leveldb基本知識

        levelDB是google開源的一個key-value存儲引擎庫，類似於開源的Lucene索引庫一樣。其他的軟件開發者可以利用該庫做二次開發，來滿足定製需求。LevelDB採用日誌式的寫方式來提高寫性能，但是犧牲了部分讀性能。爲了彌補犧牲了的讀性能，一些人提議使用SSD作爲存儲介質。

        對於本地化的Key-value存儲引擎來說，簡單的使用一般都分成三個基本的步驟：（1）打開一個數據庫實例；（2）對這個數據庫實例進行插入，修改和查詢操作；（3)最後在使用完成之後，關閉該數據庫。

Opening A Database

A leveldb database has a name which corresponds to a file system directory. All of the contents of database are stored in this directory. The following example shows how to open a database, creating it if necessary:

#include <assert>
#include "leveldb/db.h"

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

If you want to raise an error if the database already exists, add the following line before the leveldb::DB::Open call:

options.error_if_exists = true;

Status

You may have noticed the leveldb::Status type above. Values of this type are returned by most functions in leveldb that may encounter an error. You can check if such a result is ok, and also print an associated error message:

leveldb::Status s = ...;
if (!s.ok()) cerr << s.ToString() << endl;

Closing A Database

When you are done with a database, just delete the database object. Example:

... open the db as described above ...
... do something with db ...
delete db;

Reads And Writes

The database provides Put, Delete, and Get methods to modify/query the database. For example, the following code moves the value stored under key1 to key2.

std::string value;
leveldb::Status s = db->Get(leveldb::ReadOptions(), key1, &value);
if (s.ok()) s = db->Put(leveldb::WriteOptions(), key2, value);
if (s.ok()) s = db->Delete(leveldb::WriteOptions(), key1);

測試程序示例

#include <assert.h>
#include <iostream>
#include "leveldb/db.h"

using namespace std;
using namespace leveldb;

int main()
{
	DB* db;
	Options options;
	options.create_if_missing = true;
	//options.error_if_exists = true;

	Status status = DB::Open(options, "/tmp/testdb", &db);
	assert(status.ok());

	cout << status.ToString() << endl;

	string key1 = "hdu1", value1 = "123";
	string key2 = "hdu2", value2 = "456";
	string value;
	db->Put(WriteOptions(), key1, value1);
	db->Put(WriteOptions(), key2, value2);

	status = db->Get(ReadOptions(), key1, &value);
	if (status.ok())
		status = db->Put(WriteOptions(), key2, value);
	//if (status.ok())
	//	status = db->Delete(WriteOptions(), key1);
	
	status = db->Get(ReadOptions(), key1, &value);
	if (status.ok())
		cout << key1 << ": " << value << endl;
	status = db->Get(ReadOptions(), key2, &value);
	if (status.ok())
		cout << key2 << ": " << value << endl;

	delete db;

	return 0;
}

編譯程序

命令：g++ -o test test.cpp /home/luox28/leveldb/libleveldb.a -I/home/luox28/leveldb/include -lpthread

當然，在下載好了leveldb源碼後，進入到leveldb主目錄，編譯源碼。  cd leveldb && make all

參考資料

        1、leveldb自帶doc文檔

        2、leveldb入門知識