4.4 Reading Entity Providers at the Same SCN

If you have a graph which consists of multiple vertex or edge tables or both, then you can read all the vertices and edges at the same System Change Number (SCN). This helps to overcome issues such as reading edge providers at a later SCN than the SCN at which the vertices were read, as some edges may reference missing vertices.

Note that reading a graph from the database is still possible even if Flashback is not enabled on Oracle Database. In case of multiple databases, SCN can be used to maintain consistency for entity providers belonging to the same database only.

You can use the as_of flag in the graph configuration to specify at what SCN an entity provider must be read. The valid values for the as_of flag are as follows:

Table 4-5 Valid values for "as_of" Key in Graph Configuration

Value	Description
A positive long value	This is a parseable SCN value.
"<current-scn>"	The current SCN is determined at the beginning of the graph loading.
"<no-scn>"	This is to disable SCN at the time of graph loading.
null	This defaults to "<current-scn>" behavior.

If "as_of" is omitted for a vertex or an edge provider in the graph configuration file, then this follows the same behavior as "as_of": null.

Example 4-5 Graph Configuration Using "as_of" for Vertex and Edge Providers in the Same Database

The following example configuration has three vertex providers and one edge provider pointing to the same database.

{
  "name": "employee_graph",
  "vertex_providers": [
    {
      "name": "Department",
      "as_of": "<current-scn>",
      "format": "rdbms",
      "database_table_name": "DEPARTMENTS",
      "key_column": "DEPARTMENT_ID",
      "props": [
        {
          "name": "DEPARTMENT_NAME",
          "type": "string"
        }
      ]
    },
    {
      "name": "Location",
      "as_of": "28924323",
      "format": "rdbms",
      "database_table_name": "LOCATIONS",
      "key_column": "LOCATION_ID",
      "props": [
        {
          "name": "CITY",
          "type": "string"
        }
      ]
    },
    {
      "name": "Region",
      "as_of": "<no-scn>",
      "format": "rdbms",
      "database_table_name": "REGIONS",
      "key_column": "REGION_ID",
      "props": [
        {
          "name": "REGION_NAME",
          "type": "string"
        }
      ]
    }
  ],
  "edge_providers": [
    {
      "name": "LocatedAt",
      "format": "rdbms",
      "database_table_name": "DEPARTMENTS",
      "key_column": "DEPARTMENT_ID",
      "source_column": "DEPARTMENT_ID",
      "destination_column": "LOCATION_ID",
      "source_vertex_provider": "Department",
      "destination_vertex_provider": "Location"
    }
  ]
}

When reading the employee_graph using the preceding configuration file, the graph is read at the same SCN for the Department and LocatedAt entity providers. This is explained in the following table:

Table 4-6 Example Scenario Using "as_of"

Entity Provider	"as_of"	SCN Value
Department	"<current-scn>"	SCN determined automatically
Location	"28924323"	"28924323" used as SCN
Region	"<no-scn>"	No SCN used
LocatedAt	"as_of" flag is omitted	SCN determined automatically

The current SCN value of the database can be determined using one of the following options:

• Querying V$DATABASE view:

  SELECT CURRENT_SCN FROM V$DATABASE;

• Using DBMS_FLASHBACK package:

  SELECT DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER FROM DUAL;

If you do not have the required privileges to perform either of the preceding operations, then you can use:

SELECT TIMESTAMP_TO_SCN(SYSDATE) FROM DUAL;

However, note that this option is less precise than the earlier two options.

You can then read the graph into the graph server using the JSON configuration file as shown:

• JShell
• Java
• Python

opg4j> var g = session.readGraphWithProperties("employee_graph.json")

PgxGraph g = session.readGraphWithProperties("employee_graph.json");

g = session.read_graph_with_properties("employee_graph.json")