JSON - ObjectId

About

ObjectId are generated identifier (known as surrogate) with the intent to be unique for a Json.

ObjectId are custom UUID that are created from:

a counter
timestamp (milliseconds)
node id (IP address or hostname)
logical id (user, …)
random integer

The risk of collisions is almost the same than for uuid.

The id is called ObjectId because Json is the string version of a Javascript Object

Implementation Example

MongoDb Object Id

They have been popularized by the MongoDb implementation.

They are:

small (12 bytes in length)
likely unique,
fast to generate,
and ordered values

consisting of:

a 4-byte timestamp value, representing the ObjectId's creation, measured in seconds since the Unix epoch
a 5-byte random value generated once per process. This random value is unique to the machine and process.
a 3-byte incrementing counter, initialized to a random value

let mongoObjectId = function () {
    let timestamp = (new Date().getTime() / 1000 | 0).toString(16);
    return timestamp + 'xxxxxxxxxxxxxxxx'.replace(/[x]/g, function() {
        return (Math.random() * 16 | 0).toString(16);
    }).toLowerCase();
};
console.log("New Object Id: "+mongoObjectId());

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Parse Server

This example has been adapted from the Parse Server Object Id implementation.

let randomBytes = function(n) {
    let a = new Uint8Array(n), QUOTA = 65536;
    for (let i = 0; i < n; i += QUOTA) {
        crypto.getRandomValues(a.subarray(i, i + Math.min(n - i, QUOTA)));
    }
    return a;
};
let parseObjectId  = function (size = 10) {
  if (size === 0) {
    throw new Error('Zero-length randomString is useless.');
  }
  const chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ' + 'abcdefghijklmnopqrstuvwxyz' + '0123456789';
  let objectId = '';
  const bytes = randomBytes(size);
  for (let i = 0; i < bytes.length; ++i) {
    let number = bytes[i]
    objectId += chars[number % chars.length];
  }
  return objectId;
}
console.log("New Parse Object Id: "+parseObjectId());

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Instagram

Each of their IDs consists ¹⁾ of:

41 bits for time in milliseconds (41 years of IDs with a epoch and a latter custom start date)
13 bits that represent the logical shard ID (the node / computer / instance)
10 bits that represent an auto-incrementing sequence, modulus 1024. This means they can generate 1024 IDs, per shard, per millisecond

Documentation / Reference

¹⁾

Sharding & IDs at Instagram and Hackers news