Does Traefik’s sticky‑session middleware support session tracking for non‑frontend providers?

Direct answer

Yes. The sticky‑session middleware can be attached to any router created by a non‑frontend provider (Docker, Kubernetes, Consul, etc.). It is not limited to the built‑in frontend provider.

Confirmed behavior

• Session entries are stored in the Traefik process’s in‑memory map, keyed by the cookie value (or source IP).
• By default the middleware sets expiration = 0, meaning entries never expire and the cache can grow without bound.
• Each Traefik instance maintains its own cache; there is no built‑in cross‑instance synchronization.

Likely explanation for the documentation wording

The reference to the “frontend provider” in older docs reflects the historical name of the built‑in file/HTTP provider. Current providers (Docker, Kubernetes, Consul, etc.) all generate routers that accept middleware, so the sticky‑session middleware works with them as well.

Steps to verify and prevent unbounded growth

1. Confirm the middleware is active – check Traefik logs for lines containing Sticky session when a request carries the cookie.
2. Inspect the in‑memory cache size – open the dashboard (/dashboard/#/routers) or call the API endpoint /api/http/routers and look at the stickySessions count.
3. Set an expiration TTL – add expiration: "10m" (or any duration) to the middleware configuration, e.g.:

   http:
     middlewares:
       my-sticky:
         sticky:
           cookie:
             name: "traefik_sticky"
             expiration: "10m"

4. Optional: use an external store – for multi‑instance deployments configure a shared session store (Redis, Consul) via a custom middleware or sidecar; the built‑in sticky middleware does not support external stores.

One missing diagnostic detail

Are you running a single Traefik instance or multiple replicas behind a load balancer? The answer changes whether you only need an expiration TTL or also need a shared session store.