By default, HTTP reverse-proxy caches, such as Varnish (which is integral to Adobe Commerce), are configured to cache only "safe" and "idempotent" requests. The HTTP GET and HEAD methods are considered safe as they are intended only for data retrieval and should not alter the server's state. In contrast, HTTP POST and PUT methods are designed to modify data or create resources on the server. Because these actions change the state of the application, their responses are not considered cacheable by default to prevent serving stale or incorrect data. Therefore, if a custom API uses POST or PUT for an operation that should be returning cacheable data, Varnish will bypass the cache, leading to performance delays.

A. The request is using HTTP GET.

Incorrect. GET requests are idempotent and safe, meaning they are designed for data retrieval and are cacheable by default by Varnish and other HTTP caches.

B. The request is using HTTP HEAD.

Incorrect. HEAD requests are functionally similar to GET but without the response body. They are also considered safe and are cacheable by default.

1. Adobe Commerce Developer Documentation - Configure Varnish: The default Varnish Configuration Language (VCL) file provided for Adobe Commerce explicitly bypasses the cache for requests that are not GET or HEAD. The logic in the vclrecv subroutine typically includes a condition like: if (req.method != "GET" && req.method != "HEAD") { return (pass); }. This pass directive instructs Varnish to send the request directly to the backend

bypassing any cache lookup.

Source: Adobe Commerce Developer Documentation

"Get the Varnish VCL file". The default VCL files for various Varnish versions can be found in the magento/magento2 repository under app/code/Magento/PageCache/etc/varnish/.

2. RFC 7231 - Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content: This specification defines the cacheability of different HTTP methods.

Section 4.3.3 (POST): "Responses to POST requests are only cacheable when they include explicit freshness information... However

POST caching is not widely implemented." In practice

caches like Varnish do not cache POST by default.

Section 4.3.4 (PUT): The specification implies non-cacheability as it modifies a resource. Caching a response to a state-changing request is generally unsafe.

Section 4.3.1 (GET): "Responses to GET requests are cacheable..."

Source: IETF RFC 7231

Sections 4.3.1

4.3.3

4.3.4. (DOI not applicable for RFCs

can be found at https://datatracker.ietf.org/doc/html/rfc7231).