Correct definition of schemas in the Repository concept page.

Author: reid-spencer

docs/riddl/concepts/repository.md

@@ -7,16 +7,22 @@ A RIDDL repository is an abstraction for anything that can retain
 information (e.g. [messages](message.md)) for retrieval at a
 later time. This might be a relational database, NoSQL database, a data lake, 
 an API, or something not yet invented. There is no specific technology implied
-other than the retention and retrieval of information. You should think of 
-repositories more like a message-oriented version of the 
-[Java Repository Pattern](https://java-design-patterns.com/patterns/repository/#explanation)
-than any particular kind of database. 
+other than the retention and retrieval of information. Thinking of repositories
+only as a database is missing the mark. 
 
 ## Schemas
-Repositories have traditionally had data schemas as part of their design 
-definition; but RIDDL regards schemas as a technical detail of the 
-implementation and leaves them unspecified. Repositories are only defined in 
-terms of their message handling.
+As part of their intrinsic definition, repositories have schemas. Schemas in 
+implementation and leaves them unspecified. Repository schemas in RIDDL have
+little relationship to SQL because they are not relational databases and indeed
+must support the schema for any kind of data storage. The schema is specified
+more like a graph database.  A set of types defines what can be placed in 
+the database, an optional set of links defines how the fields of those types
+are related, and a set of indices defines which fields the database indexes. 
+
+A repository may define multiple schemas, each with its own _kind_. The kind of
+schema may be: flat, relational, time-series, graphical, hierarchical, star, 
+document, columnar, vector, or other. These are only suggestive of the kind of
+storage layout the repository uses. 
 
 ## Handling Messages
 A repository has a [handler](handler.md) that processes