docs(kopflos): handler pipeline

docs/apis/kopflos/explanations/_category_.yml

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+label: Explanations
+position: 3

docs/apis/kopflos/explanations/request-pipeline.md

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+---
+title: Request pipeline
+sidebar_position: 1
+---
+
+# Kopflos request pipeline
+
+```
+Incoming Request
+  │
+  └─▶ Kopflos handler
+             │
+   4**/5** ◀─┴─▶ Resource Shape Lookup
+                   │
+         4**/5** ◀─┴─▶ Resource Loader Lookup
+                         │
+               4**/5** ◀─┴─▶ Load Resource
+                               │
+                         4** ◀─┴─▶ Authorization
+                                     │
+                               400 ◀─┴─▶ Validation
+                                           │
+                                 4**/5** ◀─┴─▶ (User handler)
+                                                 │
+                                                 └─▶ Reply
+```
+
+## Incoming request
+
+Incoming request is handled by the server library, such as express or fastify and then forwarded to Kopflos.
+
+## Kopflos handler
+
+Kopflos handler is the main entry point for all incoming requests. It is responsible for orchestrating the request pipeline.
+
+## Resource Shape Lookup
+
+Resource Shape Lookup executes SPARQL against the `default` query endpoint to find the shape targeting the requested resource.
+
+:::tip
+See also: [How to Select which resources should be served by the API](../how-to/resource-shape.md)
+:::
+
+## Resource Loader Lookup + Load Resource
+
+When the Resource Shape is found, a resource loader is selected based from `kopflos:resourceLoader` property, going bottom-up from the Resource/Property Shape to the share `kopflos:Config` resource.
+
+It is used to load the requested resource's Core Representation.
+
+:::info
+The Core Representation are the triples returned by the resource loader. Typically, that would be the result of a SPARQL `DESCRIBE` query or contents of resource's "own graph".
+:::
+
+:::warning
+By default, a loader which returns the resource's own graph is used.
+:::
+
+## Authorization
+
+Not implemented yet.
+
+## Validation
+
+Not implemented yet.
+
+## User handler
+
+Finally, the handler is executed. If no handler is defined, and the request method is a GET, the resource's Core Representation is returned.
+
+The result of the handler is forwarded back to the server library to be sent as a response.