add basic ARCHITECTURE.md (#2503)

* refactor: move ApiRequest a top-level module
* refactor: rename DbStructure to SchemaCache
* refactor: GucHeader inside Response
* refactor: admin app to Workers

src/PostgREST/SchemaCache/Identifiers.hs

@@ -0,0 +1,42 @@
+{-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE DeriveGeneric  #-}
+
+module PostgREST.SchemaCache.Identifiers
+  ( QualifiedIdentifier(..)
+  , Schema
+  , TableName
+  , FieldName
+  , dumpQi
+  , toQi
+  ) where
+
+import qualified Data.Aeson as JSON
+import qualified Data.Text  as T
+
+import Protolude
+
+
+-- | Represents a pg identifier with a prepended schema name "schema.table".
+-- When qiSchema is "", the schema is defined by the pg search_path.
+data QualifiedIdentifier = QualifiedIdentifier
+  { qiSchema :: Schema
+  , qiName   :: TableName
+  }
+  deriving (Eq, Ord, Generic, JSON.ToJSON, JSON.ToJSONKey)
+
+instance Hashable QualifiedIdentifier
+
+dumpQi :: QualifiedIdentifier -> Text
+dumpQi (QualifiedIdentifier s i) =
+  (if T.null s then mempty else s <> ".") <> i
+
+-- TODO: Handle a case where the QI comes like this: "my.fav.schema"."my.identifier"
+-- Right now it only handles the schema.identifier case
+toQi :: Text -> QualifiedIdentifier
+toQi txt = case T.drop 1 <$> T.breakOn "." txt of
+  (i, "") -> QualifiedIdentifier mempty i
+  (s, i)  -> QualifiedIdentifier s i
+
+type Schema = Text
+type TableName = Text
+type FieldName = Text