作者： JeffDean,SanjayGhemawat 原文： 译者： phylips@bmy2011-8-16 译文： 打开一个数据库 一个 LevelDB 数据库有一个文件系统目录名称与之关联。该数据库的所有内容都存储在该目录下。下面的例子展示了如何打开一个数据库，或者如何在必要的时候创建一个

作者：Jeff Dean, Sanjay Ghemawat

原文：

译者：phylips@bmy 2011-8-16

译文：

一个LevelDB数据库有一个文件系统目录名称与之关联。该数据库的所有内容都存储在该目录下。下面的例子展示了如何打开一个数据库，或者如何在必要的时候创建一个：

#include

#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());

...

如果你想在数据库已经存在的情况下，让上面的代码产生一个错误，需要再Open调用之前加入如下一行：

options.error_if_exists = true;

leveldb::Status s = ...;

if (!s.ok()) cerr << s.ToString() << endl;

在完成一个数据库的处理之后，直接删除该数据库对象即可。

... open the db as described above ...

... do something with db ...

delete db;

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);

#include "leveldb/write_batch.h"

...

std::string value;

leveldb::Status s = db->Get(leveldb::ReadOptions(), key1, &value);

if (s.ok()) {

leveldb::WriteBatch batch;

batch.Delete(key1);

batch.Put(key2, value);

s = db->Write(leveldb::WriteOptions(), &batch);

}

leveldb::WriteOptions write_options;

write_options.sync = true;

db->Put(write_options, ...);

下面的例子用来说明如何打印出数据库中的所有key value对。

leveldb::Iterator* it = db->NewIterator(leveldb::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;

for (it->Seek(start);

it->Valid() && it->key().ToString() < limit;

it->Next()) {

...

}

for (it->SeekToLast(); it->Valid(); it->Prev()) {

...

}

leveldb::ReadOptions options;

options.snapshot = db->GetSnapshot();

... apply some updates to db ...

leveldb::Iterator* iter = db->NewIterator(options);

... read using iter to view the state when the snapshot was created ...

delete iter;

db->ReleaseSnapshot(options.snapshot);

leveldb::Snapshot* snapshot;

leveldb::WriteOptions write_options;

write_options.post_write_snapshot = &snapshot;

leveldb::Status status = db->Write(write_options, ...);

... perform other mutations to db ...

leveldb::ReadOptions read_options;

read_options.snapshot = snapshot;

leveldb::Iterator* iter = db->NewIterator(read_options);

... read as of the state just after the Write call returned ...

delete iter;

db->ReleaseSnapshot(snapshot);

leveldb::Slice s1 = "hello";

std::string str("world");

leveldb::Slice s2 = str;

std::string str = s1.ToString();

assert(str == std::string("hello"));

。比如下面代码的就是有问题的：

leveldb::Slice slice;

if (...) {

std::string str = ...;

slice = str;

}

Use(slice);

class TwoPartComparator : public leveldb::Comparator {

public:

// Three-way comparison function:

// if a < b: negative result

// if a > b: positive result

// else: zero result

int Compare(const leveldb::Slice& a, const leveldb::Slice& b) const {

int a1, a2, b1, b2;

ParseKey(a, &a1, &a2);

ParseKey(b, &b1, &b2);

if (a1 < b1) return -1;

if (a1 > b1) return +1;

if (a2 < b2) return -1;

if (a2 > b2) return +1;

return 0;

}

// Ignore the following methods for now:

const char* Name() const { return "TwoPartComparator"; }

void FindShortestSeparator(std::string*, const leveldb::Slice&) const { }

void FindShortSuccessor(std::string*) const { }

};

现在使用定制的比较器，创建一个数据库:

TwoPartComparator cmp;

leveldb::DB* db;

leveldb::Options options;

options.create_if_missing = true;

options.comparator = &cmp;

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

向后兼容(Backwards compatibility)

性能

可以通过改变include/leveldb/options.h里的默认值来对性能进行调整优化。