Common: explore graph extensions to Avro enriched event

[chuwy]

We may want to leap straight to this approach. The basic idea would be to express relationships between entities in the event as typed edges.

Need to review Nathan Marz's chapter on modeling graphs in Thrift in Big Data.

Here is a simple example (using self-describing JSON instead of Avro for simplicity):

{
  "nodes": {
   "n0": {
      "schema": "iglu:com.acme/player/jsonschema/1-0-0",
      "data": {
        "username": "bob",
        "alliance": "pirates"
      }
    },
    "n1": {
      "schema": "iglu:com.acme/player/jsonschema/1-0-0",
      "data": {
        "username": "sue",
        "alliance": "zombies"
      }
    },
    "n2": {
      "schema": "iglu:com.acme/kills/jsonschema/1-0-0",
      "data": {}
    },
    "n3": {
      "schema": "iglu:com.acme/level/jsonschema/1-0-0",
      "data": {
        "name": "blood gulch",
        "difficulty": 23
      }
    },
  },
  "edges": {
    "e0": {
      "schema": "iglu:com.snowplowanalytics.snowplow/verb-subject/jsonschema/1-0-0",
      "source": "n0",
      "target": "n1"
    },
   "e1":  {
      "schema": "iglu:com.snowplowanalytics.snowplow/verb-direct-object/jsonschema/1-0-0",
      "source": "n1",
      "target": "n2"
    },
   "e2":  {
      "schema": "iglu:com.acme/at-geographic/jsonschema/1-0-0",
      "source": "n2",
      "target": "n3"
    }
  }
}