ceph/doc/rados/api/librados-intro.rst

==========================
 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
========================

The ``librados`` API is written in C, with additional bindings for C++, Python
and Java. Your client app needs to bind with ``librados``, which is part
of Ceph, but there are extra packages provided for writing applications
using librados directly which must be installed on your client host first.


Getting librados development support files for C/C++
----------------------------------------------------

To install ``librados`` development support files for C/C++, execute
the following for Debian/Ubuntu distributions::

	sudo apt-get install librados-dev

For CentOS/RHEL distributions, execute the following:: 

	sudo yum install ceph-devel

Once you've installed ``librados`` for developers, you can find the required 
headers for C/C++ under ``/usr/include/rados``. ::

	ls /usr/include/rados


Getting librados for Python
---------------------------

``librados`` Python support is the ``rados.py`` module, provided in
the ``python-ceph`` package for both Debian and RedHat systems. ::

	sudo apt-get install librados-dev

For CentOS/RHEL distributions, execute the following:: 

	sudo yum install ceph-devel

You can find the module under ``/usr/share/pyshared`` on Debian systems,
or under ``/usr/lib/python2.X/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:: 

	sudo apt-get install libjna-java

   For CentOS/RHEL, execute::

	sudo yum install jna

   The JAR files are located in ``/usr/share/java``.

#. Clone the ``rados-java`` repository::

	git clone --recursive https://github.com/ceph/rados-java.git

#. Build the ``rados-java`` repository:: 

	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::

	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::

	ant docs


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 in a manner that's substantially
transparent to the client app, the client app must invoke ``librados`` to
retrieve the cluster map. Ceph Clients retrieve a :term:`Cluster Map` from a
Ceph Monitor, and write objects to pools. Ceph's CRUSH algorithm determines how
Ceph will place data. ``librados`` will do this for your client app, so your
client app doesn't need to learn about the topology of the cluster.

.. ditaa:: 
            +--------+  Retrieves  +---------------+
            | Client |------------>|  Cluster Map  |
            +--------+             +---------------+
                 |
                 v      Writes
              /-----\
              | obj |
              \-----/
                 |      To
                 v
            +--------+           +---------------+
            |  Pool  |---------->| CRUSH Ruleset |
            +--------+  Selects  +---------------+


The Ceph Storage Cluster handle encapsulates the client configuration, including:

- The `user ID`_
- The authentication key
- The monitor ID and 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``). 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

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 <string.h>
	#include <rados/librados.h>

	int main (int argc, char argv**) 
	{

		/* Declare the cluster handle and required arguments. */
		rados_t cluster;
		char cluster_name[] = "ceph";
		char user_name[] = "client.admin";
		uint64_t flags; 
	
		/* 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:: 

	gcc ceph-client.c -lrados -o ceph-client


C++ Example
-----------

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; 
	
		/* 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;
				ret = EXIT_FAILURE;
				return 1;
			} 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;
				ret = EXIT_FAILURE;
				return 1;
			} 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;
				ret = EXIT_FAILURE;
				return 1;
			} 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;
				ret = EXIT_FAILURE;
				return 1;
			} else {
				std::cout << "Connected to the cluster." << std::endl;
			}
		}
	
		return 0;
	}
	


Compile the source; then, link ``librados`` using ``-lrados``. 
For example::

	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-based errors
into Python exceptions.


.. code-block:: python

	import rados

	try:
		cluster = rados.Rados(conffile='')
	except TypeError as e:
		print 'Argument validation error: ', e
		raise e
		
	print "Created cluster handle."

	try:
		cluster.connect()
	except Exception as e:
		print "connection error: ", e
		raise e
	finally:
		print "Connected to the cluster."


Execute the example to verify that it connects to your cluster. ::

	python ceph-client.py


Java Example
------------

Java requires you to specify the user ID, 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 {
				cluster = rados.Rados(None, "client.admin", "ceph")
				print "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::

	javac CephClient.java
	java CephClient


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 ID 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 mapping of objects to
placement groups is also performed by the library 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 <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_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("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", "Hello World!")
	print "Writing XATTR 'lang' with value 'en_US' to object 'hw'"
	ioctx.set_xattr("hw", "lang", "en_US")
	
	
	print "\nWriting object 'bm' with contents 'Bonjour tout le monde!' to pool 'data'."
	ioctx.write("bm", "Bonjour tout le monde!")
	print "Writing XATTR 'lang' with value 'fr_FR' to object 'bm'"
	ioctx.set_xattr("bm", "lang", "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 "Getting 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")



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();


Python Example
--------------

.. code-block:: python

	print "\nClosing the connection."
	ioctx.close()
	
	print "Shutting down the handle."
	cluster.shutdown()





.. _user ID: ../../operations/authentication#cephx-commandline-options
.. _CAPS: ../../operations/auth-intro#ceph-authorization-caps
.. _Installation (Quick): ../../../start
.. _Smart Daemons Enable Hyperscale: ../../../architecture#smart-daemons-enable-hyperscale
.. _Calculating PG IDs: ../../../architecture#calculating-pg-ids