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Schema:
{ "fields":
    [ {"name": "version", "type": "int", "doc": "version id"},
      {"name": "pattern", "type": "string", "doc": "can be of static, white_list or black_list"},
      {"name": "subscription", {"type": "map", "values": {"type": "int"},
                                "doc": "a map from a topic or a wildcard pattern to the number of streams"}      }    ]
}

Example:
A static subscription:
{
  "version": 1,
  "pattern": "static",
  "subscription": {"topic1": 1, "topic2": 2}
}


A whitelist subscription:
{
  "version": 1,
  "pattern": "white_list",
  "subscription": {"*abc": 1}}

A blacklist subscription:
{
  "version": 1,
  "pattern": "black_list",
  "subscription": {"!abc": 1}}