Storing files on disk vs memory in Express - Hands-On Comparison

Storing Files on Disk vs Memory in Express

📖 Scenario: You are building a simple Express server that accepts file uploads. You want to learn how to store uploaded files either directly on the server's disk or temporarily in memory.This is useful when you want to process files quickly without saving them permanently, or when you want to save files for later use.

🎯 Goal: Create an Express server with two routes: one that saves uploaded files to disk, and another that stores files in memory. You will configure the file upload middleware accordingly and handle the uploaded files.

📋 What You'll Learn

Create an Express app with the express package

Use multer middleware for file uploads

Configure multer to store files on disk in a folder named uploads

Configure multer to store files in memory as buffers

Create two POST routes: /upload/disk and /upload/memory

Handle the uploaded file in each route and respond with the file's original name

💡 Why This Matters

🌍 Real World

Uploading files is common in web apps for user avatars, documents, or images. Knowing how to store files on disk or in memory helps optimize performance and storage.

💼 Career

Backend developers often handle file uploads. Understanding multer and Express routes is essential for building APIs that accept and process files.

1

Set up Express app and import multer

Create an Express app by importing express and multer. Then initialize the app with const app = express().

Express

const express = require('express')
const multer = require('multer')

const app = express()
// Your code here

Hint

Use require('express') and require('multer') to import the packages.

2

Configure multer to store files on disk

Create a multer storage configuration called storageDisk that saves files to the uploads folder on disk using multer.diskStorage(). Then create an upload middleware called uploadDisk using multer({ storage: storageDisk }).

Express

const express = require('express')
const multer = require('multer')

const app = express()

// Configure multer to store files on disk
// Your code here

Hint

Use multer.diskStorage() with destination and filename functions.

3

Configure multer to store files in memory

Create a multer upload middleware called uploadMemory that stores files in memory by setting storage to multer.memoryStorage().

Express

const express = require('express')
const multer = require('multer')

const app = express()

const storageDisk = multer.diskStorage({
  destination: function (req, file, cb) {
    cb(null, 'uploads')
  },
  filename: function (req, file, cb) {
    cb(null, file.originalname)
  }
})

const uploadDisk = multer({ storage: storageDisk })

// Configure multer to store files in memory
// Your code here

Hint

Use multer.memoryStorage() inside the multer() call.

4

Create routes to handle file uploads

Create two POST routes on the Express app: /upload/disk and /upload/memory. Use uploadDisk.single('file') middleware for the disk route and uploadMemory.single('file') middleware for the memory route. In each route handler, respond with JSON containing the uploaded file's original name using res.json({ filename: req.file.originalname }).

Express

const express = require('express')
const multer = require('multer')

const app = express()

const storageDisk = multer.diskStorage({
  destination: function (req, file, cb) {
    cb(null, 'uploads')
  },
  filename: function (req, file, cb) {
    cb(null, file.originalname)
  }
})

const uploadDisk = multer({ storage: storageDisk })

const uploadMemory = multer({ storage: multer.memoryStorage() })

// Create POST route /upload/disk using uploadDisk.single('file')
// Create POST route /upload/memory using uploadMemory.single('file')
// Respond with JSON containing the uploaded file's original name
// Your code here

Hint

Use app.post with the correct route paths and multer middleware. Access the uploaded file with req.file.originalname.

Practice

1. In Express, what is the main difference between storing uploaded files on disk versus in memory?

easy

A. Disk storage is faster than memory storage for file uploads.

B. Disk storage saves files physically on the server, memory storage keeps files temporarily in RAM.

C. Memory storage saves files permanently, disk storage deletes files after upload.

D. Disk storage only works with images, memory storage only works with text files.

Solution

1. Step 1: Understand disk storage in Express

   Disk storage saves uploaded files physically on the server's hard drive, making them persistent.

2. Step 2: Understand memory storage in Express

   Memory storage keeps files temporarily in RAM, which is faster but not persistent after server restarts.

3. Final Answer:

   Disk storage saves files physically on the server, memory storage keeps files temporarily in RAM. -> Option B

4. Quick Check:

   Disk = physical, Memory = temporary RAM [OK]

Hint: Disk = saved on server, Memory = temporary RAM [OK]

Common Mistakes:

• Thinking memory storage saves files permanently
• Assuming disk storage is always faster
• Confusing file types with storage methods

2. Which of the following is the correct way to configure multer for storing files in memory in Express?

easy

A. const upload = multer({ storage: multer.memoryStorage() });

B. const upload = multer({ storage: multer.diskStorage({}) });

C. const upload = multer({ dest: '/uploads' });

D. const upload = multer({ storage: 'memory' });

Solution

1. Step 1: Identify multer memory storage syntax

   Memory storage is set by calling multer.memoryStorage() and passing it to the storage option.

2. Step 2: Check options for correct syntax

   const upload = multer({ storage: multer.memoryStorage() }); correctly uses multer.memoryStorage() inside the storage property.

3. Final Answer:

   const upload = multer({ storage: multer.memoryStorage() }); -> Option A

4. Quick Check:

   Use multer.memoryStorage() for memory storage [OK]

Hint: Use multer.memoryStorage() to store files in RAM [OK]

Common Mistakes:

• Using string 'memory' instead of multer.memoryStorage()
• Confusing diskStorage with memoryStorage
• Setting dest property for memory storage

3. Given this Express code snippet using multer with memory storage:

const upload = multer({ storage: multer.memoryStorage() });
app.post('/upload', upload.single('file'), (req, res) => {
  console.log(req.file.buffer.length);
  res.send('File size: ' + req.file.size);
});

What will be logged and sent if a 5000-byte file is uploaded?

medium

A. Throws an error because buffer is not available

B. Logs undefined, sends 'File size: undefined'

C. Logs 0, sends 'File size: 0'

D. Logs 5000, sends 'File size: 5000'

Solution

1. Step 1: Understand multer memoryStorage behavior

   When using memoryStorage, the uploaded file is stored in req.file.buffer as a Buffer object containing the file data.

2. Step 2: Check properties used in code

   req.file.buffer.length gives the byte length of the file buffer, which will be 5000 for a 5000-byte file. req.file.size also holds the file size in bytes.

3. Final Answer:

   Logs 5000, sends 'File size: 5000' -> Option D

4. Quick Check:

   Memory storage buffer length = file size [OK]

Hint: Memory storage files have buffer and size properties [OK]

Common Mistakes:

• Assuming buffer is undefined in memoryStorage
• Confusing size with buffer length
• Expecting disk path properties in memory storage

4. You wrote this Express code to store files on disk:

const upload = multer({ storage: multer.diskStorage({
  destination: './uploads',
  filename: (req, file, cb) => cb(null, file.originalname)
}) });
app.post('/upload', upload.single('file'), (req, res) => {
  res.send('File saved');
});

But files are not saved and no error appears. What is the likely problem?

medium

A. The destination path './uploads' does not exist or lacks write permission.

B. filename callback must return a Promise instead of using cb.

C. upload.single should be upload.array for disk storage.

D. multer.diskStorage cannot be used with Express.

Solution

1. Step 1: Check diskStorage destination requirements

   The destination folder must exist and be writable by the server process; multer does not create folders automatically.

2. Step 2: Analyze why files are not saved

   If the './uploads' folder is missing or permission denied, multer silently fails to save files without throwing errors.

3. Final Answer:

   The destination path './uploads' does not exist or lacks write permission. -> Option A

4. Quick Check:

   Disk storage needs existing writable folder [OK]

Hint: Ensure upload folder exists and is writable [OK]

Common Mistakes:

• Expecting multer to create upload folders automatically
• Misusing filename callback with Promises
• Confusing single vs array upload methods

5. You want to upload large files in Express and process them quickly without saving to disk. Which approach is best and why?

hard

A. Use multer.diskStorage to save files on disk for persistence and later processing.

B. Use multer.memoryStorage to keep files in RAM for fast access but risk high memory use.

C. Use multer.memoryStorage but limit file size to avoid memory overflow.

D. Use multer.diskStorage with a temporary folder and delete files after processing.

Solution

1. Step 1: Consider large file upload challenges

   Large files can consume a lot of RAM if stored in memory, risking server crashes.

2. Step 2: Evaluate memoryStorage with file size limits

   Using memoryStorage with strict file size limits allows fast processing while preventing excessive memory use.

3. Step 3: Compare with diskStorage options

   Disk storage is persistent but slower; temporary disk storage with deletion adds complexity and latency.

4. Final Answer:

   Use multer.memoryStorage but limit file size to avoid memory overflow. -> Option C

5. Quick Check:

   MemoryStorage + size limit = fast and safe [OK]

Hint: Limit file size when using memory storage for large files [OK]

Common Mistakes:

• Ignoring memory limits causing crashes
• Assuming disk storage is always faster
• Not cleaning up temporary disk files