SSR은 왜 주목받고 있을까?

yarn run v1.22.10
$ next start
ready - started server on 0.0.0.0:3000, url: http://localhost:3000
Error: Could not find a production build in the '~/.next' directory. Try building your app with 'next build' before starting the production server. https://nextjs.org/docs/messages/production-start-no-build-id
    at NextNodeServer.getBuildId (~/node_modules/next/dist/server/next-server.js:169:23)
    at new Server (~/node_modules/next/dist/server/base-server.js:58:29)
    at new NextNodeServer (~/node_modules/next/dist/server/next-server.js:70:9)
    at NextServer.createServer (~/node_modules/next/dist/server/next.js:140:16)
    at async ~/node_modules/next/dist/server/next.js:149:31
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

// ...
// next.js
async getServer() {
        if (!this.serverPromise) {
            setTimeout(getServerImpl, 10);
            this.serverPromise = this.loadConfig().then(async (conf)=>{
                this.server = await this.createServer({
                    ...this.options,
                    conf
                });
                if (this.preparedAssetPrefix) {
                    this.server.setAssetPrefix(this.preparedAssetPrefix);
                }
                return this.server;
            });
        }
        return this.serverPromise;
    }

// ...
// next.js
 async createServer(options) {
        if (options.dev) {
            const DevServer = require("./dev/next-dev-server").default;
            return new DevServer(options);
        }
        const ServerImplementation = await getServerImpl();
        return new ServerImplementation(options);
    }

// next-server.js
getBuildId() {
    const buildIdFile = (0, _path).join(this.distDir, _constants.BUILD_ID_FILE);
    try {
        return _fs.default.readFileSync(buildIdFile, "utf8").trim();
    } catch (err) {
        // 오류시, Error 메시지를 던져주는 부분
        if (!_fs.default.existsSync(buildIdFile)) {
            throw new Error(`Could not find a production build in the '${this.distDir}' directory. Try building your app with 'next build' before starting the production server. https://nextjs.org/docs/messages/production-start-no-build-id`);
        }
        throw err;
    }
}

yarn run v1.22.10
$ next start
ready - started server on 0.0.0.0:3000, url: http://localhost:3000

// ...
"scripts": {
"dev": "next dev",
"build": "next build",
"start": "next start",
"lint": "next lint"
},
// ...

export const commands: { [command: string]: () => Promise<cliCommand> } = {
  // ...
  start: () => Promise.resolve(require("../cli/next-start").nextStart),
  // ...
};

import { startServer } from "../server/lib/start-server";
//...
const nextStart = (argv) => {
  //...
  const keepAliveTimeout = keepAliveTimeoutArg
    ? Math.ceil(keepAliveTimeoutArg)
    : undefined;
  (0, _startServer)
    .startServer({
      dir,
      hostname: host,
      port,
      keepAliveTimeout,
    })
    .then(async (app) => {
      const appUrl = `http://${app.hostname}:${app.port}`;
      // 이 부분이 터미널에 찍히는 부분인 것을 알 수 있습니다!
      // ready - started server on 0.0.0.0:3000, url: http://localhost:3000
      Log.ready(`started server on ${host}:${app.port}, url: ${appUrl}`);
      await app.prepare();
    })
    .catch((err) => {
      console.error(err);
      process.exit(1);
    });
};

exports.nextStart = nextStart;

import type { NextServerOptions, NextServer, RequestHandler } from '../next'
import { warn } from '../../build/output/log'
import http from 'http'
import next from '../next'

interface StartServerOptions extends NextServerOptions {
  allowRetry?: boolean
  keepAliveTimeout?: number
}

export function startServer(opts: StartServerOptions) {
  let requestHandler: RequestHandler

  const server = http.createServer((req, res) => {
    return requestHandler(req, res)
  })

  if (opts.keepAliveTimeout) {
    server.keepAliveTimeout = opts.keepAliveTimeout
  }

  return new Promise<NextServer>((resolve, reject) => {
    let port = opts.port
    let retryCount = 0

    server.on('error', (err: NodeJS.ErrnoException) => {
      if (
        port &&
        opts.allowRetry &&
        err.code === 'EADDRINUSE' &&
        retryCount < 10
      ) {
        warn(`Port ${port} is in use, trying ${port + 1} instead.`)
        port += 1
        retryCount += 1
        server.listen(port, opts.hostname)
      } else {
        reject(err)
      }
    })

    let upgradeHandler: any

    if (!opts.dev) {
      server.on('upgrade', (req, socket, upgrade) => {
        upgradeHandler(req, socket, upgrade)
      })
    }

    server.on('listening', () => {
      const addr = server.address()
      const hostname =
        !opts.hostname || opts.hostname === '0.0.0.0'
          ? 'localhost'
          : opts.hostname

      const app = next({
        ...opts,
        hostname,
        customServer: false,
        httpServer: server,
        port: addr && typeof addr === 'object' ? addr.port : port,
      })

      requestHandler = app.getRequestHandler()
      upgradeHandler = app.getUpgradeHandler()
      resolve(app)
    })

    server.listen(port, opts.hostname)
  })
}