for ( it -> Seek ( small ) ;
it -> Valid ( ) && it -> key ( ) . ToString ( ) < big ;
it -> Next ( ) ) {
. . .
}
assert ( it -> status ( ) . ok ( ) ) ; // Check for any errors found during the scan
反向遍历db中的元素
for ( it -> SeekToLast ( ) ; it -> Valid ( ) ; it -> Prev ( ) ) {
. . .
}
assert ( it -> status ( ) . ok ( ) ) ; // Check for any errors found during the scan
反向遍历一个指定范围的key,如(small, big]
for ( it -> SeekForPrev ( start ) ;
it -> Valid ( ) && it -> key ( ) . ToString ( ) > limit ;
it -> Prev ( ) ) {
. . .
}
assert ( it -> status ( ) . ok ( ) ) ; // Check for any errors found during the scan
迭代器的接口可以算是 rocksdb针对客户端的核心接口，主要是提供排序以及高效查找的功能。
测试代码如下:
# include <iostream>
# include <string>
# include <rocksdb/db.h>
# include <rocksdb/iterator.h>
# include <rocksdb/table.h>
# include <rocksdb/options.h>
# include <rocksdb/env.h>
using namespace std ;
static string rand_key ( unsigned long long key_range ) {
char buff [ 30 ] ;
unsigned long long n = 1 ;
for ( int i = 1 ; i <= 4 ; ++ i ) {
n * = ( unsigned long long ) rand ( ) ;
}
sprintf ( buff , "%llu" , n % key_range ) ;
string k ( buff ) ;
return k ;
}
int main ( ) {
rocksdb : : DB * db ;
rocksdb : : Options option ;
option . create_if_missing = true ;
option . compression = rocksdb : : CompressionType : : kNoCompression ;
rocksdb : : Status s = rocksdb : : DB : : Open ( option , "./iterator_db" , & db ) ;
if ( ! s . ok ( ) ) {
cout << "Open failed with " << s . ToString ( ) << endl ;
exit ( 1 ) ;
}
rocksdb : : DestroyDB ( "./iterator_db" , option ) ;
cout << "seek all keys : " << endl ;
for ( int i = 0 ; i < 5 ; i ++ ) {
rocksdb : : Status s = db -> Put ( rocksdb : : WriteOptions ( ) ,
rand_key ( 9 ) , string ( 10 , ‘a‘ + ( i % 26 ) ) ) ;
if ( ! s . ok ( ) ) {
cout << "Put failed with " << s . ToString ( ) << endl ;
exit ( 1 ) ;
}
}
/* traverse rocksdb key-value */
rocksdb : : Iterator * it = db -> NewIterator ( rocksdb : : ReadOptions ( ) ) ;
for ( it -> SeekToFirst ( ) ; it -> Valid ( ) ; it -> Next ( ) ) {
cout << it -> key ( ) . ToString ( ) << ": " << it -> value ( ) . ToString ( ) << endl ;
}
string limit = "4" ;
string start = "2" ;
cout << "seek from ‘2‘ to ‘4‘ : " << endl ;
for ( it -> Seek ( start ) ; it -> Valid ( ) && it -> key ( ) . ToString ( ) < limit ;
it -> Next ( ) ) {
cout << it -> key ( ) . ToString ( ) << ": " << it -> value ( ) . ToString ( ) << endl ;
}
assert ( it -> status ( ) . ok ( ) ) ;
cout << "seek from last to start :" << endl ;
for ( it -> SeekToLast ( ) ; it -> Valid ( ) ; it -> Prev ( ) ) {
cout << it -> key ( ) . ToString ( ) << ": " << it -> value ( ) . ToString ( ) << endl ;
}
assert ( it -> status ( ) . ok ( ) ) ;
cout << "seek from ‘4‘ to ‘2‘ :" << endl ;
for ( it -> SeekForPrev ( limit ) ; it -> Valid ( ) && it -> key ( ) . ToString ( ) > start ;
it -> Prev ( ) ) {
cout << it -> key ( ) . ToString ( ) << ": " << it -> value ( ) . ToString ( ) << endl ;
}
assert ( it -> status ( ) . ok ( ) ) ;
delete it ;
db -> Close ( ) ;
delete db ;
return 0 ;
}
输出如下:
seek all keys : 3: cccccccccc 4: dddddddddd 7: bbbbbbbbbb 8: eeeeeeeeee seek from ‘2‘ to ‘4‘ : 3: cccccccccc seek from last to start : 8: eeeeeeeeee 7: bbbbbbbbbb 4: dddddddddd 3: cccccccccc seek from ‘4‘ to ‘2‘ : 4: dddddddddd 3: cccccccccc
且上层使用rocksdb迭代器接口时一般会和 snapshot 接口一同使用，用来实现MVCC的版本控制功能。
关于snapshot的实现，我们在Rocksdb事务：隔离性的实现中有提到，感兴趣的可以看看。
关于snapshot的客户端接口主要有:
sp1 = db->GetSnapshot(); 在当前db状态下创建一个snapshot，添加到内部维护的一个全局的snapshotImpl的双向链表中，并返回该snapshot的对象 read_option.snapshot = sp1; 将获取到的snapshot 传给read_option,进行Get操作 db->ReleaseSnapshot(sp1); 释放snapshot相关的资源（从双向链表中删除该节点）隔离性的测试代码如下：
# include <iostream>
# include <string>
# include <rocksdb/db.h>
# include <rocksdb/iterator.h>
# include <rocksdb/table.h>
# include <rocksdb/options.h>
# include <rocksdb/env.h>
using namespace std ;
int main ( ) {
rocksdb : : DB * db ;
rocksdb : : Options option ;
option . create_if_missing = true ;
option . compression = rocksdb : : CompressionType : : kNoCompression ;
rocksdb : : Status s = rocksdb : : DB : : Open ( option , "./iterator_db" , & db ) ;
if ( ! s . ok ( ) ) {
cout << "Open failed with " << s . ToString ( ) << endl ;
exit ( 1 ) ;
}
// set a snapshot before put
const rocksdb : : Snapshot * sp1 = db -> GetSnapshot ( ) ;
s = db -> Put ( rocksdb : : WriteOptions ( ) , "sp2" , "value_sp2" ) ;
assert ( s . ok ( ) ) ;
// set a snapshot after put
const rocksdb : : Snapshot * sp2 = db -> GetSnapshot ( ) ;
rocksdb : : ReadOptions read_option ;
read_option . snapshot = sp1 ;
string value = "" ;
//预期获取不到sp2的value，因为这里用的是sp1的快照
s = db -> Get ( read_option , "sp2" , & value ) ;
if ( value == "" ) {
cout << "Can‘t get sp2 at sp1!" << endl ;
}
read_option . snapshot = sp2 ;
// 能够获取到，使用的是sp2的快照，其是在put之后设置的
s = db -> Get ( read_option , "sp2" , & value ) ;
assert ( s . ok ( ) ) ;
if ( value != "" ) {
cout << "Got sp2‘s value: " << value << endl ;
}
db -> ReleaseSnapshot ( sp1 ) ;
db -> ReleaseSnapshot ( sp2 ) ;
输出如下：
Can ‘t get sp2 at sp1! Got sp2‘ s value: value_sp2
当然rocksdb也提供了更为复杂的mvcc特性，来以事务的方式支持不同的隔离级别。
Rocksdb提供迭代器来来访问整个db中的数据，就像STL中的迭代器功能一样，用来访问容器中的具体的数据。
访问形式以及访问接口有如下几种：
遍历所有的key-value //打开db，并初始化一个迭代器指针
rocksdb : : Iterator * it = db -> NewIterator ( rocksdb : : ReadOptions ( ) ) ;
for ( it -> SeekToFirst ( ) ; it -> Valid ( ) ; it -> Next ( ) ) {
cout << it -> key ( ) . ToString ( ) << ": " << it -> value ( ) . ToString ( ) << endl ;
}
assert ( it -> status ( ) . ok ( ) ) ; // Check for any errors found during the scan
delete it ;
输出一个范围内的key-value，[small, big)
for ( it -> Seek ( small ) ;
it -> Valid ( ) && it -> key ( ) . ToString ( ) < big ;
it -> Next ( ) ) {
. . .
}
assert ( it -> status ( ) . ok ( ) ) ; // Check for any errors found during the scan
反向遍历db中的元素
for ( it -> SeekToLast ( ) ; it -> Valid ( ) ; it -> Prev ( ) ) {
. . .
}
assert ( it -> status ( ) . ok ( ) ) ; // Check for any errors found during the scan
反向遍历一个指定范围的key,如(small, big]
for ( it -> SeekForPrev ( start ) ;
it -> Valid ( ) && it -> key ( ) . ToString ( ) > limit ;
it -> Prev ( ) ) {
. . .
}
assert ( it -> status ( ) . ok ( ) ) ; // Check for any errors found during the scan
迭代器的接口可以算是 rocksdb针对客户端的核心接口，主要是提供排序以及高效查找的功能。
测试代码如下:
# include <iostream>
# include <string>
# include <rocksdb/db.h>
# include <rocksdb/iterator.h>
# include <rocksdb/table.h>
# include <rocksdb/options.h>
# include <rocksdb/env.h>
using namespace std ;
static string rand_key ( unsigned long long key_range ) {
char buff [ 30 ] ;
unsigned long long n = 1 ;
for ( int i = 1 ; i <= 4 ; ++ i ) {
n * = ( unsigned long long ) rand ( ) ;
}
sprintf ( buff , "%llu" , n % key_range ) ;
string k ( buff ) ;
return k ;
}
int main ( ) {
rocksdb : : DB * db ;
rocksdb : : Options option ;
option . create_if_missing = true ;
option . compression = rocksdb : : CompressionType : : kNoCompression ;
rocksdb : : Status s = rocksdb : : DB : : Open ( option , "./iterator_db" , & db ) ;
if ( ! s . ok ( ) ) {
cout << "Open failed with " << s . ToString ( ) << endl ;
exit ( 1 ) ;
}
rocksdb : : DestroyDB ( "./iterator_db" , option ) ;
cout << "seek all keys : " << endl ;
for ( int i = 0 ; i < 5 ; i ++ ) {
rocksdb : : Status s = db -> Put ( rocksdb : : WriteOptions ( ) ,
rand_key ( 9 ) , string ( 10 , ‘a‘ + ( i % 26 ) ) ) ;
if ( ! s . ok ( ) ) {
cout << "Put failed with " << s . ToString ( ) << endl ;
exit ( 1 ) ;
}
}
/* traverse rocksdb key-value */
rocksdb : : Iterator * it = db -> NewIterator ( rocksdb : : ReadOptions ( ) ) ;
for ( it -> SeekToFirst ( ) ; it -> Valid ( ) ; it -> Next ( ) ) {
cout << it -> key ( ) . ToString ( ) << ": " << it -> value ( ) . ToString ( ) << endl ;
}
string limit = "4" ;
string start = "2" ;
cout << "seek from ‘2‘ to ‘4‘ : " << endl ;
for ( it -> Seek ( start ) ; it -> Valid ( ) && it -> key ( ) . ToString ( ) < limit ;
it -> Next ( ) ) {
cout << it -> key ( ) . ToString ( ) << ": " << it -> value ( ) . ToString ( ) << endl ;
}
assert ( it -> status ( ) . ok ( ) ) ;
cout << "seek from last to start :" << endl ;
for ( it -> SeekToLast ( ) ; it -> Valid ( ) ; it -><
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