Roles reference

Author: andreyaksenov

doc/reference/configuration/configuration_reference.rst

@@ -2877,6 +2877,35 @@ This section describes configuration parameters related to roles.
 
     **Since:** :doc:`3.0.0 </release/3.0.0>`.
 
+    Application roles of an instance.
+    An application role is a Lua module that implement specific functions or logic.
+
+    There two types of application roles:
+
+    -   Built-in roles. For example, the `CRUD <https://github.com/tarantool/crud>`__ provides the ``crud-storage`` and ``crud-router`` roles that enable CRUD operations in a sharded cluster.
+    -   Custom roles. For example, you create a custom role to define a stored procedure or implement a supplementary service, such as an e-mail notifier or a replicator.
+
+     To create a custom role, you need to define the following functions and fields:
+
+    -   ``validate()``
+
+        Validate a role's configuration.
+
+    -   ``apply()``
+
+        Apply a role's configuration.
+
+    -   ``stop()``:
+
+        Stop a role.
+
+    -   (Optional) ``dependencies``
+
+        Define a role's dependencies.
+
+    Functions should throw an error if it occurs.
+
+
     |
     | Type: array
     | Default: nil