数据库连接的基础封装

两个问题

• 多次连接数据库
• 多次实例化

const Config = require("./config");

const MongoClient = require("mongodb").MongoClient;

const client = new MongoClient(Config.dbUrl, { useUnifiedTopology: true });

class Db {
  // 解决多次实例化，实例不共享的问题
  static getInstance() {
    if (!Db.instance) {
      Db.instance = new Db();
    }
    return Db.instance;
  }
  constructor() {
    this.dbClient = "";
    this.connect(); // 第一次实例化的时候，就开始连接数据库，这样效率更高
  }
  connect() {
    return new Promise((resolve, reject) => {
      // 把数据库的连接变成长连接，解决多次连接数据库的操作
      if (this.dbClient) {
        resolve(this.dbClient);
      } else {
        client.connect(err => {
          if (err) {
            reject(err);
          } else {
            this.dbClient = client.db(Config.dbName);
            resolve(this.dbClient);
          }
        });
      }
    });
  }
  find(collectionName, json) {
    return new Promise((resolve, reject) => {
      this.connect()
        .then(db => {
          const collection = db.collection(collectionName);
          collection.find(json).toArray((err, docs) => {
            if (err) {
              reject(err);
            } else {
              resolve(docs);
            }
          });
        })
        .catch(err => {
          reject(err);
        });
    });
  }
}

const db = Db.getInstance();
// const db = new Db();

console.time("db1");
db.find("student", {})
  .then(docs => {
    console.timeEnd("db1");
  })
  .catch(e => {
    console.log(e.message);
  });

setTimeout(() => {
  console.time("db2");
  db.find("student", {})
    .then(docs => {
      console.timeEnd("db2");
    })
    .catch(e => {
      console.log(e.message);
    });
}, 3000);

const db1 = Db.getInstance();
// const db1 = new Db();

setTimeout(() => {
  console.time("db3");
  db1
    .find("student", {})
    .then(docs => {
      console.timeEnd("db3");
    })
    .catch(e => {
      console.log(e.message);
    });
}, 5000);

setTimeout(() => {
  console.time("db4");
  db1
    .find("student", {})
    .then(docs => {
      console.timeEnd("db4");
    })
    .catch(e => {
      console.log(e.message);
    });
}, 7000);