In-memory caching with node-cache in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is in-memory caching in the context of a Node.js server?

In-memory caching stores data temporarily inside the server's memory (RAM) to quickly serve repeated requests without recalculating or fetching from a database.

beginner

What is the main purpose of the node-cache library?

node-cache provides a simple way to store and retrieve data in memory with expiration times, helping improve app speed by avoiding repeated heavy operations.

intermediate

How do you set a value in node-cache with a time-to-live (TTL)?

Use cache.set(key, value, ttlInSeconds). The TTL defines how long the data stays in cache before it expires.

intermediate

What happens when you try to get a key from node-cache that has expired or does not exist?

The get method returns undefined, indicating the data is not available in cache and may need to be fetched or recalculated.

beginner

Why is in-memory caching not suitable for storing very large data or permanent storage?

Because memory is limited and data is lost when the server restarts, in-memory caching is best for small, temporary data to speed up responses.

Which of the following best describes node-cache?

A A library to store data temporarily in server memory with expiration.

B A database for permanent data storage.

C A tool to compress files on the server.

D A frontend UI framework.

node-cache is used for temporary in-memory data storage with TTL, not permanent storage or UI.

How do you retrieve a cached value using node-cache?

A cache.set(key)

B cache.get(key)

C cache.delete(key)

D cache.fetch(key)

The get method retrieves the value stored under the given key.

What does TTL stand for in caching?

A Time To Live

B Total Time Limit

C Temporary Transfer Link

D Timed Token Lock

TTL means Time To Live, the duration data stays valid in cache.

If a cached item expires, what will cache.get(key) return?

A null

B empty string

C false

D undefined

Expired or missing keys return undefined in node-cache.

Which scenario is best suited for using node-cache?

A Saving large video files.

B Storing user passwords permanently.

C Caching API responses to reduce database calls.

D Hosting a website's static files.

Caching API responses temporarily speeds up repeated requests without permanent storage.

Explain how you would use node-cache to improve performance in an Express app.

Think about saving repeated data to avoid slow operations.

What are the limitations of using in-memory caching with node-cache?

Consider what happens when the server stops or restarts.

Practice

1. What is the main purpose of using node-cache in an Express app?

easy

A. To permanently save data to a database

B. To temporarily store data in memory for faster access

C. To handle user authentication securely

D. To serve static files like images and CSS

Solution

1. Step 1: Understand what caching means

   Caching means storing data temporarily to avoid repeated slow operations.

2. Step 2: Identify node-cache role in Express

   node-cache stores data in memory temporarily to speed up data retrieval.

3. Final Answer:

   To temporarily store data in memory for faster access -> Option B

4. Quick Check:

   In-memory caching = temporary fast storage [OK]

Hint: Cache stores data temporarily to speed up access [OK]

Common Mistakes:

• Confusing cache with permanent database storage
• Thinking node-cache handles authentication
• Assuming node-cache serves static files

2. Which of the following is the correct way to create a new cache instance using node-cache?

easy

A. const cache = new nodeCache();

B. const cache = nodeCache.create();

C. const cache = new NodeCache();

D. const cache = NodeCache();

Solution

1. Step 1: Recall node-cache import and usage

   node-cache is imported as NodeCache and instantiated with new keyword.

2. Step 2: Check syntax correctness

   Correct syntax is: const cache = new NodeCache();

3. Final Answer:

   const cache = new NodeCache(); -> Option C

4. Quick Check:

   Use new NodeCache() to create cache [OK]

Hint: Use 'new' keyword with NodeCache class [OK]

Common Mistakes:

• Calling nodeCache.create() which does not exist
• Using lowercase 'nodeCache' instead of 'NodeCache'
• Omitting 'new' keyword when creating instance

3. Given the code snippet below, what will be the output when accessing /data twice quickly?

const express = require('express');
const NodeCache = require('node-cache');
const app = express();
const cache = new NodeCache();

app.get('/data', (req, res) => {
  const cached = cache.get('myData');
  if (cached) {
    res.send('From cache: ' + cached);
  } else {
    const freshData = 'Hello World';
    cache.set('myData', freshData, 10);
    res.send('Fresh data: ' + freshData);
  }
});

app.listen(3000);

medium

A. First request: Fresh data: Hello World; Second request: From cache: Hello World

B. Both requests: Fresh data: Hello World

C. Both requests: From cache: Hello World

D. First request: From cache: Hello World; Second request: Fresh data: Hello World

Solution

1. Step 1: Analyze cache check on first request

   Cache is empty initially, so first request sets and returns fresh data.

2. Step 2: Analyze cache check on second request

   Second request finds data in cache and returns cached data.

3. Final Answer:

   First request: Fresh data: Hello World; Second request: From cache: Hello World -> Option A

4. Quick Check:

   Cache empty first, then hit cache second [OK]

Hint: First request sets cache, second reads from cache [OK]

Common Mistakes:

• Assuming cache is always empty
• Thinking cache returns fresh data every time
• Mixing order of fresh and cached responses

4. Identify the error in this code snippet using node-cache in Express:

const NodeCache = require('node-cache');
const cache = NodeCache();

app.get('/item', (req, res) => {
  const item = cache.get('itemKey');
  if (!item) {
    cache.set('itemKey', 'value');
    res.send('Set new item');
  } else {
    res.send('Cached item: ' + item);
  }
});

medium

A. Cache key 'itemKey' is invalid

B. Express app is not imported

C. cache.set requires a callback function

D. Missing 'new' keyword when creating cache instance

Solution

1. Step 1: Check cache instance creation

   node-cache requires 'new NodeCache()' to create an instance.

2. Step 2: Identify missing 'new' keyword

   Code uses 'NodeCache()' without 'new', causing error.

3. Final Answer:

   Missing 'new' keyword when creating cache instance -> Option D

4. Quick Check:

   Always use 'new' with NodeCache [OK]

Hint: Remember 'new' keyword for NodeCache instance [OK]

Common Mistakes:

• Forgetting 'new' keyword
• Thinking cache.set needs callback
• Assuming cache keys must be special format

5. You want to cache user profile data for 5 minutes using node-cache. Which code snippet correctly sets the cache with expiration and retrieves it safely in Express?

hard

A. cache.set('user1', profile, 300); const data = cache.get('user1'); if (data) res.send(data); else res.send('No cache');

B. cache.set('user1', profile); const data = cache.get('user1', 300); if (data) res.send(data); else res.send('No cache');

C. cache.set('user1', profile, 5); const data = cache.get('user1'); if (data) res.send(data); else res.send('No cache');

D. cache.set('user1', profile, 300000); const data = cache.get('user1'); if (data) res.send(data); else res.send('No cache');

Solution

1. Step 1: Understand TTL unit in node-cache

   TTL (time to live) is in seconds, so 5 minutes = 300 seconds.

2. Step 2: Check correct usage of set and get

   cache.set(key, value, ttl) sets with expiration; cache.get(key) retrieves value.

3. Final Answer:

   cache.set('user1', profile, 300); const data = cache.get('user1'); if (data) res.send(data); else res.send('No cache'); -> Option A

4. Quick Check:

   TTL in seconds, set with 300 for 5 minutes [OK]

Hint: TTL is seconds; 5 minutes = 300 seconds [OK]

Common Mistakes:

• Using TTL in milliseconds instead of seconds
• Passing TTL to get() method incorrectly
• Setting TTL too low (5 seconds instead of 300)