Adding upstream version 18.2.2.upstream/18.2.2

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+==========================
+ Introduction to librados
+==========================
+
+The :term:`Ceph Storage Cluster` provides the basic storage service that allows
+:term:`Ceph` to uniquely deliver **object, block, and file storage** in one
+unified system. However, you are not limited to using the RESTful, block, or
+POSIX interfaces. Based upon :abbr:`RADOS (Reliable Autonomic Distributed Object
+Store)`, the ``librados`` API enables you to create your own interface to the
+Ceph Storage Cluster.
+
+The ``librados`` API enables you to interact with the two types of daemons in
+the Ceph Storage Cluster: 
+
+- The :term:`Ceph Monitor`, which maintains a master copy of the cluster map. 
+- The :term:`Ceph OSD Daemon` (OSD), which stores data as objects on a storage node.
+
+.. ditaa::
+            +---------------------------------+
+            |  Ceph Storage Cluster Protocol  |
+            |           (librados)            |
+            +---------------------------------+
+            +---------------+ +---------------+
+            |      OSDs     | |    Monitors   |
+            +---------------+ +---------------+
+
+This guide provides a high-level introduction to using ``librados``. 
+Refer to :doc:`../../architecture` for additional details of the Ceph
+Storage Cluster. To use the API, you need a running Ceph Storage Cluster. 
+See `Installation (Quick)`_ for details.
+
+
+Step 1: Getting librados
+========================
+
+Your client application must bind with ``librados`` to connect to the Ceph
+Storage Cluster. You must install ``librados`` and any required packages to
+write applications that use ``librados``. The ``librados`` API is written in
+C++, with additional bindings for C, Python, Java and PHP. 
+
+
+Getting librados for C/C++
+--------------------------
+
+To install ``librados`` development support files for C/C++ on Debian/Ubuntu
+distributions, execute the following:
+
+.. prompt:: bash $
+
+   sudo apt-get install librados-dev
+
+To install ``librados`` development support files for C/C++ on RHEL/CentOS
+distributions, execute the following:
+
+.. prompt:: bash $
+
+   sudo yum install librados2-devel
+
+Once you install ``librados`` for developers, you can find the required 
+headers for C/C++ under ``/usr/include/rados``:
+
+.. prompt:: bash $
+
+   ls /usr/include/rados
+
+
+Getting librados for Python
+---------------------------
+
+The ``rados`` module provides ``librados`` support to Python
+applications. You may install ``python3-rados`` for Debian, Ubuntu, SLE or
+openSUSE or the ``python-rados`` package for CentOS/RHEL.
+
+To install ``librados`` development support files for Python on Debian/Ubuntu
+distributions, execute the following:
+
+.. prompt:: bash $
+
+   sudo apt-get install python3-rados
+
+To install ``librados`` development support files for Python on RHEL/CentOS
+distributions, execute the following:
+
+.. prompt:: bash $
+
+   sudo yum install python-rados
+
+To install ``librados`` development support files for Python on SLE/openSUSE
+distributions, execute the following:
+
+.. prompt:: bash $
+
+   sudo zypper install python3-rados
+
+You can find the module under ``/usr/share/pyshared`` on Debian systems,
+or under ``/usr/lib/python*/site-packages`` on CentOS/RHEL systems.
+
+
+Getting librados for Java
+-------------------------
+
+To install ``librados`` for Java, you need to execute the following procedure:
+
+#. Install ``jna.jar``. For Debian/Ubuntu, execute:
+
+   .. prompt:: bash $
+
+      sudo apt-get install libjna-java
+
+   For CentOS/RHEL, execute:
+
+   .. prompt:: bash $
+
+      sudo yum install jna
+
+   The JAR files are located in ``/usr/share/java``.
+
+#. Clone the ``rados-java`` repository:
+
+   .. prompt:: bash $
+
+      git clone --recursive https://github.com/ceph/rados-java.git
+
+#. Build the ``rados-java`` repository:
+
+   .. prompt:: bash $
+
+      cd rados-java
+      ant
+
+   The JAR file is located under ``rados-java/target``.
+
+#. Copy the JAR for RADOS to a common location (e.g., ``/usr/share/java``) and 
+   ensure that it and the JNA JAR are in your JVM's classpath. For example:
+
+   .. prompt:: bash $
+
+      sudo cp target/rados-0.1.3.jar /usr/share/java/rados-0.1.3.jar
+      sudo ln -s /usr/share/java/jna-3.2.7.jar /usr/lib/jvm/default-java/jre/lib/ext/jna-3.2.7.jar  
+      sudo ln -s /usr/share/java/rados-0.1.3.jar  /usr/lib/jvm/default-java/jre/lib/ext/rados-0.1.3.jar
+
+To build the documentation, execute the following:
+
+.. prompt:: bash $
+
+   ant docs
+
+
+Getting librados for PHP
+-------------------------
+
+To install the ``librados`` extension for PHP, you need to execute the following procedure:
+
+#. Install php-dev. For Debian/Ubuntu, execute:
+
+   .. prompt:: bash $
+
+      sudo apt-get install php5-dev build-essential
+
+   For CentOS/RHEL, execute:
+
+   .. prompt:: bash $
+
+      sudo yum install php-devel
+
+#. Clone the ``phprados`` repository:
+
+   .. prompt:: bash $
+
+      git clone https://github.com/ceph/phprados.git
+
+#. Build ``phprados``:
+
+   .. prompt:: bash $
+
+      cd phprados
+      phpize
+      ./configure
+      make
+      sudo make install
+
+#. Enable ``phprados`` by adding the following line to ``php.ini``::
+   
+      extension=rados.so
+
+
+Step 2: Configuring a Cluster Handle
+====================================
+
+A :term:`Ceph Client`, via ``librados``, interacts directly with OSDs to store
+and retrieve data. To interact with OSDs, the client app must invoke
+``librados``  and connect to a Ceph Monitor. Once connected, ``librados``
+retrieves the  :term:`Cluster Map` from the Ceph Monitor. When the client app
+wants to read or write data, it creates an I/O context and binds to a
+:term:`Pool`. The pool has an associated :term:`CRUSH rule` that defines how it
+will place data in the storage cluster. Via the I/O context, the client 
+provides the object name to ``librados``, which takes the object name
+and the cluster map (i.e., the topology of the cluster) and `computes`_ the
+placement group and `OSD`_  for locating the data. Then the client application
+can read or write data. The client app doesn't need to learn about the topology
+of the cluster directly.
+
+.. ditaa::
+            +--------+  Retrieves  +---------------+
+            | Client |------------>|  Cluster Map  |
+            +--------+             +---------------+
+                 |
+                 v      Writes
+              /-----\
+              | obj |
+              \-----/
+                 |      To
+                 v
+            +--------+           +---------------+
+            |  Pool  |---------->| CRUSH Rule |
+            +--------+  Selects  +---------------+
+
+
+The Ceph Storage Cluster handle encapsulates the client configuration, including:
+
+- The `user ID`_ for ``rados_create()`` or user name for ``rados_create2()`` 
+  (preferred).
+- The :term:`cephx` authentication key
+- The monitor ID and IP address
+- Logging levels
+- Debugging levels
+
+Thus, the first steps in using the cluster from your app are to 1) create
+a cluster handle that your app will use to connect to the storage cluster,
+and then 2) use that handle to connect. To connect to the cluster, the
+app must supply a monitor address, a username and an authentication key
+(cephx is enabled by default).
+
+.. tip:: Talking to different Ceph Storage Clusters – or to the same cluster 
+   with different users – requires different cluster handles.
+
+RADOS provides a number of ways for you to set the required values. For
+the monitor and encryption key settings, an easy way to handle them is to ensure
+that your Ceph configuration file contains a ``keyring`` path to a keyring file
+and at least one monitor address (e.g., ``mon_host``). For example::
+
+	[global]
+	mon_host = 192.168.1.1
+	keyring = /etc/ceph/ceph.client.admin.keyring
+
+Once you create the handle, you can read a Ceph configuration file to configure
+the handle. You can also pass arguments to your app and parse them with the
+function for parsing command line arguments (e.g., ``rados_conf_parse_argv()``),
+or parse Ceph environment variables (e.g., ``rados_conf_parse_env()``). Some
+wrappers may not implement convenience methods, so you may need to implement
+these capabilities. The following diagram provides a high-level flow for the
+initial connection.
+
+
+.. ditaa::
+           +---------+     +---------+
+           | Client  |     | Monitor |
+           +---------+     +---------+
+                |               |
+                |-----+ create  |
+                |     | cluster |
+                |<----+ handle  |
+                |               |
+                |-----+ read    |
+                |     | config  |
+                |<----+ file    |
+                |               |
+                |    connect    |
+                |-------------->|
+                |               |
+                |<--------------|
+                |   connected   |
+                |               |
+
+
+Once connected, your app can invoke functions that affect the whole cluster
+with only the cluster handle. For example, once you have a cluster
+handle, you can:
+
+- Get cluster statistics
+- Use Pool Operation (exists, create, list, delete)
+- Get and set the configuration
+
+
+One of the powerful features of Ceph is the ability to bind to different pools.
+Each pool may have a different number of placement groups, object replicas and
+replication strategies. For example, a pool could be set up as a "hot" pool that
+uses SSDs for frequently used objects or a "cold" pool that uses erasure coding.
+
+The main difference in the various ``librados`` bindings is between C and
+the object-oriented bindings for C++, Java and Python. The object-oriented
+bindings use objects to represent cluster handles, IO Contexts, iterators,
+exceptions, etc.
+
+
+C Example
+---------
+
+For C, creating a simple cluster handle using the ``admin`` user, configuring
+it and connecting to the cluster might look something like this: 
+
+.. code-block:: c
+
+	#include <stdio.h>
+	#include <stdlib.h>
+	#include <string.h>
+	#include <rados/librados.h>
+
+	int main (int argc, const char **argv) 
+	{
+
+		/* Declare the cluster handle and required arguments. */
+		rados_t cluster;
+		char cluster_name[] = "ceph";
+		char user_name[] = "client.admin";
+		uint64_t flags = 0;
+	
+		/* Initialize the cluster handle with the "ceph" cluster name and the "client.admin" user */  
+		int err;
+		err = rados_create2(&cluster, cluster_name, user_name, flags);
+
+		if (err < 0) {
+			fprintf(stderr, "%s: Couldn't create the cluster handle! %s\n", argv[0], strerror(-err));
+			exit(EXIT_FAILURE);
+		} else {
+			printf("\nCreated a cluster handle.\n");
+		}
+
+
+		/* Read a Ceph configuration file to configure the cluster handle. */
+		err = rados_conf_read_file(cluster, "/etc/ceph/ceph.conf");
+		if (err < 0) {
+			fprintf(stderr, "%s: cannot read config file: %s\n", argv[0], strerror(-err));
+			exit(EXIT_FAILURE);
+		} else {
+			printf("\nRead the config file.\n");
+		}
+
+		/* Read command line arguments */
+		err = rados_conf_parse_argv(cluster, argc, argv);
+		if (err < 0) {
+			fprintf(stderr, "%s: cannot parse command line arguments: %s\n", argv[0], strerror(-err));
+			exit(EXIT_FAILURE);
+		} else {
+			printf("\nRead the command line arguments.\n");
+		}
+
+		/* Connect to the cluster */
+		err = rados_connect(cluster);
+		if (err < 0) {
+			fprintf(stderr, "%s: cannot connect to cluster: %s\n", argv[0], strerror(-err));
+			exit(EXIT_FAILURE);
+		} else {
+			printf("\nConnected to the cluster.\n");
+		}
+
+	}
+
+Compile your client and link to ``librados`` using ``-lrados``. For example:
+
+.. prompt:: bash $
+
+   gcc ceph-client.c -lrados -o ceph-client
+
+
+C++ Example
+-----------
+
+The Ceph project provides a C++ example in the ``ceph/examples/librados``
+directory. For C++, a simple cluster handle using the ``admin`` user requires
+you to initialize a ``librados::Rados`` cluster handle object:
+
+.. code-block:: c++
+
+	#include <iostream>
+	#include <string>
+	#include <rados/librados.hpp>
+
+	int main(int argc, const char **argv)
+	{
+
+		int ret = 0;
+
+		/* Declare the cluster handle and required variables. */	
+		librados::Rados cluster;
+		char cluster_name[] = "ceph";
+		char user_name[] = "client.admin";
+		uint64_t flags = 0; 
+	
+		/* Initialize the cluster handle with the "ceph" cluster name and "client.admin" user */ 
+		{
+			ret = cluster.init2(user_name, cluster_name, flags);
+			if (ret < 0) {
+				std::cerr << "Couldn't initialize the cluster handle! error " << ret << std::endl;
+				return EXIT_FAILURE;
+			} else {
+				std::cout << "Created a cluster handle." << std::endl;
+			}
+		}
+
+		/* Read a Ceph configuration file to configure the cluster handle. */	
+		{	
+			ret = cluster.conf_read_file("/etc/ceph/ceph.conf");	
+			if (ret < 0) {
+				std::cerr << "Couldn't read the Ceph configuration file! error " << ret << std::endl;
+				return EXIT_FAILURE;
+			} else {
+				std::cout << "Read the Ceph configuration file." << std::endl;
+			}
+		}
+		
+		/* Read command line arguments */
+		{
+			ret = cluster.conf_parse_argv(argc, argv);
+			if (ret < 0) {
+				std::cerr << "Couldn't parse command line options! error " << ret << std::endl;
+				return EXIT_FAILURE;
+			} else {
+				std::cout << "Parsed command line options." << std::endl;
+			}
+		}
+	
+		/* Connect to the cluster */
+		{
+			ret = cluster.connect();
+			if (ret < 0) {
+				std::cerr << "Couldn't connect to cluster! error " << ret << std::endl;
+				return EXIT_FAILURE;
+			} else {
+				std::cout << "Connected to the cluster." << std::endl;
+			}
+		}
+	
+		return 0;
+	}
+	
+
+Compile the source; then, link ``librados`` using ``-lrados``. 
+For example:
+
+.. prompt:: bash $
+
+   g++ -g -c ceph-client.cc -o ceph-client.o
+   g++ -g ceph-client.o -lrados -o ceph-client
+
+
+
+Python Example
+--------------
+
+Python uses the ``admin`` id and the ``ceph`` cluster name by default, and
+will read the standard ``ceph.conf`` file if the conffile parameter is
+set to the empty string. The Python binding converts C++ errors
+into exceptions.
+
+
+.. code-block:: python
+
+	import rados
+
+	try:
+		cluster = rados.Rados(conffile='')
+	except TypeError as e:
+		print('Argument validation error: {}'.format(e))
+		raise e
+		
+	print("Created cluster handle.")
+
+	try:
+		cluster.connect()
+	except Exception as e:
+		print("connection error: {}".format(e))
+		raise e
+	finally:
+		print("Connected to the cluster.")
+
+
+Execute the example to verify that it connects to your cluster:
+
+.. prompt:: bash $
+
+   python ceph-client.py
+
+
+Java Example
+------------
+
+Java requires you to specify the user ID (``admin``) or user name
+(``client.admin``), and uses the ``ceph`` cluster name by default . The Java
+binding converts C++-based errors into exceptions.
+
+.. code-block:: java
+
+	import com.ceph.rados.Rados;
+	import com.ceph.rados.RadosException;
+	
+	import java.io.File;
+	
+	public class CephClient {
+		public static void main (String args[]){
+	
+			try {
+				Rados cluster = new Rados("admin");
+				System.out.println("Created cluster handle.");
+	            
+				File f = new File("/etc/ceph/ceph.conf");
+				cluster.confReadFile(f);
+				System.out.println("Read the configuration file.");
+
+				cluster.connect();
+				System.out.println("Connected to the cluster.");            
+
+			} catch (RadosException e) {
+				System.out.println(e.getMessage() + ": " + e.getReturnValue());
+			}
+		}
+	}
+
+
+Compile the source; then, run it. If you have copied the JAR to
+``/usr/share/java`` and sym linked from your ``ext`` directory, you won't need
+to specify the classpath. For example:
+
+.. prompt:: bash $
+
+   javac CephClient.java
+   java CephClient
+
+
+PHP Example
+------------
+
+With the RADOS extension enabled in PHP you can start creating a new cluster handle very easily:
+
+.. code-block:: php
+
+	<?php
+
+	$r = rados_create();
+	rados_conf_read_file($r, '/etc/ceph/ceph.conf');
+	if (!rados_connect($r)) {
+		echo "Failed to connect to Ceph cluster";
+	} else {
+		echo "Successfully connected to Ceph cluster";
+	}
+
+
+Save this as rados.php and run the code:
+
+.. prompt:: bash $
+
+   php rados.php
+
+
+Step 3: Creating an I/O Context
+===============================
+
+Once your app has a cluster handle and a connection to a Ceph Storage Cluster,
+you may create an I/O Context and begin reading and writing data. An I/O Context
+binds the connection to a specific pool. The user must have appropriate
+`CAPS`_ permissions to access the specified pool. For example, a user with read
+access but not write access will only be able to read data. I/O Context 
+functionality includes:
+
+- Write/read data and extended attributes
+- List and iterate over objects and extended attributes
+- Snapshot pools, list snapshots, etc.
+
+
+.. ditaa::
+           +---------+     +---------+     +---------+
+           | Client  |     | Monitor |     |   OSD   |
+           +---------+     +---------+     +---------+
+                |               |               |
+                |-----+ create  |               |
+                |     | I/O     |               | 
+                |<----+ context |               |              
+                |               |               |
+                |  write data   |               |
+                |---------------+-------------->|
+                |               |               |
+                |  write ack    |               |
+                |<--------------+---------------|
+                |               |               |
+                |  write xattr  |               |
+                |---------------+-------------->|
+                |               |               |
+                |  xattr ack    |               |
+                |<--------------+---------------|
+                |               |               |
+                |   read data   |               |
+                |---------------+-------------->|
+                |               |               |
+                |   read ack    |               |
+                |<--------------+---------------|
+                |               |               |
+                |  remove data  |               |
+                |---------------+-------------->|
+                |               |               |
+                |  remove ack   |               |
+                |<--------------+---------------|
+
+
+
+RADOS enables you to interact both synchronously and asynchronously. Once your
+app has an I/O Context, read/write operations only require you to know the
+object/xattr name. The CRUSH algorithm encapsulated in ``librados`` uses the
+cluster map to identify the appropriate OSD. OSD daemons handle the replication,
+as described in `Smart Daemons Enable Hyperscale`_. The ``librados`` library also 
+maps objects to placement groups, as described in  `Calculating PG IDs`_.
+
+The following examples use the default ``data`` pool. However, you may also
+use the API to list pools, ensure they exist, or create and delete pools. For 
+the write operations, the examples illustrate how to use synchronous mode. For
+the read operations, the examples illustrate how to use asynchronous mode.
+
+.. important:: Use caution when deleting pools with this API. If you delete
+   a pool, the pool and ALL DATA in the pool will be lost.
+
+
+C Example
+---------
+
+
+.. code-block:: c
+
+	#include <stdio.h>
+	#include <stdlib.h>
+	#include <string.h>
+	#include <rados/librados.h>
+
+	int main (int argc, const char **argv) 
+	{
+		/* 
+		 * Continued from previous C example, where cluster handle and
+		 * connection are established. First declare an I/O Context. 
+		 */
+
+		rados_ioctx_t io;
+		char *poolname = "data";
+	
+		err = rados_ioctx_create(cluster, poolname, &io);
+		if (err < 0) {
+			fprintf(stderr, "%s: cannot open rados pool %s: %s\n", argv[0], poolname, strerror(-err));
+			rados_shutdown(cluster);
+			exit(EXIT_FAILURE);
+		} else {
+			printf("\nCreated I/O context.\n");
+		}
+
+		/* Write data to the cluster synchronously. */	
+		err = rados_write(io, "hw", "Hello World!", 12, 0);
+		if (err < 0) {
+			fprintf(stderr, "%s: Cannot write object \"hw\" to pool %s: %s\n", argv[0], poolname, strerror(-err));
+			rados_ioctx_destroy(io);
+			rados_shutdown(cluster);
+			exit(1);
+		} else {
+			printf("\nWrote \"Hello World\" to object \"hw\".\n");
+		}
+	
+		char xattr[] = "en_US";
+		err = rados_setxattr(io, "hw", "lang", xattr, 5);
+		if (err < 0) {
+			fprintf(stderr, "%s: Cannot write xattr to pool %s: %s\n", argv[0], poolname, strerror(-err));
+			rados_ioctx_destroy(io);
+			rados_shutdown(cluster);
+			exit(1);
+		} else {
+			printf("\nWrote \"en_US\" to xattr \"lang\" for object \"hw\".\n");
+		}
+	
+		/*
+		 * Read data from the cluster asynchronously. 
+		 * First, set up asynchronous I/O completion.
+		 */
+		rados_completion_t comp;
+		err = rados_aio_create_completion(NULL, NULL, NULL, &comp);
+		if (err < 0) {
+			fprintf(stderr, "%s: Could not create aio completion: %s\n", argv[0], strerror(-err));
+			rados_ioctx_destroy(io);
+			rados_shutdown(cluster);
+			exit(1);
+		} else {
+			printf("\nCreated AIO completion.\n");
+		}
+
+		/* Next, read data using rados_aio_read. */
+		char read_res[100];
+		err = rados_aio_read(io, "hw", comp, read_res, 12, 0);
+		if (err < 0) {
+			fprintf(stderr, "%s: Cannot read object. %s %s\n", argv[0], poolname, strerror(-err));
+			rados_ioctx_destroy(io);
+			rados_shutdown(cluster);
+			exit(1);
+		} else {
+			printf("\nRead object \"hw\". The contents are:\n %s \n", read_res);
+		}
+		
+		/* Wait for the operation to complete */
+		rados_aio_wait_for_complete(comp);
+		
+		/* Release the asynchronous I/O complete handle to avoid memory leaks. */
+		rados_aio_release(comp);		
+		
+	
+		char xattr_res[100];
+		err = rados_getxattr(io, "hw", "lang", xattr_res, 5);
+		if (err < 0) {
+			fprintf(stderr, "%s: Cannot read xattr. %s %s\n", argv[0], poolname, strerror(-err));
+			rados_ioctx_destroy(io);
+			rados_shutdown(cluster);
+			exit(1);
+		} else {
+			printf("\nRead xattr \"lang\" for object \"hw\". The contents are:\n %s \n", xattr_res);
+		}
+
+		err = rados_rmxattr(io, "hw", "lang");
+		if (err < 0) {
+			fprintf(stderr, "%s: Cannot remove xattr. %s %s\n", argv[0], poolname, strerror(-err));
+			rados_ioctx_destroy(io);
+			rados_shutdown(cluster);
+			exit(1);
+		} else {
+			printf("\nRemoved xattr \"lang\" for object \"hw\".\n");
+		}
+
+		err = rados_remove(io, "hw");
+		if (err < 0) {
+			fprintf(stderr, "%s: Cannot remove object. %s %s\n", argv[0], poolname, strerror(-err));
+			rados_ioctx_destroy(io);
+			rados_shutdown(cluster);
+			exit(1);
+		} else {
+			printf("\nRemoved object \"hw\".\n");
+		}
+
+	}
+
+
+
+C++ Example
+-----------
+
+
+.. code-block:: c++
+
+	#include <iostream>
+	#include <string>
+	#include <rados/librados.hpp>
+
+	int main(int argc, const char **argv)
+	{
+
+		/* Continued from previous C++ example, where cluster handle and
+		 * connection are established. First declare an I/O Context. 
+		 */
+
+		librados::IoCtx io_ctx;
+		const char *pool_name = "data";
+		
+		{
+			ret = cluster.ioctx_create(pool_name, io_ctx);
+			if (ret < 0) {
+				std::cerr << "Couldn't set up ioctx! error " << ret << std::endl;
+				exit(EXIT_FAILURE);
+			} else {
+				std::cout << "Created an ioctx for the pool." << std::endl;
+			}
+		}
+		
+
+		/* Write an object synchronously. */
+		{
+			librados::bufferlist bl;
+			bl.append("Hello World!");
+			ret = io_ctx.write_full("hw", bl);
+			if (ret < 0) {
+				std::cerr << "Couldn't write object! error " << ret << std::endl;
+				exit(EXIT_FAILURE);
+			} else {
+				std::cout << "Wrote new object 'hw' " << std::endl;
+			}
+		}
+		
+		
+		/*
+		 * Add an xattr to the object.
+		 */
+		{
+			librados::bufferlist lang_bl;
+			lang_bl.append("en_US");
+			ret = io_ctx.setxattr("hw", "lang", lang_bl);
+			if (ret < 0) {
+				std::cerr << "failed to set xattr version entry! error "
+				<< ret << std::endl;
+				exit(EXIT_FAILURE);
+			} else {
+				std::cout << "Set the xattr 'lang' on our object!" << std::endl;
+			}
+		}
+		
+		
+		/*
+		 * Read the object back asynchronously.
+		 */
+		{
+			librados::bufferlist read_buf;
+			int read_len = 4194304;
+
+			//Create I/O Completion.
+			librados::AioCompletion *read_completion = librados::Rados::aio_create_completion();
+			
+			//Send read request.
+			ret = io_ctx.aio_read("hw", read_completion, &read_buf, read_len, 0);
+			if (ret < 0) {
+				std::cerr << "Couldn't start read object! error " << ret << std::endl;
+				exit(EXIT_FAILURE);
+			}
+
+			// Wait for the request to complete, and check that it succeeded.
+			read_completion->wait_for_complete();
+			ret = read_completion->get_return_value();
+			if (ret < 0) {
+				std::cerr << "Couldn't read object! error " << ret << std::endl;
+				exit(EXIT_FAILURE);
+			} else {
+				std::cout << "Read object hw asynchronously with contents.\n"
+				<< read_buf.c_str() << std::endl;
+			}
+		}
+		
+		
+		/*
+		 * Read the xattr.
+		 */
+		{
+			librados::bufferlist lang_res;
+			ret = io_ctx.getxattr("hw", "lang", lang_res);
+			if (ret < 0) {
+				std::cerr << "failed to get xattr version entry! error "
+				<< ret << std::endl;
+				exit(EXIT_FAILURE);
+			} else {
+				std::cout << "Got the xattr 'lang' from object hw!"
+				<< lang_res.c_str() << std::endl;
+			}
+		}
+		
+		
+		/*
+		 * Remove the xattr.
+		 */
+		{
+			ret = io_ctx.rmxattr("hw", "lang");
+			if (ret < 0) {
+				std::cerr << "Failed to remove xattr! error "
+				<< ret << std::endl;
+				exit(EXIT_FAILURE);
+			} else {
+				std::cout << "Removed the xattr 'lang' from our object!" << std::endl;
+			}
+		}
+		
+		/*
+		 * Remove the object.
+		 */
+		{
+			ret = io_ctx.remove("hw");
+			if (ret < 0) {
+				std::cerr << "Couldn't remove object! error " << ret << std::endl;
+				exit(EXIT_FAILURE);
+			} else {
+				std::cout << "Removed object 'hw'." << std::endl;
+			}
+		}
+	}
+
+
+
+Python Example
+--------------
+
+.. code-block:: python
+
+	print("\n\nI/O Context and Object Operations")
+	print("=================================")
+	
+	print("\nCreating a context for the 'data' pool")
+	if not cluster.pool_exists('data'):
+		raise RuntimeError('No data pool exists')
+	ioctx = cluster.open_ioctx('data')
+	
+	print("\nWriting object 'hw' with contents 'Hello World!' to pool 'data'.")
+	ioctx.write("hw", b"Hello World!")
+	print("Writing XATTR 'lang' with value 'en_US' to object 'hw'")
+	ioctx.set_xattr("hw", "lang", b"en_US")
+	
+	
+	print("\nWriting object 'bm' with contents 'Bonjour tout le monde!' to pool
+	'data'.")
+	ioctx.write("bm", b"Bonjour tout le monde!")
+	print("Writing XATTR 'lang' with value 'fr_FR' to object 'bm'")
+	ioctx.set_xattr("bm", "lang", b"fr_FR")
+	
+	print("\nContents of object 'hw'\n------------------------")
+	print(ioctx.read("hw"))
+	
+	print("\n\nGetting XATTR 'lang' from object 'hw'")
+	print(ioctx.get_xattr("hw", "lang"))
+	
+	print("\nContents of object 'bm'\n------------------------")
+	print(ioctx.read("bm"))
+	
+	print("\n\nGetting XATTR 'lang' from object 'bm'")
+	print(ioctx.get_xattr("bm", "lang"))
+	
+	
+	print("\nRemoving object 'hw'")
+	ioctx.remove_object("hw")
+	
+	print("Removing object 'bm'")
+	ioctx.remove_object("bm")
+
+
+Java-Example
+------------
+
+.. code-block:: java
+
+	import com.ceph.rados.Rados;
+	import com.ceph.rados.RadosException;
+
+	import java.io.File;
+	import com.ceph.rados.IoCTX;
+
+	public class CephClient {
+        	public static void main (String args[]){
+
+                	try {
+				Rados cluster = new Rados("admin");
+				System.out.println("Created cluster handle.");
+
+                        	File f = new File("/etc/ceph/ceph.conf");
+                        	cluster.confReadFile(f);
+                        	System.out.println("Read the configuration file.");
+
+                        	cluster.connect();
+                        	System.out.println("Connected to the cluster.");
+
+				IoCTX io = cluster.ioCtxCreate("data");
+
+				String oidone = "hw";
+				String contentone = "Hello World!";
+				io.write(oidone, contentone); 
+
+				String oidtwo = "bm";
+				String contenttwo = "Bonjour tout le monde!";
+				io.write(oidtwo, contenttwo); 
+
+				String[] objects = io.listObjects();
+                       		for (String object: objects)
+					System.out.println(object);
+
+				io.remove(oidone);
+				io.remove(oidtwo);
+
+				cluster.ioCtxDestroy(io);
+
+                	} catch (RadosException e) {
+                        	System.out.println(e.getMessage() + ": " + e.getReturnValue());
+                	}
+        	}
+	}
+
+
+PHP Example
+-----------
+
+.. code-block:: php
+
+	<?php
+
+	$io = rados_ioctx_create($r, "mypool");
+	rados_write_full($io, "oidOne", "mycontents");
+	rados_remove("oidOne");
+	rados_ioctx_destroy($io);
+
+
+Step 4: Closing Sessions
+========================
+
+Once your app finishes with the I/O Context and cluster handle, the app should
+close the connection and shutdown the handle. For asynchronous I/O, the app
+should also ensure that pending asynchronous operations have completed.
+
+
+C Example
+---------
+
+.. code-block:: c
+
+	rados_ioctx_destroy(io);
+	rados_shutdown(cluster);	
+
+
+C++ Example
+-----------
+
+.. code-block:: c++
+
+	io_ctx.close();
+	cluster.shutdown();
+
+
+Java Example
+--------------
+
+.. code-block:: java
+
+	cluster.ioCtxDestroy(io);
+	cluster.shutDown();
+	
+	
+Python Example
+--------------
+
+.. code-block:: python
+
+	print("\nClosing the connection.")
+	ioctx.close()
+	
+	print("Shutting down the handle.")
+	cluster.shutdown()
+
+PHP Example
+-----------
+
+.. code-block:: php
+
+	rados_shutdown($r);
+
+
+
+.. _user ID: ../../operations/user-management#command-line-usage
+.. _CAPS: ../../operations/user-management#authorization-capabilities
+.. _Installation (Quick): ../../../start
+.. _Smart Daemons Enable Hyperscale: ../../../architecture#smart-daemons-enable-hyperscale
+.. _Calculating PG IDs: ../../../architecture#calculating-pg-ids
+.. _computes: ../../../architecture#calculating-pg-ids
+.. _OSD: ../../../architecture#mapping-pgs-to-osds