How does the ESI invalidation specification compare to Web Cache Invalidation Protocol (WCIP)?
WCIP and similar solutions try to solve a different problem than ESI invalidation. Although both are a means of communicating invalidations (and possibly other information) from a Web site to caches and CDNs, they have different scopes and applications. WCIP allows a large number of clients to subscribe to a channel, on which they’ll get invalidations and other kinds of updates. The primary problems that WCIP addresses are a) scaling to a very large number of clients and b) making the service to all of those clients reliable. ESI invalidation, on the other hand, has a one-to-one relationship; there is no channel mechanism. Instead, an invalidation message passes from the origin server directly to the local cache or CDN. This is more appropriate for sending invalidations to a small number of servers, or when the origin server doesn’t have responsibility for propagating the message (such as when a CDN is used). Both WCIP and ESI invalidation might share a common payload (that is, the inv
