Real-time notification pattern in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to publish a message to a Redis channel named 'alerts'.

Redis

redis_client.publish('alerts', [1])

Drag options to blanks, or click blank then click option'

A'New alert!'

Balerts

Cpublish

D'alerts'

Attempts:

3 left

💡 Hint

Common Mistakes

Using the channel name as the message instead of a string message.

Forgetting to put quotes around the message.

The publish method requires the channel name and the message. The message here is a string 'New alert!'.

2fill in blank

medium

Complete the code to subscribe to the Redis channel 'alerts' using a Redis client.

Redis

redis_client.subscribe([1])

Drag options to blanks, or click blank then click option'

Aalerts

B'alerts'

C'messages'

D'subscribe'

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the channel name without quotes.

Using the wrong channel name.

To listen for messages, you subscribe to the channel named 'alerts' as a string.

3fill in blank

hard

Fix the error in the code to correctly handle incoming messages from the 'alerts' channel.

Redis

redis_client.on('message', function(channel, [1]) {
  console.log('Received:', [1])
});

Drag options to blanks, or click blank then click option'

Amsg

Bmessage

Cdata

Dalert

Attempts:

3 left

💡 Hint

Common Mistakes

Using a parameter name that does not match inside the function.

Confusing the channel name with the message parameter.

The callback function receives the channel and the message as parameters. 'msg' is a common name for the message parameter.

4fill in blank

hard

Fill both blanks to publish a JSON string notification with a type and content to the 'notifications' channel.

Redis

const notification = JSON.stringify({ type: [1], content: [2] });
redis_client.publish('notifications', notification);

Drag options to blanks, or click blank then click option'

A'info'

B'Hello, user!'

C'alert'

D'Welcome message'

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around string values.

Mixing up type and content values.

The notification object needs a type and content as strings. 'info' and 'Hello, user!' are valid values.

5fill in blank

hard

Fill all three blanks to subscribe to the 'notifications' channel and log parsed JSON messages with type and content.

Redis

redis_client.subscribe([1]);
redis_client.on('message', (channel, [2]) => {
  const data = JSON.parse([3]);
  console.log(`Type: ${data.type}, Content: ${data.content}`);
});

Drag options to blanks, or click blank then click option'

A'notifications'

Bmsg

Cmessage

D'alerts'

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong channel name in subscribe.

Parsing the wrong variable inside the callback.

Subscribe to 'notifications' channel, use 'msg' as the message parameter, and parse 'msg' to get the JSON data.

Practice

1. What is the main purpose of the PUBLISH command in Redis real-time notification pattern?

easy

A. To store data permanently

B. To send messages to a specific channel

C. To listen for messages on a channel

D. To delete a channel

Solution

1. Step 1: Understand the role of PUBLISH

   The PUBLISH command is used to send messages to a channel in Redis.

2. Step 2: Differentiate from SUBSCRIBE

   SUBSCRIBE listens for messages, but PUBLISH sends them.

3. Final Answer:

   To send messages to a specific channel -> Option B

4. Quick Check:

   PUBLISH sends messages [OK]

Hint: PUBLISH sends, SUBSCRIBE listens [OK]

Common Mistakes:

• Confusing PUBLISH with SUBSCRIBE
• Thinking PUBLISH stores data
• Assuming PUBLISH deletes channels

2. Which of the following is the correct syntax to subscribe to a channel named news in Redis?

easy

A. LISTEN news

B. PUBLISH news

C. SUBSCRIBE('news')

D. SUBSCRIBE news

Solution

1. Step 1: Recall Redis SUBSCRIBE syntax

   The correct syntax is SUBSCRIBE channel_name without parentheses.

2. Step 2: Check each option

   SUBSCRIBE news matches the correct syntax. SUBSCRIBE('news') uses parentheses which is invalid in Redis CLI.

3. Final Answer:

   SUBSCRIBE news -> Option D

4. Quick Check:

   SUBSCRIBE channel_name [OK]

Hint: SUBSCRIBE uses plain command and channel name [OK]

Common Mistakes:

• Using parentheses like a function call
• Confusing PUBLISH with SUBSCRIBE
• Using LISTEN which is not a Redis command

3. Consider this Redis command sequence:

SUBSCRIBE updates
PUBLISH updates "New message arrived"

What will the subscriber receive?

medium

A. "New message arrived" on the updates channel

B. No message because SUBSCRIBE blocks commands

C. An error because PUBLISH cannot be used after SUBSCRIBE

D. The message on a different channel

Solution

1. Step 1: Understand SUBSCRIBE behavior

   SUBSCRIBE listens to messages on the specified channel and receives published messages.

2. Step 2: Analyze PUBLISH effect

   PUBLISH sends the message "New message arrived" to the updates channel, which the subscriber listens to.

3. Final Answer:

   "New message arrived" on the updates channel -> Option A

4. Quick Check:

   SUBSCRIBE receives PUBLISH messages [OK]

Hint: SUBSCRIBE gets messages published to its channel [OK]

Common Mistakes:

• Thinking SUBSCRIBE blocks all commands
• Believing PUBLISH causes errors after SUBSCRIBE
• Assuming messages go to wrong channels

4. You wrote this Redis command sequence:

SUBSCRIBE updates
PUBLISH updates "Hello"
UNSUBSCRIBE updates
PUBLISH updates "Bye"

Why does the subscriber not receive the "Bye" message?

medium

A. Because SUBSCRIBE blocks PUBLISH commands

B. Because PUBLISH only works before SUBSCRIBE

C. Because UNSUBSCRIBE stops listening to the channel

D. Because messages are lost after first publish

Solution

1. Step 1: Understand UNSUBSCRIBE effect

   UNSUBSCRIBE removes the subscriber from the channel, so it stops receiving messages.

2. Step 2: Analyze message delivery after UNSUBSCRIBE

   After unsubscribing, the subscriber no longer listens, so "Bye" is not received.

3. Final Answer:

   Because UNSUBSCRIBE stops listening to the channel -> Option C

4. Quick Check:

   UNSUBSCRIBE stops message reception [OK]

Hint: UNSUBSCRIBE stops listening, no more messages [OK]

Common Mistakes:

• Thinking PUBLISH only works before SUBSCRIBE
• Assuming messages are lost after first publish
• Believing SUBSCRIBE blocks PUBLISH commands

5. You want to build a real-time notification system using Redis where multiple users subscribe to different channels. Which approach ensures users only get messages for their subscribed channels without missing any messages?

hard

A. Use PUBLISH/SUBSCRIBE pattern and keep subscribers connected continuously

B. Use Redis lists to push messages and clients poll periodically

C. Use PUBLISH to send messages and clients fetch keys manually

D. Use Redis transactions to lock channels during message send

Solution

1. Step 1: Understand real-time delivery needs

   Real-time notifications require instant delivery without polling delays.

2. Step 2: Evaluate options for message delivery

   PUBLISH/SUBSCRIBE keeps clients connected and delivers messages instantly to subscribed channels.

3. Step 3: Compare with alternatives

   Polling (Use Redis lists to push messages and clients poll periodically) causes delays; fetching keys manually (Use PUBLISH to send messages and clients fetch keys manually) is inefficient; transactions (Use Redis transactions to lock channels during message send) do not help with message delivery.

4. Final Answer:

   Use PUBLISH/SUBSCRIBE pattern and keep subscribers connected continuously -> Option A

5. Quick Check:

   Continuous SUBSCRIBE ensures real-time messages [OK]

Hint: Keep subscribers connected with PUBLISH/SUBSCRIBE [OK]

Common Mistakes:

• Using polling instead of real-time subscription
• Trying to fetch messages manually
• Misusing transactions for message delivery