Understanding common cache-related HTTP response headers
Having a basic understanding of caching is a requirement of being a professional Drupal developer. Unfortunately, there can be many layers of caching which can make it challenging to figure out exactly how best to configure cache settings and troubleshoot potential caching issues. Web page caching can be thought of as moats around the castle, where each moat is a caching layer and the castle can be thought of as the site's web, database, and other origin servers. HTTP headers provide a mechanism for the client (usually a web browser) and a server (usually a web server) to exchange information