What is the transmission of CDN between edge, center and source?

The first case (edge-center-source-center-edge): suppose that the DNS of CDN vendor distributes the request to Edge Node A through intelligent scheduling. Because there is no corresponding cache file in edge A, edge A sends a request to the center, and when the center has no cache, it returns to the source to obtain the file content and cache it as an independent new file, and returns to edge A synchronously and caches it.

The second case (edge-center-edge): When the central node caches the corresponding file and the new edge node requests the file again, in general, the edge node C directly obtains the cached file from the central node and caches it to the edge.

The third case (edge-source-edge): Under special circumstances, when the CDN system judges that it is faster for Node B to return to the source than to obtain the file from the central node, or the domain name of Edge B does not have a central node, Node B directly returns to the source to obtain the file content and caches it as an independent new file. At this time, because the central node is skipped, the cache of the central node is still the data returned to the source last time. If necessary, this situation can be completely avoided by deploying mandatory central nodes (edge-center-source).