===========================
 Rados Gateway Data Layout
===========================

Although the source code is the ultimate guide, this document helps
users and
new developers get up to speed with the implementation details.

Introduction
------------

Swift offers something called a *container*, which we use interchangeably with
the S3 term *bucket*, so we say that RGW's buckets implement Swift containers.

This document does not consider how RGW _operates_ on these structures,
e.g. the use of ``encode()`` and ``decode()`` methods for serialization.

Conceptual View
---------------

Although RADOS only knows about pools and objects with their xattrs and
omap[1], conceptually RGW maintains three types of information:
metadata, bucket indexes, and (payload) data.

Metadata
^^^^^^^^

We have three 'sections' of metadata: 'user', 'bucket', and 'bucket.instance'.
You can use the following commands to inspect metadata entries: ::

    $ radosgw-admin metadata list
    $ radosgw-admin metadata list bucket
    $ radosgw-admin metadata list bucket.instance
    $ radosgw-admin metadata list user

    $ radosgw-admin metadata get bucket:<bucket>
    $ radosgw-admin metadata get bucket.instance:<bucket>:<bucket_id>
    $ radosgw-admin metadata get user:<user>   # get or set
    
Some variables have been used in above commands, they are:

- _user_: Holds user information
- _bucket_: Holds a mapping between bucket name and bucket instance id
- _bucket.instance_: Holds bucket instance information[2]

Each metadata entry is kept on a single RADOS object. See below for implementation details.

Note that the metadata is not indexed. When listing a metadata section we do a
RADOS ``pgls`` operation on the containing pool.

Bucket Index
^^^^^^^^^^^^

The bucket index is a different kind of metadata, and is kept separately. The bucket index holds
a key-value map attached to RADOS objects. By default it is a single RADOS object per
bucket, but it is possible since Hammer to shard that map over multiple RADOS
objects. The map itself is kept in omap, associated with each RADOS object.
The key of each omap is the name of the object, and the value holds some basic
metadata of that object -- metadata that shows up when listing the bucket.
Also, each omap holds a header, and we keep some bucket accounting metadata
in that header (number of objects, total size, etc.).

Note that we also hold other information in the bucket index, which is kept in
other key namespaces. We can hold the bucket index log there, and for versioned
objects there is more information that we keep on other keys.

Data
^^^^

Object data is kept in one or more RADOS objects for each RGW object.

Object Lookup Path
------------------

When accessing S3/Swift objects, REST APIs come to RGW with three parameters:
account information (access key in S3 or account name in Swift),
bucket or container name, and object name (or key). At present, RGW only
uses account information to find out the user ID and for access control.
Only the bucket name and object key are used to address the object in a pool.

The user ID in RGW is a string, typically the actual user name from the user
credentials and not a hashed or mapped identifier.

When accessing a user's data, the user record is loaded from an object
named ``<user_id>`` in pool ``default.rgw.meta`` with namespace ``users.uid``.

Bucket names are represented in the pool ``default.rgw.meta`` with namespace
``root``. The bucket record is
loaded in order to obtain the so-called marker, which serves as a bucket ID.

S3/Swift objects are located in a pool named like ``default.rgw.buckets.data``.
RADOS object names are ``<marker>_<key>``,
for example ``default.7593.4_image.png``, where the marker is ``default.7593.4``
and the key is ``image.png``. Since these concatenated names are not parsed,
only passed down to RADOS, the choice of the separator is not important and
causes no ambiguity. For the same reason, slashes are permitted in object
names (keys).

It is possible to create multiple data pools and make it so that
different users\` buckets will be created in different RADOS pools by default,
thus providing the necessary scaling. The layout and naming of these pools
is controlled by a 'policy' setting.[3]

An RGW object may comprise multiple RADOS objects, the first of which
is the ``HEAD`` that contains metadata including manifest, ACLs, content type,
ETag, and user-defined metadata. The metadata is stored in xattrs.
The ``HEAD` object may also inline up to :confval:`rgw_max_chunk_size` of object data, for efficiency
and atomicity.  This enables a convenenient tiering strategy:  index pools
are necessarily replicated (cannot be EC) and should be placed on fast SSD
OSDs.  With a mix of small/hot RGW objects and larger, warm/cold RGW
objects like video files, the larger objects will automatically be placed
in the ``buckets.data`` pool, which may be EC and/or slower storage like
HDDs or QLC SSDs.

The manifest describes how each RGW object is laid out across RADOS
objects.

Bucket and Object Listing
-------------------------

Buckets that belong to a given user are listed in an omap of a RADOS object named
``<user_id>.buckets`` (for example, ``foo.buckets``) in pool ``default.rgw.meta``
with namespace ``users.uid``.
These objects are accessed when listing buckets, when updating bucket
contents, and updating and retrieving bucket statistics (e.g. for quota).

See the user-visible, encoded class ``cls_user_bucket_entry`` and its
nested class ``cls_user_bucket`` for the values of these omap entries.

These listings are kept consistent with buckets in the pool named ``.rgw``.

Objects that belong to a given bucket are listed in a bucket index,
as discussed in sub-section 'Bucket Index' above. The default naming
for index objects is ``.dir.<marker>`` in pool ``default.rgw.buckets.index``.

Footnotes
---------

[1] Omap is a key-value store, associated with an object, in a way similar
to how Extended Attributes (XATTRs) are associated with a POSIX file. An object's omap
is not physically colocated with the object's payload data, and its precise
implementation is invisible to and immaterial to RGW daemons.

[2] Before the Dumpling release, the 'bucket.instance' metadata did not
exist and the 'bucket' metadata contained its information. It is possible
to encounter such buckets in old installations.

[3] Pool names changed with the Infernalis release.
If you are looking at an older setup, some details may be different. In
particular there was a different pool for each of the namespaces that are
now combined inside the ``default.root.meta`` pool.

Appendix: Compendium
--------------------

Known pools:

``.rgw.root``
  Region, zone, and global information records, one per object.

``<zone>.rgw.control``
  notify.<N>

``<zone>.rgw.meta``
  Multiple namespaces with different kinds of metadata:

  namespace: ``root``
    <bucket>
    ``.bucket.meta.<bucket>:<marker>``   # see put_bucket_instance_info()

    The tenant is used to disambiguate buckets, but not bucket instances.
    Example::

      .bucket.meta.prodtx:test%25star:default.84099.6
      .bucket.meta.testcont:default.4126.1
      .bucket.meta.prodtx:testcont:default.84099.4
      prodtx/testcont
      prodtx/test%25star
      testcont

  namespace: ``users.uid``
    Contains _both_ per-user information (RGWUserInfo) in "<user>" objects
    and per-user lists of buckets in omaps of "<user>.buckets" objects.
    The "<user>" may contain the tenant if non-empty, for example::

      prodtx$prodt
      test2.buckets
      prodtx$prodt.buckets
      test2

  namespace: ``users.email``
    Unimportant

  namespace: ``users.keys``
    example: ``47UA98JSTJZ9YAN3OS3O``

    This allows ``radosgw`` to look up users by their access keys during authentication.

  namespace: ``users.swift``
    test:tester

``<zone>.rgw.buckets.index``
  Objects are named ``.dir.<marker>``: each contains a bucket index.
  If the index is sharded, each shard appends the shard index after
  the marker.

``<zone>.rgw.buckets.data``
  example: ``default.7593.4__shadow_.488urDFerTYXavx4yAd-Op8mxehnvTI_1``
  <marker>_<key>

An example of a marker would be ``default.16004.1`` or `default.7593.4``.
The current format is ``<zone>.<instance_id>.<bucket_id>``. But once
generated, a marker is not parsed again, so its format may change
freely in the future.
