Cloud storage integration concept in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when uploading a file using Express with Multer and AWS S3?

Consider this Express route that uses Multer middleware to handle file uploads and then uploads the file to AWS S3. What is the expected behavior after a successful upload?

Express

const express = require('express');
const multer = require('multer');
const AWS = require('aws-sdk');
const app = express();
const upload = multer({ storage: multer.memoryStorage() });
const s3 = new AWS.S3();

app.post('/upload', upload.single('file'), async (req, res) => {
  const params = {
    Bucket: 'my-bucket',
    Key: req.file.originalname,
    Body: req.file.buffer
  };
  try {
    await s3.upload(params).promise();
    res.status(200).send('File uploaded successfully');
  } catch (err) {
    res.status(500).send('Upload failed');
  }
});

A The file is stored in memory temporarily, then uploaded to the S3 bucket with the original filename, and the client receives a success message.

B The file is saved to disk first, then uploaded to S3, and the client receives the file URL in the response.

C The file is uploaded directly to S3 without being processed by Multer, and the client receives an error.

D The file is stored in memory but never uploaded to S3, and the client receives a timeout error.

💡 Hint

Think about how Multer's memoryStorage works and what the AWS S3 upload method expects.

Multer with memoryStorage keeps the file in memory as a buffer. The AWS S3 upload method uses this buffer to upload the file to the specified bucket with the original filename. If successful, the client gets a success message.

📝 Syntax

intermediate

Identify the syntax error in this Express route integrating Google Cloud Storage

Which option correctly fixes the syntax error in this Express route that uploads a file to Google Cloud Storage?

Express

const express = require('express');
const {Storage} = require('@google-cloud/storage');
const multer = require('multer');
const app = express();
const storage = new Storage();
const bucket = storage.bucket('my-bucket');
const upload = multer({ storage: multer.memoryStorage() });

app.post('/upload', upload.single('file'), async (req, res) => {
  const blob = bucket.file(req.file.originalname);
  const blobStream = blob.createWriteStream();

  blobStream.on('error', (err) => {
    res.status(500).send(err.message);
  });

  blobStream.on('finish', () => {
    res.status(200).send('Upload complete');
  });

  blobStream.end(req.file.buffer);
});

A Add a return statement before res.status(200).send('Upload complete') to fix the missing return error.

B Change blobStream.end(req.file.buffer) to blobStream.write(req.file.buffer) to fix the stream usage.

C Add a semicolon after bucket.file(req.file.originalname) to fix the missing semicolon error.

D Add async keyword to the callback function to fix the missing async error.

💡 Hint

Look carefully at the line that creates the blob variable.

The line creating the blob variable is missing a semicolon, which causes a syntax error. Adding the semicolon fixes the syntax issue. The other options do not address syntax errors.

🔧 Debug

advanced

Why does this Express app fail to upload files to Azure Blob Storage?

Given this Express route, why does the file upload to Azure Blob Storage fail with a 500 error?

Express

const express = require('express');
const multer = require('multer');
const { BlobServiceClient } = require('@azure/storage-blob');
const app = express();
const upload = multer({ storage: multer.memoryStorage() });
const blobServiceClient = BlobServiceClient.fromConnectionString(process.env.AZURE_CONN_STRING);
const containerClient = blobServiceClient.getContainerClient('my-container');

app.post('/upload', upload.single('file'), async (req, res) => {
  try {
    const blockBlobClient = containerClient.getBlockBlobClient(req.file.originalname);
    await blockBlobClient.upload(req.file.buffer, req.file.buffer.length);
    res.status(200).send('Upload successful');
  } catch (error) {
    res.status(500).send('Upload failed');
  }
});

A The container name 'my-container' is invalid and causes the upload to fail.

B The upload method requires the buffer length as a second argument, which is missing.

C Multer's memoryStorage does not provide a buffer, so req.file.buffer is undefined.

D The BlobServiceClient is not initialized correctly without a SAS token.

💡 Hint

Check the Azure Blob Storage SDK documentation for the upload method signature.

The Azure Blob Storage upload method requires two arguments: the data buffer and its length. Omitting the length causes the upload to fail. Adding req.file.buffer.length as the second argument fixes the issue.

❓ state_output

advanced

What is the output of this Express route using AWS S3 upload with callback?

What response does the client receive when this Express route uploads a file to AWS S3 using a callback?

Express

const express = require('express');
const multer = require('multer');
const AWS = require('aws-sdk');
const app = express();
const upload = multer({ storage: multer.memoryStorage() });
const s3 = new AWS.S3();

app.post('/upload', upload.single('file'), (req, res) => {
  const params = {
    Bucket: 'my-bucket',
    Key: req.file.originalname,
    Body: req.file.buffer
  };
  s3.upload(params, (err, data) => {
    if (err) {
      res.status(500).send('Upload error');
    }
  });
  res.status(200).send('Upload started');
});

A The client immediately receives 'Upload started' before the upload finishes.

B The client receives 'Upload error' if the upload fails, otherwise 'Upload started'.

C The client waits until the upload finishes and then receives 'Upload started'.

D The client never receives a response because res.send is called twice.

💡 Hint

Consider the asynchronous nature of the s3.upload callback and when res.send is called.

The res.status(200).send('Upload started') is called immediately after s3.upload is initiated, without waiting for the callback. So the client gets this message right away. The callback's res.send is never called, which can cause issues but does not affect this immediate response.

🧠 Conceptual

expert

Which option best describes the advantage of using signed URLs for cloud storage uploads in Express?

When integrating cloud storage with Express, what is the main benefit of generating signed URLs for clients to upload files directly?

A It enables the server to store files temporarily before forwarding them to cloud storage.

B It encrypts the files automatically during upload to enhance security.

C It allows the Express server to validate file contents before upload to cloud storage.

D It reduces server load by allowing clients to upload files directly to cloud storage without passing through the Express server.

💡 Hint

Think about how signed URLs change the upload flow between client, server, and cloud storage.

Signed URLs let clients upload files directly to cloud storage, bypassing the Express server. This reduces server bandwidth and processing load. The other options describe unrelated or incorrect benefits.