against production, capturing traffic, or pre-warming a cache. The mirror is dispatched on a detached background task; its response and any error are ignored, and the request path is never blocked or affected by it. Place it before upstream so it observes the request as it will be proxied. It always continues through the success port and never errors.

Configuration

Key	Type	Default	Description
host	string	— (required)	Shadow base URL — scheme + host + optional port, e.g. http://shadow:8080. Must start with http:// or https://; no trailing path.
path	string	—	Overrides the mirrored request path; when unset the original request path is mirrored. The original query string is always appended.
sample_ratio	number	1.0	Fraction of requests to mirror, in 0.0..=1.0. 1.0 mirrors every request; 0.0 mirrors none.

type: proxy-mirror
config:
  host: "http://shadow:8080"
  path: "/mirror"
  sample_ratio: 0.5

Behavior

On each request the plugin decides whether to mirror per sample_ratio: >= 1.0 always mirrors and 0.0 never mirrors; otherwise a pseudo-random draw in [0.0, 1.0) is compared against the ratio. The draw uses a freshly seeded hasher (the same casual, non-cryptographic source the fault-injection plugin uses) — it is not a deterministic sequence and not suitable for security decisions.

When a request is sampled, the plugin builds a mirror request (method, headers, and body copied from the incoming request; URL is host + the path override or the original path + the original query string) and spawns a detached background task that sends it via the shared outbound HTTP client. Everything the task needs is cloned before it is spawned, so:

• The mirror never blocks the request path — execute returns immediately with the Context unchanged and continues through the success port.
• The mirror's response and any transport/timeout error are dropped (best-effort). A failed or unreachable shadow host has no effect on the client's response.

The mirrored call uses a 60-second whole-call deadline; since it runs on a detached task it does not hold up request handling.