ZJSON Specification

1. Introduction

The Zed data model is based on richly typed records with a deterministic field order, as is implemented by the ZSON, ZNG, and VNG formats. Given the ubiquity of JSON, it is desirable to also be able to serialize Zed data into the JSON format. However, encoding Zed data values directly as JSON values would not work without loss of information.

For example, consider this Zed data as ZSON:

{
    ts: 2018-03-24T17:15:21.926018012Z,
    a: "hello, world",
    b: {
        x: 4611686018427387904,
        y: 127.0.0.1
    }
}

A straightforward translation to JSON might look like this:

{
  "ts": 1521911721.926018012,
  "a": "hello, world",
  "b": {
    "x": 4611686018427387904,
    "y": "127.0.0.1"
  }
}

But, when this JSON is transmitted to a JavaScript client and parsed, the result looks something like this:

{
  "ts": 1521911721.926018,
  "a": "hello, world",
  "b": {
    "x": 4611686018427388000,
    "y": "127.0.0.1"
  }
}

The good news is the a field came through just fine, but there are a few problems with the remaining fields:

• the timestamp lost precision (due to 53 bits of mantissa in a JavaScript IEEE 754 floating point number) and was converted from a time type to a number,
• the int64 lost precision for the same reason, and
• the IP address has been converted to a string.

As a comparison, Python's json module handles the 64-bit integer to full precision, but loses precision on the floating point timestamp. Also, it is at the whim of a JSON implementation whether or not the order of object keys is preserved.

While JSON is well suited for data exchange of generic information, it is not so appropriate for a super-structured data model like Zed. That said, JSON can be used as an encoding format for Zed by mapping Zed data onto a JSON-based protocol. This allows clients like web apps or Electron apps to receive and understand Zed and, with the help of client libraries like zed-js, to manipulate the rich, structured Zed types that are implemented on top of the basic JavaScript types.

In other words, because JSON objects do not have a deterministic field order nor does JSON in general have typing beyond the basics (i.e., strings, floating point numbers, objects, arrays, and booleans), we decided to encode Zed data with its embedded type model all in a layer above regular JSON.

2. The Format

The format for representing Zed in JSON is called ZJSON. Converting ZSON, ZNG, or VNG to ZJSON and back results in a complete and accurate restoration of the original Zed data.

A ZJSON stream is defined as a sequence of JSON objects where each object represents a Zed value and has the form:

{
  "type": <type>,
  "value": <value>
}

The type and value fields are encoded as defined below.

2.1 Type Encoding

The type encoding for a primitive type is simply its Zed type name e.g., "int32" or "string".

Complex types are encoded with small-integer identifiers. The first instance of a unique type defines the binding between the integer identifier and its definition, where the definition may recursively refer to earlier complex types by their identifiers.

For example, the Zed type {s:string,x:int32} has this ZJSON format:

{
  "id": 123,
  "kind": "record",
  "fields": [
    {
      "name": "s",
      "type": {
        "kind": "primitive",
        "name": "string"
      }
    },
    {
      "name": "x",
      "type": {
        "kind": "primitive",
        "name": "int32"
      }
    }
  ]
}

A previously defined complex type may be referred to using a reference of the form:

{
  "kind": "ref",
  "id": 123
}

2.1.1 Record Type

A record type is a JSON object of the form

{
  "id": <number>,
  "kind": "record",
  "fields": [ <field>, <field>, ... ]
}

where each of the fields has the form

{
  "name": <name>,
  "type": <type>,
}

and <name> is a string defining the field name and <type> is a recursively encoded type.

2.1.2 Array Type

An array type is defined by a JSON object having the form

{
  "id": <number>,
  "kind": "array",
  "type": <type>
}

where <type> is a recursively encoded type.

2.1.3 Set Type

A set type is defined by a JSON object having the form

{
  "id": <number>,
  "kind": "set",
  "type": <type>
}

where <type> is a recursively encoded type.

2.1.4 Map Type

A map type is defined by a JSON object of the form

{
  "id": <number>,
  "kind": "map",
  "key_type": <type>,
  "val_type": <type>
}

where each <type> is a recursively encoded type.

2.1.5 Union type

A union type is defined by a JSON object having the form

{
  "id": <number>,
  "kind": "union",
  "types": [ <type>, <type>, ... ]
}

where the list of types comprise the types of the union and and each <type>is a recursively encoded type.

2.1.6 Enum Type

An enum type is a JSON object of the form

{
  "id": <number>,
  "kind": "enum",
  "symbols": [ <string>, <string>, ... ]
}

where the unique <string> values define a finite set of symbols.

2.1.7 Error Type

An error type is a JSON object of the form

{
  "id": <number>,
  "kind": "error",
  "type": <type>
}

where <type> is a recursively encoded type.

2.1.8 Named Type

A named type is encoded as a binding between a name and a Zed type and represents a new type so named. A type definition type has the form

{
  "id": <number>,
  "kind": "named",
  "name": <id>,
  "type": <type>,
}

where <id> is a JSON string representing the newly defined type name and <type> is a recursively encoded type.

2.2 Value Encoding

The primitive values comprising an arbitrarily complex Zed data value are encoded as a JSON array of strings mixed with nested JSON arrays whose structure conforms to the nested structure of the value's schema as follows:

• each record, array, and set is encoded as a JSON array of its composite values,
• a union is encoded as a string of the form <tag>:<value> where tag is an integer string representing the positional index in the union's list of types that specifies the type of <value>, which is a JSON string or array as described recursively herein,
• a map is encoded as a JSON array of two-element arrays of the form [ <key>, <value> ] where key and value are recursively encoded,
• a type value is encoded as above,
• each primitive that is not a type value is encoded as a string conforming to its ZSON representation, as described in the corresponding section of the ZSON specification.

For example, a record with three fields --- a string, an array of integers, and an array of union of string, and float64 --- might have a value that looks like this:

[ "hello, world", ["1","2","3","4"], ["1:foo", "0:10" ] ]

3. Object Framing

A ZJSON file is composed of ZJSON objects formatted as newline delimited JSON (NDJSON). e.g., the zq CLI command writes its ZJSON output as lines of NDJSON.

4. Example

Here is an example that illustrates values of a repeated type, nesting, records, array, and union. Consider the file input.zson:

{s:"hello",r:{a:1,b:2}}
{s:"world",r:{a:3,b:4}}
{s:"hello",r:{a:[1,2,3]}}
{s:"goodnight",r:{x:{u:"foo"((string,int64))}}}
{s:"gracie",r:{x:{u:12((string,int64))}}}

This data is represented in ZJSON as follows:

zq -f zjson input.zson | jq .

{
  "type": {
    "kind": "record",
    "id": 31,
    "fields": [
      {
        "name": "s",
        "type": {
          "kind": "primitive",
          "name": "string"
        }
      },
      {
        "name": "r",
        "type": {
          "kind": "record",
          "id": 30,
          "fields": [
            {
              "name": "a",
              "type": {
                "kind": "primitive",
                "name": "int64"
              }
            },
            {
              "name": "b",
              "type": {
                "kind": "primitive",
                "name": "int64"
              }
            }
          ]
        }
      }
    ]
  },
  "value": [
    "hello",
    [
      "1",
      "2"
    ]
  ]
}
{
  "type": {
    "kind": "ref",
    "id": 31
  },
  "value": [
    "world",
    [
      "3",
      "4"
    ]
  ]
}
{
  "type": {
    "kind": "record",
    "id": 34,
    "fields": [
      {
        "name": "s",
        "type": {
          "kind": "primitive",
          "name": "string"
        }
      },
      {
        "name": "r",
        "type": {
          "kind": "record",
          "id": 33,
          "fields": [
            {
              "name": "a",
              "type": {
                "kind": "array",
                "id": 32,
                "type": {
                  "kind": "primitive",
                  "name": "int64"
                }
              }
            }
          ]
        }
      }
    ]
  },
  "value": [
    "hello",
    [
      [
        "1",
        "2",
        "3"
      ]
    ]
  ]
}
{
  "type": {
    "kind": "record",
    "id": 38,
    "fields": [
      {
        "name": "s",
        "type": {
          "kind": "primitive",
          "name": "string"
        }
      },
      {
        "name": "r",
        "type": {
          "kind": "record",
          "id": 37,
          "fields": [
            {
              "name": "x",
              "type": {
                "kind": "record",
                "id": 36,
                "fields": [
                  {
                    "name": "u",
                    "type": {
                      "kind": "union",
                      "id": 35,
                      "types": [
                        {
                          "kind": "primitive",
                          "name": "int64"
                        },
                        {
                          "kind": "primitive",
                          "name": "string"
                        }
                      ]
                    }
                  }
                ]
              }
            }
          ]
        }
      }
    ]
  },
  "value": [
    "goodnight",
    [
      [
        [
          "1",
          "foo"
        ]
      ]
    ]
  ]
}
{
  "type": {
    "kind": "ref",
    "id": 38
  },
  "value": [
    "gracie",
    [
      [
        [
          "0",
          "12"
        ]
      ]
    ]
  ]
}