Redis for distributed caching in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Redis caching code snippet?

Consider this Node.js code using Redis to cache a value and then retrieve it:

import { createClient } from 'redis';

async function cacheExample() {
  const client = createClient();
  await client.connect();

  await client.set('key1', 'value1', { EX: 10 });
  const result = await client.get('key1');

  await client.disconnect();
  return result;
}

cacheExample().then(console.log);

What will be printed to the console?

Node.js

import { createClient } from 'redis';

async function cacheExample() {
  const client = createClient();
  await client.connect();

  await client.set('key1', 'value1', { EX: 10 });
  const result = await client.get('key1');

  await client.disconnect();
  return result;
}

cacheExample().then(console.log);

A "value1"

B null

C undefined

D Throws a connection error

💡 Hint

Think about what happens immediately after setting a key with expiration.

The code sets a key 'key1' with value 'value1' and expiration of 10 seconds. Since the get happens immediately after setting, the key exists and returns 'value1'.

📝 Syntax

intermediate

Which option correctly sets a Redis key with a 5-minute expiration in Node.js?

Given the Redis client in Node.js, which code snippet correctly sets a key 'session' with value 'abc123' that expires after 5 minutes?

A await client.set('session', 'abc123', { EX: 300 });

B await client.set('session', 'abc123', { PX: 300000 });

C await client.set('session', 'abc123', 'EX', 300);

D await client.set('session', 'abc123', { EX: '300' });

💡 Hint

Check the official Redis client method signature for expiration options.

Option A uses the correct object syntax with EX (seconds) as a number. Option A is invalid syntax. Option A uses PX (milliseconds) but 300 ms is too short for 5 minutes; it should be 300000 ms. Option A passes EX as a string which is invalid.

🔧 Debug

advanced

Why does this Redis cache retrieval code throw an error?

Examine this code snippet:

import { createClient } from 'redis';

const client = createClient();

async function getCache() {
  const value = await client.get('missingKey');
  console.log(value.length);
}

getCache();

What error will this code throw and why?

Node.js

import { createClient } from 'redis';

const client = createClient();

async function getCache() {
  const value = await client.get('missingKey');
  console.log(value.length);
}

getCache();

A No error, prints 0

B ReferenceError: client is not defined

C SyntaxError: Unexpected token 'await'

D TypeError: Cannot read property 'length' of null

💡 Hint

What does Redis return when a key does not exist?

If the key does not exist, Redis returns null. Trying to access .length on null causes a TypeError.

❓ state_output

advanced

What is the value of 'count' after running this Redis increment code?

Consider this code snippet:

import { createClient } from 'redis';

async function incrementCounter() {
  const client = createClient();
  await client.connect();

  await client.set('count', '5');
  const count = await client.incr('count');

  await client.disconnect();
  return count;
}

incrementCounter().then(console.log);

What will be printed to the console?

Node.js

import { createClient } from 'redis';

async function incrementCounter() {
  const client = createClient();
  await client.connect();

  await client.set('count', '5');
  const count = await client.incr('count');

  await client.disconnect();
  return count;
}

incrementCounter().then(console.log);

A null

B 5

C 6

D Throws a TypeError

💡 Hint

What does the Redis INCR command do to the string value?

Redis INCR converts the string to a number, increments it by 1, and returns the new value. So '5' becomes 6.

🧠 Conceptual

expert

Which statement best explains Redis distributed caching behavior in a multi-instance Node.js app?

You have a Node.js app running on multiple servers. Each server connects to the same Redis instance for caching. Which statement is true about the cache behavior?

A Each server has its own isolated cache in Redis, so cache data is not shared across servers.

B All servers share the same cache data, so a cache set by one server is immediately visible to others.

C Redis automatically replicates cache data to each server's local memory for faster access.

D Cache data is only visible to the server that set it until Redis is restarted.

💡 Hint

Think about how Redis works as a centralized cache.

Redis is a centralized cache server. All connected clients share the same data. So when one server sets a cache key, others see it immediately.