Identity of the Fastly cache servers processing the response.
Fastly writes this header into responses. It is proprietary to Fastly.
This non-standard header is set by Fastly by default on all responses that we process, and contains the identity of the cache server acting as the delivery node. In services that use shielding or Next-gen WAF at Edge there may be more than one server identity listed, separated by commas.
The format of each entry in the header is:
WARNING: Cache nodes are moved in and out of service continually, and cache IDs may be reused. Constructing logic that acts on the cache ID is discouraged. Data centers are also subject to decommissioning, and data center codes may also be reused. The
X-Served-By value is therefore accurate at the moment it is generated but should not be compared to another value captured at a different point in time.