leveldb学习笔记（一） -- 整体架构和基本操作

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

using namespace std;
 
void test_leveldb()
{
    leveldb::DB* db;
    leveldb::Options options;
    options.create_if_missing = true;
    leveldb::Status status = leveldb::DB::Open(options, "/tmp/testdb", &db);
    assert(status.ok());
}

int main(int argc, char *argv[])
{
    test_leveldb();
    return 0;
}

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void test_leveldb_rw()
{
    leveldb::DB* db;

    // 初始化
    leveldb::Options options;
    options.create_if_missing = true;
    // 若已存在则DB::Open会报错退出
    // options.error_if_exists = true;
    leveldb::Status status = leveldb::DB::Open(options, "/tmp/testdb", &db);
    assert(status.ok());

    // 读写操作
    std::string value = "hello-leveldb";
    string key1 = "xdkey1";
    string key2 = "xdkey2";

    // 设置key1
    // leveldb::WriteOptions()默认构造一个sync为false的选项结构体
    // 声明为：Status Put(const WriteOptions& options, const Slice& key, const Slice& value);
    // class Slice是一个简单的包含指针和数据大小的类结构，可以通过char*/string来构造初始化
    leveldb::Status s = db->Put(leveldb::WriteOptions(), key1, value);
    assert(s.ok());
    cout << "set key:" << key1 << ", value:" << value << " ok" << endl;
    // 获取key1
    value="";
    // 声明为：Status Get(const ReadOptions& options, const Slice& key, std::string* value)
    s = db->Get(leveldb::ReadOptions(), key1, &value);
    assert(s.ok());
    cout << "get key:" << key1 << ", value:" << value << endl;

    // 设置key2的value为key1对应的value
    s = db->Put(leveldb::WriteOptions(), key2, value);
    assert(s.ok());
    cout << "set key:" << key2 << ", value:" << value << " ok" << endl;

    // 删除key1
    // 声明为：Status Delete(const WriteOptions& options, const Slice& key) 
    s = db->Delete(leveldb::WriteOptions(), key1);
    assert(s.ok());
    cout << "del key:" << key1 << " ok"<< endl;

    // 尝试获取key1
    s = db->Get(leveldb::ReadOptions(), key1, &value);
    if (!s.ok()) {
        cout << "get key:" << key1 << " error! errmsg: " << s.ToString() << endl;
    }

    // 获取key2
    value = "";
    s = db->Get(leveldb::ReadOptions(), key2, &value);
    if (!s.ok()){
        cout << "get key:" << key2 << " error! errmsg: " << s.ToString() << endl;
    }else{
        cout << "get key:" << key2 << ", value:" << value << endl;
    }

    // 清理数据库
    delete db;
}

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

    // 准备数据
    string key1 = "xdkey1";
    std::string value = "test-atomic-update";
    leveldb::Status s = db->Put(leveldb::WriteOptions(), key1, value);
    assert(s.ok());
    cout << "set key:" << key1 << ", value:" << value << endl;

    string key2 = "xdkey2";
    value = "";
    s = db->Get(leveldb::ReadOptions(), key1, &value);
    assert(s.ok());

    // 使用 WriteBatch
    leveldb::WriteBatch batch;
    batch.Delete(key1);
    batch.Put(key2, value);
    s = db->Write(leveldb::WriteOptions(), &batch);
    assert(s.ok());
    cout << "move key:" << key1 << " to key:" << key2 << endl;

    s = db->Get(leveldb::ReadOptions(), key1, &value);
    cout << "get key: " << key1 << " result:" << s.ToString() << endl;
    s = db->Get(leveldb::ReadOptions(), key2, &value);
    cout << "get key: " << key2 << " result:" << s.ToString() << ", value:" << value << endl;

    // 清理数据库
    delete db;
}

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

    s = db->Put(leveldb::WriteOptions(), "xdkey1", "itv1");
    assert(s.ok());
    s = db->Put(leveldb::WriteOptions(), "xdkey2", "itv2");
    assert(s.ok());
    s = db->Put(leveldb::WriteOptions(), "xdkey3", "itv3");
    assert(s.ok());
    s = db->Put(leveldb::WriteOptions(), "xdkey4", "itv4");
    assert(s.ok());

    // 创建 Iterator
    // 创建后是未初始化的，必须先调用某种Seek再使用它
    leveldb::Iterator* it = db->NewIterator(leveldb::ReadOptions());
    // 从头开始遍历所有记录，`SeekToFirst`
    cout << "scan first to last..." << endl;
    for (it->SeekToFirst(); it->Valid(); it->Next()) {
        // 注意Slice类需要调用下ToString()才转成std::string
        cout << it->key().ToString() << ": "  << it->value().ToString() << endl;
        assert(it->status().ok());  // Check for any errors found during the scan
    }

    // 也可以从尾部开始向前遍历所有记录，`SeekToLast`
    cout << "scan last to first..." << endl;
    for (it->SeekToLast(); it->Valid(); it->Prev()) {
        cout << it->key().ToString() << ": "  << it->value().ToString() << endl;
        assert(it->status().ok());  // Check for any errors found during the scan
    }

    // 也可以指定key的范围遍历，`Seek`并给定结束条件
    string start = "xdkey2";
    string end = "xdkey3";
    cout << "scan rang [" << start << ", " << end << "]..." << endl;
    // 自行控制结束条件
    for (it->Seek(start); it->Valid() && it->key().ToString() <= end; it->Next()) {
        cout << it->key().ToString() << ": "  << it->value().ToString() << endl;
        assert(it->status().ok());  // Check for any errors found during the scan
    }

    delete it;

    // 清理数据库
    delete db;
}

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// 文件位置：include/leveldb/db.h
  // Return a handle to the current DB state.  Iterators created with
  // this handle will all observe a stable snapshot of the current DB
  // state.  The caller must call ReleaseSnapshot(result) when the
  // snapshot is no longer needed.
  virtual const Snapshot* GetSnapshot() = 0;

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void test_snapshot()
{
    leveldb::DB* db;
    leveldb::Options options;
    options.create_if_missing = true;
    // 为防止之前记录影响观察，用个单独的数据库
    leveldb::Status s = leveldb::DB::Open(options, "/tmp/testdb_for_snapshot", &db);
    assert(s.ok());

    // 初始化数据
    s = db->Put(leveldb::WriteOptions(), "xdkey1", "itv1");
    assert(s.ok());
    s = db->Put(leveldb::WriteOptions(), "xdkey2", "itv2");
    assert(s.ok());
    s = db->Put(leveldb::WriteOptions(), "xdkey3", "itv3");
    assert(s.ok());
    s = db->Put(leveldb::WriteOptions(), "xdkey4", "itv4");
    assert(s.ok());

    // options.snapshot
    leveldb::ReadOptions options_sp;
    // GetSnapshot生成快照
    // 声明为：const Snapshot* GetSnapshot()，返回一个当前db状态的处理句柄
    // 当不再使用时，必须通过 ReleaseSnapshot(result) 释放
    options_sp.snapshot = db->GetSnapshot();
    cout << "get snapshot" << endl;

    // 快照后做些增删改之类的操作，用作对比
    db->Put(leveldb::WriteOptions(), "xdkey5", "itv5");
    cout << "[add xdkey5:itv5]" << endl;
    assert(s.ok());
    db->Put(leveldb::WriteOptions(), "xdkey1", "itv1_modify");
    cout << "[modify xdkey1:itv1_modify]" << endl;
    assert(s.ok());
    db->Delete(leveldb::WriteOptions(), "xdkey2");
    cout << "[delete xdkey2]" << endl;
    assert(s.ok());
    
    // 此处使用的ReadOptions是设置了快照的，所以只有当时snapshot的记录状态
    cout << "scan snapshot..." << endl;
    leveldb::Iterator* it = db->NewIterator(options_sp);
    for (it->SeekToFirst(); it->Valid(); it->Next()) {
        cout << it->key().ToString() << ":" << it->value().ToString() << endl;
    }

    // 使用普通ReadOptions
    cout << "scan ordinarily..." << endl;
    leveldb::Iterator* it_od = db->NewIterator(leveldb::ReadOptions());
    for (it_od->SeekToFirst(); it_od->Valid(); it_od->Next()) {
        cout << it_od->key().ToString() << ":" << it_od->value().ToString() << endl;
    }
    delete it;

    // 释放快照
    db->ReleaseSnapshot(options_sp.snapshot);

    // 清理数据库
    delete db;
}