GLB-X-Seen-By
The HTTP GLB-X-Seen-By response header is an unofficial HTTP header sent by WordPress.com, recording which global load balancer node handled the request at the outermost layer of Automattic's infrastructure.
Note
The "X-" naming convention for HTTP headers, "X" referring to "experimental", has been deprecated and needs to be transitioned to the formal naming convention for HTTP headers.
Usage
Automattic operates a global load balancer (GLB) tier in front of its application infrastructure. When a request arrives at WordPress.com, the GLB layer processes the request first before routing to regional application servers and caches.
The GLB-X-Seen-By header is set by the GLB layer and contains a single Base64-encoded hash identifying the specific global load balancer node receiving the request. The "GLB" prefix distinguishes this header from X-Seen-By, which records nodes deeper in the infrastructure stack.
Both headers serve the same diagnostic purpose: giving Automattic engineers a trace of where in the infrastructure a request traveled. The values are opaque identifiers with no meaning to external clients. The two headers commonly appear together on WordPress.com responses, providing a layered view from the global entry point through to the internal application nodes.
Values
Global load balancer node hash
The header value is a single Base64-encoded hash identifying the GLB node. Unlike X-Seen-By, which accumulates multiple node identifiers, GLB-X-Seen-By reflects only the single entry point at the global load balancing layer.
Example
The value is a Base64-encoded string mapping to a specific node in Automattic's global load balancer fleet. The value is opaque and stable for a given node, but meaningless outside Automattic's internal systems.
GLB-X-Seen-By: bS8wRlGzu0Hc+WrYuHB8QIg44yfcdCMJRkBoQ1h6Vjc=
The header typically appears alongside X-Seen-By on the same response. Together they provide a trace from the global entry point through the internal application infrastructure.
GLB-X-Seen-By: bS8wRlGzu0Hc+WrYuHB8QIg44yfcdCMJRkBoQ1h6Vjc=
X-Seen-By: yvSunuo/8ld62ehjr5B7kA==,T7xPrjRFKDMHVv938PYVfx9slopJdhD+WySraMrpIY8=
Takeaway
The GLB-X-Seen-By header identifies the Automattic global load balancer node receiving a WordPress.com request. The value is a single opaque Base64 hash used for internal infrastructure diagnostics, and appears alongside X-Seen-By to form a complete request trace.