[WIP]feat: docs about storage refactor

docs/storage-refactor.md

@@ -0,0 +1,84 @@
+## Background
+
+### Current design
+
++ `TuStoreContext` stores reference to:
+  + `NodeStoreAPI`
+  + `RelationshipStoreAPI`
++ `storage` package:
+  + details about the underline storage system, such as rocksdb storage
++ `relationship` package:
+  + `RelationshipStoreSPI` and `RelationshipStoreAPI` (extends `RelationshipStoreSPI`)
+  + `Relationship` defines a relationship
+    + `StoredRelationship`
+    + `StoredRelationshipWithProperty`
+    + `TuRelationship`
+    + `LynxRelationshipId`
+    + `RelationshipDirectionStore`
+    + `RelationshipLabelIndex`
+    + `RelationshipPropertyStore`
++ `node` package:
+  + `StoredNode`
+  + `StoredNodeWithProperty`
+  + `LynxNodeId`
+  + `TuNode`
+  + `LazyTuNode`
+  + `NodeLabelStore`
+  + `NodeStore`
+  + `NodeStoreSPI` and `NodeStoreAPI` (extends `NodeStoreSPI`)
++ `index` package:
+  + `EmptyIndexServerImpl`, `MemoryIndexServerImpl` and `RocksIndexServerImpl`
+  + `IndexStoreKVImpl`
+  + `PropertyIndex`, `PropertyIndexStore`
++ `meta` package:
+  + `ConfigNameMap`: object wraps property name and its value
+  + `DbNameMap`: object wraps property name and its value
+  + `IdGenerator`
+  + `NameStore`: store information about an `id` and a string map
+  + `NodeLabelNameStore`: extends `NameStore`
+  + `PropertyNameStore`: extends `NameStore`
+  + `RelationshipTypeNameStore`: extends `NameStore`
+  + `TuDBStatistics`
+  + `TypeManager`: aliases to different types of objects
+
+Problems:
+
++ Graph abstractions are coupled with storage abstractions.
++ API calls are scattered around a lot of files which makes it hard to maintain.
+
+## Proposed design
+
++ Storage abstraction wraps underline storage.
++ Graph abstraction wraps node, relationship, meta of a graph, it has no directly
+interaction with storage
++ Engine abstraction wraps graph and storage and it provides services to other packages.
+
+### Storage
+
+The storage packages is similar to current design
+
+### Graph
+
++ Node
+  + Label
+  + Properties
++ Relationship:
+  + Label
+  + Properties
++ Meta
++ Serializer
+
+### Engine
+
+It holds references to a graph and a storage.
+
++ Serialize data to bytes and store them
++ Deserialize from data read from storage
++ Provide CRUD support using graph and storage handler
+
+## Others
+
++ Reference is not restricted, such `NodeStoreAPI` and `RelationshipStoreAPI` is
+created and used directly.
++ Other functions are called in a lot of places.
+