基本接口

1. Put/Get

   • 使用key进行插入或查询

   •   //opening a database
       leveldb::DB* db;
       leveldb::Options options;
       options.create_if_missing = true;
       leveldb::Status status = leveldb::DB::Open(options, "testdb", &db);
       assert(status.ok());
     
       std::string key1="book";
       std::string value1="algorithm";
       std::string value;
       db->Put(WriteOptions(),key1,value1);
       db->Get(ReadOptions(),key1,&value); 
       std::cout<<"key:"<<key1<<",value:"<<value<<std::endl;
     

2. Snapshot

   • 可以查询历史版本的值，使用readoptions指定

   • std::string key1="fruit";
     std::string value1="apple";
     status=db->Put(leveldb::WriteOptions(),key1,value1);
     assert(status.ok());
     leveldb::ReadOptions readoptions;
     readoptions.snapshot=db->GetSnapshot();
     
     std::string value2="orange";
     status=db->Put(leveldb::WriteOptions(),key1,value2);
     assert(status.ok());
     std::string value;
     //status=db->Get(leveldb::ReadOptions(),key1,&value);
     // snapshot
     status=db->Get(readoptions,key1,&value);
     assert(status.ok());
     

3. Writebatch

   • 使用batch的方式进行写入，使用WriteBatch类进行write操作

   • std::string key2="fruit";
     std::string value2="apple";
     if (status.ok()) {
         leveldb::WriteBatch batch;
         batch.Delete(key1);
         batch.Put(key2, value);
         status = db->Write(leveldb::WriteOptions(), &batch);
     }
     

练习

1. 从文件读入数据并写入数据库（Batch）

   • std::string id, mail;
     std::ifstream inFile;
     leveldb::WriteBatch batch;
     inFile.open("/home/hadoop/Documents/data/PJ2/inputFile.csv");
     if (status.ok()) {
         while (getline(inFile, id, ',')) {
     //      std::cout << "ID: " << id << " ";
           getline(inFile, mail);
     //      std::cout << "mail: " << mail << std::endl;
           batch.Put(id, mail);
         }
       }
     inFile.close();
     // batch write
     status = db->Write(leveldb::WriteOptions(), &batch);
     

2. 通过Get接口获取读入数据库的指定属性ID号和邮箱

   • int fibonacci[1000];
     fibonacci[0] = 0;
     fibonacci[1] = 1;
     fibonacci[2] = 1;
     
     for (int i = 2; fibonacci[i-1] < 1000; i++) {
         fibonacci[i] = fibonacci[i - 1] + fibonacci[i - 2];
         status = db->Get(leveldb::ReadOptions(), std::to_string(fibonacci[i-1]), &mail);
         assert(status.ok());
         std::cout << fibonacci[i-1] << "\t" << mail << std::endl;
       }