EVAL command for Lua execution in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to run a Lua script that returns the value of key 'mykey'.

Redis

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

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

Amykey"

Bmykey

C"mykey"

D'mykey'

Attempts:

3 left

💡 Hint

Common Mistakes

Using the key name without quotes causes a syntax error.

Using mismatched quotes breaks the script.

The Lua script needs the key name as a string, so it must be enclosed in single quotes inside the double-quoted script.

2fill in blank

medium

Complete the code to run a Lua script that increments the value of key 'counter' by 1.

Redis

EVAL "return redis.call('INCR', [1])" 1 counter

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

Acounter'

Bcounter

C'counter'

D"counter"

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting quotes around the key name.

Using mismatched or incorrect quotes.

The key name must be a string inside the Lua script, so it needs single quotes.

3fill in blank

hard

Fix the error in the Lua script that returns the length of the value stored at key 'data'.

Redis

EVAL "return string.len(redis.call('GET', [1]))" 1 data

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

A'data'

Bdata

C"data"

Ddata"

Attempts:

3 left

💡 Hint

Common Mistakes

Using unquoted key names causing syntax errors.

Mismatched quotes inside the script.

The key name must be a string literal inside the Lua script, so it needs single quotes.

4fill in blank

hard

Fill all three blanks to run a Lua script that sets key 'name' to 'Alice' and returns the new value.

Redis

EVAL "redis.call([1], [2], [3]); return redis.call('GET', 'name')" 1 name

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

A'SET'

B'name'

C'Alice'

D'GET'

Attempts:

3 left

💡 Hint

Common Mistakes

Using unquoted command or key names.

Swapping the order of command and key.

The first argument to redis.call is the command 'SET' as a string, the second is the key name 'name' as a string, the third is the value 'Alice' as a string.

5fill in blank

hard

Fill all three blanks to run a Lua script that sets key 'age' to 30 and returns the value as a number.

Redis

EVAL "redis.call([1], [2], [3]); return tonumber(redis.call('GET', 'age'))" 1 age

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

A'SET'

B'age'

C'30'

D'GET'

Attempts:

3 left

💡 Hint

Common Mistakes

Not quoting the value '30'.

Using wrong order of arguments.

Forgetting to convert the returned value to number.

The command is 'SET', the key is 'age', and the value is '30' as a string. Then the script returns the numeric value.

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