MSET and MGET for bulk operations in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this MGET command?

Given the following Redis commands executed in order:

1. MSET user:1 "Alice" user:2 "Bob" user:3 "Charlie"
2. MGET user:1 user:2 user:4

What is the output of the MGET command?

Redis

MSET user:1 "Alice" user:2 "Bob" user:3 "Charlie"
MGET user:1 user:2 user:4

A ["Alice", "Bob", null]

B ["Alice", "Bob", ""]

C ["Alice", "Bob", "Charlie"]

D [null, null, null]

💡 Hint

MGET returns values for keys in order; missing keys return null.

MGET returns the values for the requested keys in the same order. Since user:4 does not exist, its value is null.

📝 Syntax

intermediate

Which MSET command is syntactically correct?

Choose the syntactically correct MSET command to set keys 'a' to '1' and 'b' to '2'.

A MSET [a,1,b,2]

B MSET {a:1, b:2}

C MSET (a=1, b=2)

D MSET a 1 b 2

💡 Hint

MSET expects key and value pairs as separate arguments.

MSET requires keys and values as separate arguments, not as objects or arrays.

❓ optimization

advanced

Why use MSET instead of multiple SET commands?

Which is the main advantage of using MSET over multiple individual SET commands in Redis?

A MSET encrypts keys and values for security.

B MSET executes all sets atomically and reduces network overhead.

C MSET allows setting keys with expiration times.

D MSET automatically compresses values to save memory.

💡 Hint

Think about network calls and atomicity.

MSET sends all key-value pairs in one command, reducing network round-trips and ensuring atomicity.

🔧 Debug

advanced

Identify the error in this MGET usage

What error occurs when running this command?

MGET user:1 user:2 user:3 user:4 user:5 user:6 user:7 user:8 user:9 user:10 user:11 user:12 user:13 user:14 user:15 user:16 user:17 user:18 user:19 user:20 user:21 user:22 user:23 user:24 user:25 user:26 user:27 user:28 user:29 user:30 user:31 user:32 user:33 user:34 user:35 user:36 user:37 user:38 user:39 user:40 user:41 user:42 user:43 user:44 user:45 user:46 user:47 user:48 user:49 user:50 user:51 user:52 user:53 user:54 user:55 user:56 user:57 user:58 user:59 user:60 user:61 user:62 user:63 user:64 user:65 user:66 user:67 user:68 user:69 user:70 user:71 user:72 user:73 user:74 user:75 user:76 user:77 user:78 user:79 user:80 user:81 user:82 user:83 user:84 user:85 user:86 user:87 user:88 user:89 user:90 user:91 user:92 user:93 user:94 user:95 user:96 user:97 user:98 user:99 user:100 user:101

Redis

MGET user:1 user:2 user:3 user:4 user:5 user:6 user:7 user:8 user:9 user:10 user:11 user:12 user:13 user:14 user:15 user:16 user:17 user:18 user:19 user:20 user:21 user:22 user:23 user:24 user:25 user:26 user:27 user:28 user:29 user:30 user:31 user:32 user:33 user:34 user:35 user:36 user:37 user:38 user:39 user:40 user:41 user:42 user:43 user:44 user:45 user:46 user:47 user:48 user:49 user:50 user:51 user:52 user:53 user:54 user:55 user:56 user:57 user:58 user:59 user:60 user:61 user:62 user:63 user:64 user:65 user:66 user:67 user:68 user:69 user:70 user:71 user:72 user:73 user:74 user:75 user:76 user:77 user:78 user:79 user:80 user:81 user:82 user:83 user:84 user:85 user:86 user:87 user:88 user:89 user:90 user:91 user:92 user:93 user:94 user:95 user:96 user:97 user:98 user:99 user:100 user:101

A Returns null for keys beyond 100

B Error: MGET supports a maximum of 100 keys per call

C No error, returns values for all keys

D SyntaxError: Too many arguments for MGET

💡 Hint

Redis MGET has no hard limit of 100 keys; it is limited practically by memory, network buffers, and performance considerations.

Redis MGET supports more than 100 keys per call. There is no error for 101 keys; it returns an array with values or null for each key in order.

🧠 Conceptual

expert

What happens if MSET is interrupted mid-operation?

Consider a Redis server crash occurring during an MSET command that sets multiple keys.

Which statement best describes the state of the keys after the server restarts?

A Either all keys are set or none are set, due to atomicity of MSET.

B Some keys may be set while others remain unchanged, partial update possible.

C Redis rolls back all changes automatically on crash.

D All keys are set but with empty string values.

💡 Hint

Think about atomicity guarantees of MSET in Redis.

MSET is atomic in Redis, so either all keys are set or none are set, even if interrupted.

Practice

1.

What does the Redis command MSET do?

easy

A. Increments multiple numeric keys

B. Stores multiple key-value pairs in one command

C. Deletes multiple keys at once

D. Retrieves multiple values for given keys

Solution

1. Step 1: Understand MSET purpose

   The MSET command is used to save many keys and their values in one go, making it efficient for bulk storage.

2. Step 2: Compare with other commands

   MGET retrieves values, DEL deletes keys, and INCR increments numeric keys, so these are not what MSET does.

3. Final Answer:

   Stores multiple key-value pairs in one command -> Option B

4. Quick Check:

   MSET stores many keys at once = D [OK]

Hint: MSET saves many keys and values together [OK]

Common Mistakes:

• Confusing MSET with MGET
• Thinking MSET deletes keys
• Assuming MSET increments values

2.

Which of the following is the correct syntax to set multiple keys key1 and key2 with values val1 and val2 using MSET?

?

easy

A. MSET {key1: val1, key2: val2}

B. MSET (key1, val1), (key2, val2)

C. MSET key1 val1 key2 val2

D. MSET [key1, val1, key2, val2]

Solution

1. Step 1: Recall MSET syntax

   The correct syntax for MSET is to list keys and values in sequence separated by spaces: MSET key1 val1 key2 val2.

2. Step 2: Identify incorrect formats

   Parentheses, braces, or brackets are not used in Redis commands for MSET. So options B, C, and D are invalid syntax.

3. Final Answer:

   MSET key1 val1 key2 val2 -> Option C

4. Quick Check:

   MSET uses space-separated key value pairs = A [OK]

Hint: MSET uses space-separated key value pairs [OK]

Common Mistakes:

• Using parentheses or braces in command
• Separating pairs with commas
• Trying to pass a dictionary or list

3.

Given the commands below, what will be the output of the MGET command?

MSET name Alice age 30 city Paris
MGET name age city

medium

A. ["Alice", "30", "Paris"]

B. ["name", "age", "city"]

C. ["Alice", 30, "Paris"]

D. ["Alice", null, "Paris"]

Solution

1. Step 1: Understand MSET effect

   The MSET command sets keys name, age, and city with values "Alice", "30", and "Paris" respectively as strings.

2. Step 2: Predict MGET output

   MGET name age city returns the values for these keys in order as strings: ["Alice", "30", "Paris"].

3. Final Answer:

   ["Alice", "30", "Paris"] -> Option A

4. Quick Check:

   MGET returns values in order = B [OK]

Hint: MGET returns values as strings in key order [OK]

Common Mistakes:

• Expecting numeric 30 instead of string "30"
• Confusing keys with values in output
• Assuming missing keys return null

4.

What is wrong with this Redis command sequence?

MSET key1 val1 key2
MGET key1 key2

medium

A. MSET is missing a value for key2, causing an error

B. MGET syntax is incorrect, keys must be comma-separated

C. MSET keys must be quoted strings

D. MGET cannot retrieve multiple keys

Solution

1. Step 1: Check MSET command completeness

   The MSET command requires pairs of keys and values. Here, key2 has no value, so the command is incomplete and causes an error.

2. Step 2: Verify MGET syntax

   MGET accepts space-separated keys, so the syntax is correct. Quoting keys is not required in Redis commands.

3. Final Answer:

   MSET is missing a value for key2, causing an error -> Option A

4. Quick Check:

   MSET needs key-value pairs = A [OK]

Hint: MSET needs pairs: key then value [OK]

Common Mistakes:

• Leaving out a value for a key in MSET
• Using commas in MGET keys
• Quoting keys unnecessarily

5.

You want to store user data for three users with keys user:1, user:2, and user:3 and their names Alice, Bob, and Carol. Which command correctly sets all three users at once and then fetches their names?

hard

A. SET user:1 Alice
SET user:2 Bob
SET user:3 Carol
GET user:1 user:2 user:3

B. MSET {user:1: Alice, user:2: Bob, user:3: Carol}
MGET [user:1, user:2, user:3]

C. MSET user:1=Alice user:2=Bob user:3=Carol
MGET user:1 user:2 user:3

D. MSET user:1 Alice user:2 Bob user:3 Carol
MGET user:1 user:2 user:3

Solution

1. Step 1: Use correct MSET syntax for multiple keys

   The MSET command requires keys and values listed in sequence separated by spaces, like MSET user:1 Alice user:2 Bob user:3 Carol.

2. Step 2: Use MGET with space-separated keys

   MGET fetches values for multiple keys separated by spaces, so MGET user:1 user:2 user:3 is correct.

3. Step 3: Identify incorrect options

   Options B and D use invalid syntax with braces or equals signs. SET user:1 Alice SET user:2 Bob SET user:3 Carol GET user:1 user:2 user:3 uses multiple SET commands and an invalid GET with multiple keys, which is not supported.

4. Final Answer:

   MSET user:1 Alice user:2 Bob user:3 Carol MGET user:1 user:2 user:3 -> Option D

5. Quick Check:

   MSET and MGET use space-separated keys and values = C [OK]

Hint: MSET and MGET use space-separated keys and values [OK]

Common Mistakes:

• Using braces or equals signs in MSET
• Trying to GET multiple keys at once
• Using multiple SET commands instead of MSET