APPEND for string concatenation in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the APPEND command do in Redis?

APPEND adds the specified value to the end of the string stored at a given key. If the key does not exist, it creates it with the value.

beginner

How does Redis handle APPEND if the key does not exist?

If the key does not exist, APPEND creates the key and sets its value to the appended string.

intermediate

What is the return value of the APPEND command in Redis?

APPEND returns the length of the string after the append operation.

intermediate

Can APPEND be used to concatenate multiple strings in Redis?

Yes, by calling APPEND multiple times on the same key, you can concatenate multiple strings step by step.

advanced

Is APPEND in Redis atomic, and why is that important?

Yes, APPEND is atomic, meaning it completes fully without interruption. This ensures safe string concatenation even with multiple clients.

What happens if you APPEND a string to a non-existing key in Redis?

A Redis returns an error.

B Redis creates the key with the appended string as its value.

C Redis ignores the command.

D Redis sets the key to an empty string.

APPEND creates the key with the given string if the key does not exist.

What does the APPEND command return after execution?

A The length of the string after appending.

B The original string length.

C The appended string itself.

D The number of keys in the database.

APPEND returns the new length of the string stored at the key.

Is APPEND in Redis an atomic operation?

A No, it can be interrupted.

B Only if used inside a transaction.

C Yes, it is atomic.

D Only on certain Redis versions.

APPEND is atomic, so it completes fully without interruption.

Which data type does APPEND work with in Redis?

A Lists

B Hashes

C Sets

D Strings

APPEND works only with string values in Redis.

How can you concatenate multiple strings using APPEND?

A By calling APPEND multiple times on the same key.

B By passing multiple strings in one APPEND command.

C By using APPEND with a list.

D By using APPEND with a hash.

You concatenate strings by calling APPEND repeatedly on the same key.

Explain how the APPEND command works in Redis and what happens if the key does not exist.

Think about adding text to a note that might be empty.

Describe why APPEND being atomic is important when multiple clients use Redis.

Imagine many people writing on the same paper at once.

Practice

1. What does the Redis APPEND command do when used on a string key?

easy

A. Returns the current string value without changes.

B. Replaces the entire string value with the new text.

C. Deletes the string key from the database.

D. Adds the given text to the end of the existing string value.

Solution

1. Step 1: Understand the APPEND command purpose

   The APPEND command adds new text to the end of an existing string without removing the old text.

2. Step 2: Compare options with command behavior

   Only Adds the given text to the end of the existing string value. correctly describes adding text to the end, not replacing or deleting.

3. Final Answer:

   Adds the given text to the end of the existing string value. -> Option D

4. Quick Check:

   APPEND adds text to end [OK]

Hint: APPEND never replaces, it always adds at the end [OK]

Common Mistakes:

• Thinking APPEND replaces the string
• Confusing APPEND with DELETE command
• Assuming APPEND returns the string value

2. Which of the following is the correct syntax to append the string 'world' to the key 'greeting' in Redis?

easy

A. APPEND greeting world

B. APPEND 'greeting' world

C. APPEND greeting 'world'

D. APPEND 'greeting' 'world'

Solution

1. Step 1: Recall Redis command syntax

   Redis commands use the format: COMMAND key value without quotes unless the value contains spaces.

2. Step 2: Identify correct option

   APPEND greeting world uses correct syntax: APPEND greeting world (no quotes needed here).

3. Final Answer:

   APPEND greeting world -> Option A

4. Quick Check:

   Redis commands: APPEND key value [OK]

Hint: Use APPEND key value without quotes unless spaces exist [OK]

Common Mistakes:

• Adding quotes unnecessarily around key or value
• Swapping key and value order
• Using commas or other separators

3. Given the following Redis commands executed in order:

SET key1 "Hello"
APPEND key1 " World"
APPEND key1 "!"

What will be the final length returned by the last APPEND command?

medium

A. 12

B. 13

C. 6

D. 11

Solution

1. Step 1: Calculate initial string length

   SET key1 "Hello" sets string length to 5 characters.

2. Step 2: Add " World" with APPEND

   " World" has 6 characters (including space), new length = 5 + 6 = 11.

3. Step 3: Add "!" with APPEND

   Adding "!" (1 character) makes length 11 + 1 = 12, but note quotes are not stored.

4. Step 4: Recount carefully

   "Hello" (5) + " World" (6) + "!" (1) = 12 characters total.

5. Final Answer:

   12 -> Option A

6. Quick Check:

   5 + 6 + 1 = 12 [OK]

Hint: Count characters added stepwise, including spaces [OK]

Common Mistakes:

• Counting quotes as characters
• Forgetting spaces in appended strings
• Adding lengths incorrectly

4. You run the command APPEND mykey "text" but get an error: "WRONGTYPE Operation against a key holding the wrong kind of value". What is the most likely cause?

medium

A. The key 'mykey' does not exist yet.

B. The key 'mykey' holds a non-string value like a list or set.

C. The APPEND command syntax is incorrect.

D. Redis server is down.

Solution

1. Step 1: Understand the error message

   "WRONGTYPE" means the key exists but is not a string type.

2. Step 2: Analyze possible causes

   If the key held a string or did not exist, APPEND would work or create it. Syntax error or server down would cause different errors.

3. Final Answer:

   The key 'mykey' holds a non-string value like a list or set. -> Option B

4. Quick Check:

   WRONGTYPE means key type mismatch [OK]

Hint: WRONGTYPE means key is not a string [OK]

Common Mistakes:

• Assuming key absence causes WRONGTYPE
• Blaming syntax for WRONGTYPE error
• Ignoring key data type

5. You want to build a Redis string key 'log' that starts empty and appends multiple messages separated by commas. Which sequence of commands correctly achieves this without leading commas?

hard

A. APPEND log ",message1" APPEND log ",message2" APPEND log ",message3"

B. APPEND log "message1" APPEND log "message2" APPEND log "message3"

C. SET log "message1" APPEND log ",message2" APPEND log ",message3"

D. SET log "message1," APPEND log "message2," APPEND log "message3,"

Solution

1. Step 1: Initialize the key without leading comma

   Use SET to start 'log' with "message1" (no comma at start).

2. Step 2: Append subsequent messages with leading commas

   Use APPEND to add ",message2" and ",message3" to separate messages properly.

3. Step 3: Verify no leading comma and correct separators

   SET log "message1" APPEND log ",message2" APPEND log ",message3" correctly avoids leading comma and separates messages with commas.

4. Final Answer:

   SET log "message1" APPEND log ",message2" APPEND log ",message3" -> Option C

5. Quick Check:

   Start with SET, then APPEND with commas [OK]

Hint: Start with SET, then APPEND with commas to separate [OK]

Common Mistakes:

• Starting with APPEND adding commas first
• Appending without commas causing merged words
• Adding trailing commas unnecessarily