实现 memcached 客户端：TCP、连接池、一致性哈希、自定义协议

const HashRing = require('hashring');

// 输入集群地址构造 hash ring
const ring = new HashRing(['127.0.0.1:11211', '127.0.0.2:11211']);

// 输入 key 获取指定节点
const host = ring.get(key);

const net = require('net');

const socket = new net.Socket();
socket.connect({
    host: host,                     // 目标主机
    port: port,                     // 目标端口
    // localAddress: localAddress,  // 本地地址
    // localPort: localPort,        // 本地端口
});

socket.setKeepAlive(true);      // 保活

// 连接相关
socket.on('connect', () => {
    console.log(`socket connected`);
});
socket.on('error', error => {
    console.log(`socket error: ${error}`);
});
socket.on('close', hadError => {
    console.log(`socket closed, transmission error: ${hadError}`);
});

socket.on('data', data => {
    // 接受数据
});

socket.write(data); // 发送数据

const net = require('net');
const genericPool = require('generic-pool');

// 自定义创建连接池的函数
function _buildPool(remote_server) {
    const factory = {
        create: function () {
            return new Promise((resolve, reject) => {
                const host = remote_server.split(':')[0];
                const port = remote_server.split(':')[1];
                const socket = new net.Socket();
                socket.connect({
                    host: host, // 目标主机
                    port: port, // 目标端口
                });
                socket.setKeepAlive(true);
                socket.on('connect', () => {
                    console.log(`socket connected: ${remote_server} , local: ${socket.localAddress}:${socket.localPort}`);
                    resolve(socket);
                });
                socket.on('error', error => {
                    console.log(`socket error: ${remote_server} , ${error}`);
                    reject(error);
                });
                socket.on('close', hadError => {
                    console.log(`socket closed: ${remote_server} , transmission error: ${hadError}`);
                });
            });
        },
        destroy: function (socket) {
            return new Promise((resolve) => {
                socket.destroy();
                resolve();
            });
        },
        validate: function (socket) { // validate socket
            return new Promise((resolve) => {
                if (socket.connecting || socket.destroyed || !socket.readable || !socket.writable) {
                    return resolve(false);
                } else {
                    return resolve(true);
                }
            });
        }
    };
    const pool = genericPool.createPool(factory, {
        max: 10,            // 最大连接数
        min: 0,             // 最小连接数
        testOnBorrow: true, // 从池中取连接时进行 validate 函数验证
    });
    return pool;
}

// 连接池基本使用
const pool = _buildPool('127.0.0.1:11211'); // 构建连接池

const s = await pool.acquire();             // 从连接池中取连接

await pool.release(s);                      // 使用完成后释放连接

get <key>\r\n

VALUE <key> <flags> <bytes>\r\n
<data block>\r\n

// 定义一个请求方法并返回响应数据
function _request(command) {
    return new Promise(async (resolve, reject) => {
        try {
            // ...这里省略了连接池构建相关部分
            const s = await pool.acquire(); // 取连接

            const bufs = [];
            s.on('data', async buf => { // 监听 data 事件接受响应数据
                bufs.push(buf);

                const END_BUF = Buffer.from('\r\n'); // 数据接受完成的结束位
                if (END_BUF.equals(buf.slice(-2))) {
                    s.removeAllListeners('data'); // 移除监听
                    try {
                        await pool.release(s); // 释放连接
                    } catch (error) { }
                    const data = Buffer.concat(bufs).toString();
                    return resolve(data);
                }
            });

            s.write(command);
        } catch (error) {
            return reject(error);
        }
    });
}


// get
function get(key) {
    return new Promise(async (resolve, reject) => {
        try {
            const command = `get ${key}\r\n`;
            const data = await _request(key, command);

            // ...响应数据的处理，注意有省略

            // key not exist
            if (data === 'END\r\n') {
                return resolve(undefined);
            }

            /*
                VALUE <key> <flags> <bytesLength>\r\n
                <data block>\r\n
            */
            const data_arr = data.split('\r\n');
            const response_line = data_arr[0].split(' ');
            const value_flag = response_line[2];
            const value_length = Number(response_line[3]);

            let value = data_arr.slice(1, -2).join('');
            value = unescapeValue(value); // unescape \r\n
            // ...有省略
            return resolve(value);
        } catch (error) {
            return reject(error);
        }
    });
}

class Memcached {
    constructor(serverLocations, options) {
        this._configs = {
            ...{
                pool: {
                    max: 1,
                    min: 0,
                    idle: 30000,             // 30000 ms.
                },
                timeout: 5000,              // timeout for every command, 5000 ms.
                retries: 5,                 // max retry times for failed request.
                maxWaitingClients: 10000,   // maximum number of queued requests allowed
            }, ...options
        };
        this._hashring = new HashRing(serverLocations);
        this._pools = {}; // 通过 k-v 的形式存储具体的地址及它的连接池
    }

    _buildPool(remote_server) {
        // ...
    }

    _request(key, command) {
        // ...
    }


    // get
    async get(key) {
        // ...
    }

    // ... 其他方法
}

// 使用实例
const memcached = new Memcached(['127.0.0.1:11211'], {
    pool: {
        max: 10,
        min: 0
    }
});

const key = 'testkey';
const result = await memcached.get(key);