[cds^9] cds.middlewares.after + PATCH vs. PUT vs. Replace + Base Protocol Adapter

node.js/cds-serve.md

@@ -208,7 +208,7 @@ srv/cat-service.js   #> service implementation used by default
 For each service served at a certain protocol, the framework registers a configurable set of express middlewares by default like so:
 
 ```js
-app.use (cds.middlewares.before, protocol_adapter)
+app.use (cds.middlewares.before, protocol_adapter, cds.middlewares.after)
 ```
 
 The standard set of middlewares uses the following order:
@@ -222,6 +222,16 @@ cds.middlewares.before = [
 ]
 ```
 
+```js
+cds.middlewares.after = [
+  cds_error_handler(), // provides final error handling
+]
+```
+
+::: tip Custom error middleware before `cds_error_handler`
+To invoke a custom error middleware successfully, you must register it _before_ the built-in `cds_error_handler`. You can achieve that, for example, by adding the middleware using `cds.middlewares.after.unshift()`.
+:::
+
 ::: warning _Be aware of the interdependencies of middlewares_ <!--  -->
 _ctx_model_ requires that _cds.context_ middleware has run before.
 _ctx_auth_ requires that _authentication_ has run before.
@@ -400,6 +410,31 @@ service CatalogService {}
 
 Be aware that using an absolute path will disallow serving the service at multiple protocols.
 
+### PATCH vs. PUT vs. Replace
+
+The HTTP method `PATCH` is meant for partial modification of an _existing resource_.
+`PUT`, on the other hand, is meant for ensuring a resource exists
+That is, if it doesn't yet exists, it gets created.
+If it does exist, it gets updated to reflect the request's content.
+
+This content, however, may be incomplete.
+By default, the values for not listed keys are not touched.
+The rationale being that default values are known and clients have the option to send full representations, if necessary.
+
+The following table shows the Node.js runtime's configuration options and their respective default value:
+
+| Flag                                         | Behavior                                 | Default |
+|----------------------------------------------|------------------------------------------|---------|
+| <Config keyOnly>cds.runtime.patch_as_upsert = false</Config> | Create resource if it does not yet exist   | false   |
+| <Config keyOnly>cds.runtime.put_as_upsert = true</Config>     | Create resource if it does not yet exist    | true    |
+| <Config keyOnly>cds.runtime.put_as_replace = false</Config>   | Payload is enriched with default values  | false   |
+
+### Base Protocol Adapter
+
+All CAP-own protocol adapters extend a base protocol adapter that mounts the following middlewares:
+1. `http_log`: log all incoming requests
+2. `requires_check`: check the required roles for the respective service
+
 ### Custom Protocol Adapter
 
 Similar to the configuration of the GraphQL Adapter, you can plug in your own protocol.