docs: add first version for selectors high-level decisions

docs/decisions/0019-selectors.rst

@@ -1,4 +1,39 @@
 Selectors for Dynamically Selecting Content
 ===========================================
 
-WIP
+Context
+-------
+
+This ADR proposes a way to represent dynamic members of a container, where dynamic means selecting members from a specified pool. Some examples of dynamic selection are:
+
+1. A/B Testing: testing two different groups of components to see which perform better.
+2. Per Student: randomly selecting three problems from a set of 20 per student.
+
+And any other custom use case to dynamically select members for a container. This proposal introduces the concepts of selectors and variants to implement this type of dynamic selection.
+
+1. Core Structure
+=================
+
+This section explains the concepts and behaviors used to build dynamic selection, selectors and variants.
+
+- Selectors determine what the container should display based on the selector type. For example, based on the nature an A/B split test or a randomization selector members of the container would vary.
+- Selectors are designed to dynamically select 0-N publishable entities (e.g., components).
+- Each version represents a set of members at a particular time.
+- The logic for pushing members into variants depends on the selection version's selection method.
+- Variants hold the members selected for a container based on what the selection method is.
+- Variants are build on the parent-child relationship between containers and their members, storing the dynamically selected content as an ordered list.
+
+2. Selector Types and Selecting Content
+=======================================
+
+This section describes how different types of selectors work and how they handle the selection of dynamic content.
+
+- A selector can be of any type, which means it can implement any method to select members from a pool.
+- Selection versions encode the rules and holds useful details for the selection process like: where to get members from, number of items to select, and other criteria.
+- Depending on the size of the pool of members, variants can be generated at publishing time or on-demand. This behavior should be determined by the selector version based on high vs low permutation scenarios.
+- A compositor is responsible for populating the variants but will not be implemented as part of the authoring application.
+
+3. Versioning
+=============
+
+A new version of a selector is created whenever the pool of concent changes by adding, removing or reordering existing members.