Rocksdb iterator和snapshot 接口

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->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。