[Discussion] use nlohmann::ordered_json to preserve insert order?

[zhjwpku]

The current implementation utilizes nlohmann::json for serialization, which does not maintain the insertion order of elements. In contrast, the Java implementation[0] appears to preserve the order of elements. Notably, nlohmann offers an ordered_json[1] variant, so I'd like to know should we take the effort to preserve the insert order?

[0] https://github.com/apache/iceberg/blob/main/core/src/main/java/org/apache/iceberg/SnapshotParser.java#L57-L110
[1] https://json.nlohmann.me/api/ordered_json/

[wgtmac]

That makes sense. Let's do this!

[zhjwpku]

Quote from JSON standard [1]:

   JavaScript Object Notation (JSON) is a text format for the
   serialization of structured data.  It is derived from the object
   literals of JavaScript, as defined in the ECMAScript Programming
   Language Standard, Third Edition [[ECMA-262](https://datatracker.ietf.org/doc/html/rfc8259.html#ref-ECMA-262)].

   JSON can represent four primitive types (strings, numbers, booleans,
   and null) and two structured types (objects and arrays).

   A string is a sequence of zero or more Unicode characters [[UNICODE](https://datatracker.ietf.org/doc/html/rfc8259.html#ref-UNICODE)].
   Note that this citation references the latest version of Unicode
   rather than a specific release.  It is not expected that future
   changes in the Unicode specification will impact the syntax of JSON.

   An object is an unordered collection of zero or more name/value
   pairs, where a name is a string and a value is a string, number,
   boolean, null, object, or array.

   An array is an ordered sequence of zero or more values.

Object is unordered, so there is no need to preserve any specific order of object keys, using nlohmann::ordered_json should not be necessary, thus closed.

[1] https://datatracker.ietf.org/doc/html/rfc8259.html