2.3 RADOS与LIBRADOS
LIBRADOS模块是客户端用来访问RADOS对象存储设备的。Ceph存储集群提供了消息传递层协议，用于客户端与Ceph Monitor与OSD交互，LIBRADOS以库形式为Ceph Client提供了这个功能，LIBRADOS就是操作RADOS对象存储的接口。所有Ceph客户端可以用LIBRADOS或LIBRADOS里封装的相同功能和对象存储交互，LIBRBD和LIBCEPHFS就利用了此功能。你可以用LIBRADOS直接和Ceph交互（如与Ceph兼容的应用程序、Ceph接口等）。下面是简单描述的步骤。
第1步：获取LIBRADOS。
第2步：配置集群句柄。
第3步：创建IO上下文。
第4步：关闭连接。
LIBRADOS架构图，如图2-5所示。
先根据配置文件调用librados创建一个rados，接下来为这个rados创建一个radosclient，radosclient包含3个主要模块（finisher、Messager、Objector）。再根据pool创建对应的ioctx，在ioctx中能够找到radosclient。再调用osdc对生成对应osd请求，与OSD进行通信响应请求。
下面分别介绍LIBRADOS的C语言、Java语言和Python语言示例。
1. LIBRADOS C语言示例
下面是LIBRADOS C语言示例。
#include <stdio.h>
#include <string.h>
#include <rados/librados.h>
int main (int argc, char argv**)
{
/*声明集群句柄以及所需参数 */
rados_t cluster;
char cluster_name[] = “ceph”; //集群名称
char user_name[] = “client.admin”; //指定访问集群的用户，这里用admin
uint64_t flags;
rados_ioctx_t io; //rados上下文句柄
char *poolname = “data”; //目标pool名
char read_res[100];
char xattr[] = “en_US”;
/* 指定参数初始化句柄*/
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”);
}
/* 读取配置文件用来配置句柄*/
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”);
}
/* 分解参数 */
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”);
}
/* 连接集群*/
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”);
}
//创建rados句柄上下文
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”);
}
//写对象
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”);
}
//设置对象属性
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”);
}
rados_completion_t comp;
//确认异步rados句柄成功创建
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. */
//异步读对象
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);
}
//等待对象上的操作完成
rados_wait_for_complete(comp);
// 释放complete句柄
rados_aio_release(comp);
char xattr_res[100];
//获取对象
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);
}
//移除对象属性
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”);
}
//删除对象
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”);
}
rados_ioctx_destroy(io); //销毁io上下文
rados_shutdown(cluster); //销毁句柄
}
2. LIBRADOS Java语言示例
下面是LIBRADOS Java语言示例。
import com.ceph.rados.Rados;
import com.ceph.rados.RadosException;
import java.io.File;
public class CephClient {
public static void main (String args[]){
try {
//获取句柄
Rados cluster = new Rados(“admin”);
System.out.println(“Created cluster handle.”);
File f = new File(“/etc/ceph/ceph.conf”);
//读取配置文件
cluster.confReadFile(f);
System.out.println(“Read the configuration file.”);
//连接集群
cluster.connect();
System.out.println(“Connected to the cluster.”);
} catch (RadosException e) {
System.out.println(e.getMessage()+”:”+e.getReturnValue());
}
}
}
3. LIBRADOS Python语言示例
下面是LIBRADOS Python语言示例。
#!/usr/bin/python
#encoding=utf-8
import rados, sys
cluster = rados.Rados(conffile=’/etc/ceph/ceph.conf’) #获取句柄
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’) #获取pool的io上下文句柄
print “\nWriting object ‘hw’ with contents ‘Hello World!’ to pool ‘data’.”
ioctx.write(“hw”, “Hello World!”) #写入对象
print “Writing XATTR ‘lang’ with value ‘en_US’ to object ‘hw'”
ioctx.set_xattr(“hw”, “lang”, “en_US”) #设置对象的属性
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”) #读取对象
print “\n\nGetting XATTR ‘lang’ from object ‘hw'”
print ioctx.get_xattr(“hw”, “lang”) #读取对象属性
print “\nContents of object ‘bm’\n————————“
print ioctx.read(“bm”)
print “\nClosing the connection.”
ioctx.close() #关闭io上下文
print “Shutting down the handle.”
cluster.shutdown() #销毁句柄
