.. _rfc4122.version6:

=========================
Version 6: Reordered Time
=========================

.. note::

    Version 6, reordered time UUIDs are a new format of UUID, proposed in an
    `Internet-Draft under review`_ at the IETF. While the draft is still going
    through the IETF process, the version 6 format is not expected to change
    in any way that breaks compatibility.

.. attention::

    If you need a time-based UUID, and you don't need the other features
    included in version 6 UUIDs, we recommend using
    :ref:`version 7 UUIDs <rfc4122.version7>`.

Version 6 UUIDs solve `two problems that have long existed`_ with the use of
:ref:`version 1 <rfc4122.version1>` UUIDs:

1. Scattered database records
2. Inability to sort by an identifier in a meaningful way (i.e., insert order)

To overcome these issues, we need the ability to generate UUIDs that are
*monotonically increasing* while still providing all the benefits of version
1 UUIDs.

Version 6 UUIDs do this by storing the time in standard byte order, instead of
breaking it up and rearranging the time bytes, according to the `RFC 4122`_
definition. All other fields remain the same, and the version maintains its
position, according to RFC 4122.

In all other ways, version 6 UUIDs function like version 1 UUIDs.

.. tip::

    Prior to version 4.0.0, ramsey/uuid provided a solution for this with the
    :ref:`ordered-time codec <customize.ordered-time-codec>`. Use of the
    ordered-time codec is still valid and acceptable. However, you may replace
    UUIDs generated using the ordered-time codec with version 6 UUIDs. Keep
    reading to find out how.

.. code-block:: php
    :caption: Generate a version 6, reordered time UUID
    :name: rfc4122.version6.example

    use Ramsey\Uuid\Uuid;

    $uuid = Uuid::uuid6();

    printf(
        "UUID: %s\nVersion: %d\nDate: %s\nNode: %s\n",
        $uuid->toString(),
        $uuid->getFields()->getVersion(),
        $uuid->getDateTime()->format('r'),
        $uuid->getFields()->getNode()->toString()
    );

This will generate a version 6 UUID and print out its string representation, the
time the UUID was created, and the node used to create the UUID.

It will look something like this:

.. code-block:: text

    UUID: 1ea60f56-b67b-61fc-829a-0242ac130003
    Version: 6
    Date: Sun, 08 Mar 2020 04:29:37 +0000
    Node: 0242ac130003

You may provide custom values for version 6 UUIDs, including node and clock
sequence.

.. code-block:: php
    :caption: Provide custom node and clock sequence to create a version 6,
              reordered time UUID
    :name: rfc4122.version6.custom-example

    use Ramsey\Uuid\Provider\Node\StaticNodeProvider;
    use Ramsey\Uuid\Type\Hexadecimal;
    use Ramsey\Uuid\Uuid;

    $nodeProvider = new StaticNodeProvider(new Hexadecimal('121212121212'));
    $clockSequence = 16383;

    $uuid = Uuid::uuid6($nodeProvider->getNode(), $clockSequence);

.. tip::

    Version 6 UUIDs generated in ramsey/uuid are instances of UuidV6. Check out
    the :php:class:`Ramsey\\Uuid\\Rfc4122\\UuidV6` API documentation to
    learn more about what you can do with a UuidV6 instance.


.. _rfc4122.version6.nodes:

Custom and Random Nodes
#######################

In the :ref:`example above <rfc4122.version6.custom-example>`, we provided a
custom node when generating a version 6 UUID. You may also generate random
node values.

To learn more, see the :ref:`rfc4122.version1.custom` and
:ref:`rfc4122.version1.random` sections under :ref:`rfc4122.version1`.


.. _rfc4122.version6.clock:

Clock Sequence
##############

In a version 6 UUID, the clock sequence serves the same purpose as in a version
1 UUID. See :ref:`rfc4122.version1.clock` to learn more.


.. _rfc4122.version6.version1-conversion:

Version 1-to-6 Conversion
#########################

It is possible to convert back-and-forth between version 6 and version 1 UUIDs.

.. code-block:: php
    :caption: Convert a version 1 UUID to a version 6 UUID
    :name: rfc4122.version6.convert-version1-example

    use Ramsey\Uuid\Rfc4122\UuidV1;
    use Ramsey\Uuid\Rfc4122\UuidV6;
    use Ramsey\Uuid\Uuid;

    $uuid1 = Uuid::fromString('3960c5d8-60f8-11ea-bc55-0242ac130003');

    if ($uuid1 instanceof UuidV1) {
        $uuid6 = UuidV6::fromUuidV1($uuid1);
    }

.. code-block:: php
    :caption: Convert a version 6 UUID to a version 1 UUID
    :name: rfc4122.version6.convert-version6-example

    use Ramsey\Uuid\Rfc4122\UuidV6;
    use Ramsey\Uuid\Uuid;

    $uuid6 = Uuid::fromString('1ea60f83-960c-65d8-bc55-0242ac130003');

    if ($uuid6 instanceof UuidV6) {
        $uuid1 = $uuid6->toUuidV1();
    }


.. _rfc4122.version6.ordered-time-conversion:

Ordered-time to Version 6 Conversion
####################################

You may convert UUIDs previously generated and stored using the
:ref:`ordered-time codec <customize.ordered-time-codec>` into version 6 UUIDs.

.. caution::

    If you perform this conversion, the bytes and string representation of your
    UUIDs will change. This will break any software that expects your
    identifiers to be fixed.

.. code-block:: php
    :caption: Convert an ordered-time codec encoded UUID to a version 6 UUID
    :name: rfc4122.version6.convert-ordered-time-example

    use Ramsey\Uuid\Codec\OrderedTimeCodec;
    use Ramsey\Uuid\Rfc4122\UuidV1;
    use Ramsey\Uuid\Rfc4122\UuidV6;
    use Ramsey\Uuid\UuidFactory;

    // The bytes of a version 1 UUID previously stored in some datastore
    // after encoding to bytes with the OrderedTimeCodec.
    $bytes = hex2bin('11ea60faf17c8af6ad23acde48001122');

    $factory = new UuidFactory();
    $codec = new OrderedTimeCodec($factory->getUuidBuilder());

    $factory->setCodec($codec);

    $orderedTimeUuid = $factory->fromBytes($bytes);

    if ($orderedTimeUuid instanceof UuidV1) {
        $uuid6 = UuidV6::fromUuidV1($orderedTimeUuid);
    }


.. _rfc4122.version6.privacy:

Privacy Concerns
################

Like :ref:`version 1 UUIDs <rfc4122.version1>`, version 6 UUIDs use a MAC
address from a local hardware network interface. This means it is possible to
uniquely identify the machine on which a version 6 UUID was created.

If the value provided by the timestamp of a version 6 UUID is important to you,
but you do not wish to expose the interface address of any of your local
machines, see :ref:`rfc4122.version6.nodes`.

If you do not need an identifier with a node value embedded in it, but you still
need the benefit of a monotonically increasing unique identifier, see
:ref:`rfc4122.version7`.


.. _Internet-Draft under review: https://datatracker.ietf.org/doc/html/draft-ietf-uuidrev-rfc4122bis-00#section-5.6
.. _two problems that have long existed: https://www.percona.com/blog/2014/12/19/store-uuid-optimized-way/
.. _RFC 4122: https://tools.ietf.org/html/rfc4122
