The importance of a data format: Part I – Current state problems

JSON is a really simple format. It make it very easy to work with it, interchange it, read it, etc. Here is the full JSON format definition:

• object = {} | { members }
• members = pair | pair, members
• pair = string : value
• array = [] | [ elements ]
• elements = value | value , elements
• value = string | number | object | array | true | false | null

So far, so good. But JSON also has a few major issues. In particular, JSON require that you’ll read and parse the entire document (at least until the part you actually care about) before you can do something with it. Reading JSON documents into memory and actually working with them means loading and parsing the whole thing, and typically require the use of dictionaries to get fast access to the data. Let us look at this typical document:

{
  "firstName": "John",
  "lastName": "Smith",
  "address": {
    "state": "NY",
    "postalCode": "10021-3100"
  },
  "children": [{"firstName": "Alice"}]
}

How would this look in memory after parsing?

• Dictionary (root)
  • firstName –> John
  • lastName –> Smith
  • address –> Dictionary
    • state –> NY
    • postalCode –> 10021-3100
  • children –> array
    • [0] –> Dictionary
      • firstName –> Alice

So that is three dictionaries and an array (even assuming we ignore all the strings). Using Netwonsoft.Json, the above document takes 3,840 bytes in managed memory (measured using objsize in WinDBG). The size of the document is 126 bytes as text. The reason for the different sizes is dictionaries. Here is 320 bytes allocation:

new Dictionary<string,Object>{ {"test", "tube"} };