PSUBSCRIBE for pattern matching in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to subscribe to all channels starting with 'news.'.

Redis

PSUBSCRIBE [1]

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

Anews+

Bnews?

Cnews.*

Dnews#

Attempts:

3 left

💡 Hint

Common Mistakes

Using '?' which matches exactly one character, not any sequence.

Using '+' or '#' which are not valid Redis pattern wildcards.

The pattern 'news.*' matches all channels starting with 'news.' followed by any characters.

2fill in blank

medium

Complete the code to subscribe to all channels ending with '.update'.

Redis

PSUBSCRIBE [1]

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

A+.update

B?update

C#.update

D*.update

Attempts:

3 left

💡 Hint

Common Mistakes

Using '?' which matches only one character.

Using '+' or '#' which are not Redis wildcards.

The pattern '*.update' matches any channel name that ends with '.update'.

3fill in blank

hard

Fix the error in the pattern to subscribe to channels with exactly one character after 'log'.

Redis

PSUBSCRIBE [1]

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

Alog?

Blog.*

Clog+

Dlog#

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' which matches any number of characters, not exactly one.

Using '+' or '#' which are invalid in Redis patterns.

The '?' wildcard matches exactly one character, so 'log?' matches channels like 'log1' or 'loga'.

4fill in blank

hard

Fill both blanks to subscribe to the channels 'chat.room1' and 'chat.room2'.

Redis

PSUBSCRIBE [1] [2]

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

Achat.room1

Bchat.room2

Cchat.*

D*.room1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'chat.*' which matches all channels starting with 'chat.', not just room1 or room2.

Using '*.room1' which matches any channel ending with 'room1', not starting with 'chat.'.

To subscribe to specific channels 'chat.room1' and 'chat.room2', list both patterns explicitly.

5fill in blank

hard

Fill all three blanks to subscribe to channels matching these patterns: any channel starting with 'user.', any channel ending with '.alert', and any channel containing 'error'.

Redis

PSUBSCRIBE [1] [2] [3]

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

Auser.*

B*.alert

C*error*

Derror.*

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'error.*' which matches channels starting with 'error.', not containing 'error' anywhere.

Using patterns without '*' which won't match variable parts.

The patterns 'user.*', '*.alert', and '*error*' match the required channel names for subscription.

Practice

1. What does the Redis command PSUBSCRIBE do?

easy

A. It subscribes to channels matching a pattern with wildcards.

B. It unsubscribes from all channels.

C. It publishes messages to a channel.

D. It deletes keys matching a pattern.

Solution

1. Step 1: Understand the purpose of PSUBSCRIBE

   PSUBSCRIBE listens to messages from channels that match a given pattern, using wildcards like '*'.

2. Step 2: Compare with other commands

   Unlike PUBLISH or UNSUBSCRIBE, PSUBSCRIBE focuses on pattern-based subscription.

3. Final Answer:

   It subscribes to channels matching a pattern with wildcards. -> Option A

4. Quick Check:

   PSUBSCRIBE = pattern subscription [OK]

Hint: PSUBSCRIBE listens to patterns, not single channels [OK]

Common Mistakes:

• Confusing PSUBSCRIBE with PUBLISH
• Thinking PSUBSCRIBE deletes keys
• Mixing PSUBSCRIBE with UNSUBSCRIBE

2. Which of the following is the correct syntax to subscribe to all channels starting with 'news.' using PSUBSCRIBE?

easy

A. PSUBSCRIBE news?

B. PSUBSCRIBE news.*

C. PSUBSCRIBE news#

D. PSUBSCRIBE news+

Solution

1. Step 1: Identify the wildcard for pattern matching

   In Redis PSUBSCRIBE, '*' matches any sequence of characters.

2. Step 2: Match the pattern for channels starting with 'news.'

   The pattern 'news.*' matches all channels beginning with 'news.' followed by anything.

3. Final Answer:

   PSUBSCRIBE news.* -> Option B

4. Quick Check:

   '*' wildcard = correct syntax [OK]

Hint: Use '*' to match any characters in PSUBSCRIBE patterns [OK]

Common Mistakes:

• Using '?' which matches exactly one character
• Using '#' or '+' which are not Redis wildcards
• Missing the dot after 'news'

3. Given the following Redis commands executed in order:

PSUBSCRIBE sports.*
PUBLISH sports.football "Goal!"
PUBLISH sports.basketball "Score!"
PUBLISH news.weather "Rain"

Which messages will the subscriber receive?

medium

A. "Goal!" and "Score!"

B. "Goal!" only

C. "Rain" only

D. All three messages

Solution

1. Step 1: Understand the subscription pattern

   The subscriber listens to channels matching 'sports.*', meaning any channel starting with 'sports.'.

2. Step 2: Check published messages against the pattern

   Messages on 'sports.football' and 'sports.basketball' match the pattern; 'news.weather' does not.

3. Final Answer:

   "Goal!" and "Score!" -> Option A

4. Quick Check:

   Pattern matches sports.* channels only [OK]

Hint: Only messages on matching channels are received [OK]

Common Mistakes:

• Assuming all published messages are received
• Confusing exact channel with pattern
• Ignoring the pattern prefix

4. You run the command PSUBSCRIBE sports.* but receive no messages when publishing to sports.football. What is the most likely mistake?

medium

A. You published messages before subscribing.

B. You used SUBSCRIBE instead of PSUBSCRIBE.

C. You subscribed to sports.* but published to sportsfootball without a dot.

D. You used the wrong Redis server version.

Solution

1. Step 1: Check the channel names for exact pattern match

   The pattern 'sports.*' matches channels starting with 'sports.' including the dot.

2. Step 2: Identify mismatch in published channel

   Publishing to 'sportsfootball' (no dot) does not match 'sports.*' pattern.

3. Final Answer:

   You subscribed to sports.* but published to sportsfootball without a dot. -> Option C

4. Quick Check:

   Pattern requires exact prefix including dot [OK]

Hint: Check exact channel names match pattern including dots [OK]

Common Mistakes:

• Confusing SUBSCRIBE with PSUBSCRIBE
• Ignoring dot in channel names
• Assuming order of commands affects delivery

5. You want to listen to all channels that contain the word 'chat' anywhere in their name using PSUBSCRIBE. Which pattern should you use?

hard

A. chat

B. chat*

C. *chat

D. *chat*

Solution

1. Step 1: Understand the wildcard '*' usage

   '*' matches any sequence of characters, including none.

2. Step 2: Create pattern to match 'chat' anywhere

   Using '*chat*' matches any channel with 'chat' anywhere in its name.

3. Final Answer:

   *chat* -> Option D

4. Quick Check:

   Use '*' before and after 'chat' to match anywhere [OK]

Hint: Use '*' before and after word to match anywhere [OK]

Common Mistakes:

• Using 'chat' alone matches exact channel only
• Using 'chat*' matches only channels starting with 'chat'
• Using '*chat' matches only channels ending with 'chat'