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109 lines
2.9 KiB
ReStructuredText
109 lines
2.9 KiB
ReStructuredText
=======================
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Starting to use RADOS
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=======================
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.. highlight:: python
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.. index:: RADOS, object
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Introduction
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============
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`RADOS` is the object storage component of Ceph.
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An object, in this context, means a named entity that has
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- a `name`: a sequence of bytes, unique within its container, that is
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used to locate and access the object
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- `content`: sequence of bytes
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- `metadata`: a mapping from keys to values, for example ``color:
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blue, importance: low``
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None of these have any prescribed meaning to Ceph, and can be freely
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chosen by the user.
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`RADOS` takes care of distributing the objects across the whole
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storage cluster and replicating them for fault tolerance.
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Installation
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============
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To use `RADOS`, you need to install a Ceph cluster. Follow the
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instructions in :doc:`/ops/install/index`. Continue with these
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instructions once you have a healthy cluster running.
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Setup
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=====
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First, we need to create a `pool` that will hold our assets. Follow
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the instructions in :ref:`create-new-pool`. Let's name the pool
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``assets``.
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Then, we need a ``client`` key that is authorized to access that
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pool. Follow the instructions in :ref:`add-new-key`. Let's set the
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``id`` of the key to be ``webapp``. You could set up one key per
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machine running the web service, or let them share a single key; your
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call. Make sure the keyring containing the new key is available on the
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machine running the asset management system.
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Then, authorize the key to access the new pool. Follow the
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instructions in :ref:`auth-pool`.
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Usage
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=====
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`RADOS` is accessed via a network protocol, implemented in the
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:doc:`/api/librados` and :doc:`/api/libradospp` libraries. There are
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also wrappers for other languages.
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.. todo:: link to python, phprados here
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Instead of a low-level programming library, you can also use a
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higher-level service, with user accounts, access control and such
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features, via the :ref:`radosgw` HTTP service. See :doc:`/ops/radosgw`
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for more.
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.. rubric:: Example: Asset management
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Let's say we write our asset management system in Python. We'll use
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the ``rados`` Python module for accessing `RADOS`.
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.. todo:: link to rados.py, where ever it'll be documented
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With the key we created in Setup_, we'll be able to open a RADOS
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connection::
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import rados
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r=rados.Rados('webapp')
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r.conf_read_file()
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r.connect()
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ioctx = r.open_ioctx('assets')
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and then write an object::
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# holding content fully in memory to make the example simpler;
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# see API docs for how to do this better
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ioctx.write_full('1.jpg', 'jpeg-content-goes-here')
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and read it back::
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# holding content fully in memory to make the example simpler;
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# see API docs for how to do this better
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content = ioctx.write_full('1.jpg')
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We can also manipulate the metadata related to the object::
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ioctx.set_xattr('1.jpg', 'content-type', 'image/jpeg')
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Now you can use these as fits the web server framework of your choice,
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passing the ``ioctx`` variable from initialization to the request
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serving function.
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