Enable clustering.

Fastly reads this header from requests. It is proprietary to Fastly.

In most cases, Fastly will relay requests from the first server that handles the request to a second server that acts as the primary storage location for the resource being requested. This process is called clustering. If a service configuration executes a restart during request or response processing, the clustering mechanism is automatically disabled. This header exists to re-enable it if desired.

vcl_recv { ... }
Fastly VCL
set req.http.Fastly-Force-Shield = "1";

WARNING: Despite the name, this header has nothing to do with shielding.