levelDB

目录

1. Reads And Writes
2. Atomic Update
3. Concurrency
4. Iteration
5. Snapshots
6. Slice
7. Comparators
8. Performance

#levelDB简介
LevelDB is a fast key-value storage library written at Google that provides an ordered mapping from string keys to string values.

source code: google/leveldb
具体特性参考上面的Github说明页。

levelDB使用说明文档

#示例操作

#include "leveldb/db.h"
#include "leveldb/write_batch.h"
#include <iostream>
using namespace std;

int main()
{
    leveldb::DB *db;
    leveldb::Options options;
    options.create_if_missing = true;
    leveldb::Status status = leveldb::DB::Open(options, "/tmp/testdb", &db);
    assert(status.ok());

    //Reads And Writes
    leveldb::Status s = db->Put(leveldb::WriteOptions(), "os", "mac os"); 
    if(!s.ok())
    {
        cerr << s.ToString() << endl;  
        delete db; 
        return -1;
    }
    string value;
    s = db->Get(leveldb::ReadOptions(), "os", &value);
    if(!s.ok())
    {
        delete db;
        return -1;
    }

    cout << "os: " << value << endl;

    //Atomic Updates
        
    delete db;
    return 0;
}

#源码分析

Reads And Writes

read – Get
Get操作支持区间查找’Approximate Sizes’

leveldb::Range ranges[2];
ranges[0] = leveldb::Range("a", "c");
ranges[1] = leveldb::Range("x", "z");
uint64_t sizes[2];
leveldb::Status s = db->GetApproximateSizes(ranges, 2, sizes);

write – Put
其中Write操作又分为Synchronous Writes和asynchronous writes

Atomic Update

使用leveldb::WriteBatch类实现

Concurrency

leveldb支持单进程中多线程的并发，不支持多进程之间的并发.

Iteration

迭代器遍历levelDB

Snapshots

Slice

leveldb存储类型的底层实现为一个Slice结构

Comparators

支持自定义Comparators来对k/v进行排序

Performance

底层存储技术涉及memtable, sstable，Compression, Cache

##Filter

参考资料：
LevelDB Cache实现机制分析