EVAL command for Lua execution in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of a simple Lua script with EVAL

What is the output of this Redis EVAL command?

EVAL "return ARGV[1] .. ARGV[2]" 0 Hello World

Redis

EVAL "return ARGV[1] .. ARGV[2]" 0 Hello World

A "Hello World"

B "ARGV[1]ARGV[2]"

C Error: wrong number of arguments

D "HelloWorld"

💡 Hint

Remember that ARGV is a list of arguments passed after the key count, and concatenation in Lua uses .. operator.

The Lua script concatenates ARGV[1] and ARGV[2] without spaces, so the output is "HelloWorld".

📝 Syntax

intermediate

Identify the syntax error in Lua script for EVAL

Which option contains a syntax error when used as the Lua script in Redis EVAL?

A return redis.call('incr', KEYS[1])

B return redis.call('get', KEYS[1])

C return redis.call('set', KEYS[1], ARGV[1])

D return redis.call('del', KEYS[1])

💡 Hint

Check for missing or mismatched parentheses in the Lua code.

Option A is missing a closing parenthesis after KEYS[1], causing a syntax error.

❓ optimization

advanced

Optimizing Lua script to increment multiple keys

You want to increment the values of multiple keys atomically using EVAL. Which Lua script is the most efficient?

A for i=1,#KEYS do redis.call('incr', KEYS[i]) end return 'OK'

B redis.call('incr', KEYS[1]) redis.call('incr', KEYS[2]) return 'OK'

C return redis.call('incr', KEYS[1]) + redis.call('incr', KEYS[2])

D for i=1,#ARGV do redis.call('incr', ARGV[i]) end return 'OK'

💡 Hint

Use KEYS array to loop over keys, not ARGV.

Option A loops over all keys passed in KEYS and increments each one, which is efficient and scalable.

🧠 Conceptual

advanced

Understanding KEYS and ARGV in EVAL

In Redis EVAL command, what is the main difference between KEYS and ARGV?

A KEYS are only used for read operations; ARGV only for write operations.

B KEYS are the keys the script will access; ARGV are additional arguments passed to the script.

C KEYS are arguments passed after ARGV; ARGV are keys to access.

D KEYS and ARGV are interchangeable and mean the same.

💡 Hint

Think about how Redis separates keys from other parameters for scripting.

KEYS contains the list of keys the script will access, while ARGV contains other arguments passed to the script.

🔧 Debug

expert

Debugging a Lua script that returns nil unexpectedly

Given this EVAL command:

EVAL "return redis.call('get', KEYS[1])" 1 mykey

If the key 'mykey' does not exist, what will the script return?

Redis

EVAL "return redis.call('get', KEYS[1])" 1 mykey

A "" (empty string)

B Error: key does not exist

C nil

D 0

💡 Hint

Consider what Redis returns when getting a non-existent key.

Redis returns nil when a key does not exist, so the Lua script returns nil.

Practice

1. What is the main purpose of the EVAL command in Redis?

easy

A. To run Lua scripts inside Redis for atomic operations

B. To execute SQL queries on Redis data

C. To backup Redis data to a file

D. To create new Redis databases

Solution

1. Step 1: Understand Redis scripting

   Redis uses Lua scripts to perform complex or multiple operations atomically.

2. Step 2: Identify EVAL command role

   The EVAL command runs Lua scripts inside Redis, ensuring atomicity and reducing network calls.

3. Final Answer:

   To run Lua scripts inside Redis for atomic operations -> Option A

4. Quick Check:

   EVAL runs Lua scripts = D [OK]

Hint: EVAL runs Lua scripts atomically inside Redis [OK]

Common Mistakes:

• Confusing EVAL with SQL execution
• Thinking EVAL backs up data
• Assuming EVAL creates databases

2. Which of the following is the correct syntax to run a Lua script with EVAL in Redis?

easy

A. EVAL script numkeys key [key ...] arg [arg ...]

B. EVAL numkeys script key [key ...] arg [arg ...]

C. EVAL script key numkeys arg [arg ...]

D. EVAL script arg numkeys key [key ...]

Solution

1. Step 1: Recall EVAL syntax

   The correct syntax is EVAL script numkeys key [key ...] arg [arg ...], where numkeys tells how many keys follow.

2. Step 2: Match options

   Only EVAL script numkeys key [key ...] arg [arg ...] matches this exact order and structure.

3. Final Answer:

   EVAL script numkeys key [key ...] arg [arg ...] -> Option A

4. Quick Check:

   Script, numkeys, keys, args order = A [OK]

Hint: Remember: script, numkeys, keys, then args in EVAL [OK]

Common Mistakes:

• Placing numkeys after keys or args
• Mixing order of keys and args
• Omitting numkeys parameter

3. Given the Lua script return redis.call('GET', KEYS[1]) and the command EVAL script 1 mykey, what will Redis return if mykey holds the value 'hello'?

medium

A. An error because arguments are missing

B. nil

C. 'hello'

D. The string 'mykey'

Solution

1. Step 1: Understand the Lua script

   The script calls redis.call('GET', KEYS[1]), which fetches the value of the first key passed.

2. Step 2: Analyze the command

   The command passes 1 key: mykey. If mykey holds 'hello', the script returns 'hello'.

3. Final Answer:

   'hello' -> Option C

4. Quick Check:

   GET mykey = 'hello' [OK]

Hint: Lua script returns value of first key passed [OK]

Common Mistakes:

• Confusing keys and args
• Expecting key name instead of value
• Assuming nil if key exists

4. What is wrong with this Redis command? EVAL 'return redis.call('GET', KEYS[1])' 1 mykey

medium

A. Missing number of keys argument

B. The Lua script string uses conflicting single quotes causing syntax error

C. Keys and args are swapped

D. The command is correct and will run successfully

Solution

1. Step 1: Inspect Lua script quoting

   The Lua script is enclosed in single quotes, but inside it also uses single quotes for 'GET', causing quote conflict.

2. Step 2: Identify syntax error

   This causes a syntax error because the shell or Redis cannot parse the string properly.

3. Final Answer:

   The Lua script string uses conflicting single quotes causing syntax error -> Option B

4. Quick Check:

   Quote conflict in script string = A [OK]

Hint: Use different quotes or escape inner quotes in Lua script [OK]

Common Mistakes:

• Ignoring quote conflicts
• Forgetting to escape inner quotes
• Miscounting number of keys

5. You want to atomically increment a Redis key counter by a value passed as an argument using EVAL. Which Lua script and command correctly achieve this?

hard

A. Script: return redis.call('INCRBY', ARGV[1], KEYS[1])Command: EVAL script 1 counter 5

B. Script: return redis.call('INCRBY', KEYS[1], ARGV[1])Command: EVAL script 2 counter 5

C. Script: return redis.call('INCRBY', KEYS[1], ARGV[1])Command: EVAL script 0 counter 5

D. Script: return redis.call('INCRBY', KEYS[1], ARGV[1])Command: EVAL script 1 counter 5

Solution

1. Step 1: Understand INCRBY usage in Lua script

   The command redis.call('INCRBY', KEYS[1], ARGV[1]) increments the key by the argument value.

2. Step 2: Check command arguments

   Passing 1 as number of keys, then counter as key, and 5 as argument matches the script.

3. Final Answer:

   Script: return redis.call('INCRBY', KEYS[1], ARGV[1]) and Command: EVAL script 1 counter 5 -> Option D

4. Quick Check:

   INCRBY key arg with correct keys count = B [OK]

Hint: Keys count must match keys passed; args follow keys [OK]

Common Mistakes:

• Swapping keys and args order
• Incorrect number of keys argument
• Using zero keys when keys are needed