Draft catalog flatbuffers schema (#82)

* Catalog schema draft proposal (scratch)

* Update #1

* Reordered some fields

* Add some clarifying comments

* Remove field that goes outside scope of type management

* Improve notes in comments and changed a column type based on feedback

* More field name and comment updates

scratch/laurentiu/catalog/catalog.fbs

@@ -0,0 +1,78 @@
+/////////////////////////////////////////////
+// Copyright (c) Gaia Platform LLC
+// All rights reserved.
+/////////////////////////////////////////////
+
+// -*- mode: java -*- // Java mode works for emacs to edit flatbuffers.
+
+// Identifiers are stored outside the payload.
+// Foreign keys are represented as int64 because flatbuffers does not yet support GUIDs.
+// If data size is not a concern, all integers can be made 64bit.
+
+namespace gaia;
+
+// This tracks high level information about types.
+// Scalar types have hard-coded entries.
+table types
+{
+    name: string;
+
+    // For indicating struct or table in flatbuffers.
+    //
+    // Note: this catalog representation does not support union and enum.
+    // Support for them can be added post-Q2, if needed.
+    //
+    // 0 = scalar, 1 = table, 2 = struct, 3 = enum, 4 = union
+    //
+    // Builtin basic types would be marked as scalar.
+    category: ubyte;
+}
+
+// Any complex type is defined as a set of fields,
+// with fields possibly being of complex type themselves
+// (although this may not be used currently).
+table fields
+{
+    name: string;
+
+    // The type of the field.
+    type_id: int64;
+
+    // The type that owns this field.
+    //
+    // This enables us to collect fields under a type.
+    // It also enables us to describe complex, hierarchical types.
+    owning_type_id: int64;
+
+    // Indicates whether this is an array type and if yes, what kind of size it has.
+    //
+    // 0 = variable size, 1 = not array, > 1 = fixed size.
+    //
+    // Do we need to support arrays larger than 4B?
+    repeated_count: uint;
+
+    // Monotonically assigned identifier, to maintain the order of fields in flatbuffers schema.
+    //
+    // Do we need to support more than 65k fields?
+    ordering_id: ushort;
+
+    // For deprecating fields.
+    deprecated: bool;
+
+    // Indicates whether the field accepts null values.
+    // Will require some flatbuffers modification to support null values.
+    // Mapping null to missing does not work for structs
+    // and may not be possible for tables either
+    // because we will probably avoid missing values to allow updating fields in-place.
+    nullable: bool;
+}
+
+// A table will have its own identifier as well as a specific type.
+// Several tables can share the same type.
+table tables
+{
+    name: string;
+
+    // Indicates the type of its payload, which gives us all the fields.
+    type_id: int64;
+}