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 
#include 
#include 
#include 
#include 
#include 
#include 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 
#include 
#include 
#include 
#include 
#include 
#include 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 putconst rocksdb::Snapshot *sp1 = db->GetSnapshot(); s = db->Put(rocksdb::WriteOptions(), "sp2", "value_sp2");assert(s.ok());// set a snapshot after putconst 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。