Lua script syntax in Redis - Time & Space Complexity

Time Complexity: Lua script syntax in Redis

O(n)

Understanding Time Complexity

When running Lua scripts in Redis, it's important to know how the time taken grows as the script processes more data.

We want to understand how the script's execution time changes when the input size changes.

Scenario Under Consideration

Analyze the time complexity of the following Lua script in Redis.

local key = KEYS[1]
local count = tonumber(ARGV[1])
for i = 1, count do
  redis.call('LPUSH', key, i)
end
return redis.call('LLEN', key)
    

This script pushes numbers from 1 to count into a Redis list and then returns the list length.

Identify Repeating Operations

Look for repeated actions in the script.

• Primary operation: The loop runs redis.call('LPUSH') repeatedly.
• How many times: Exactly count times, once for each number pushed.

How Execution Grows With Input

As the count grows, the number of push operations grows the same way.

Input Size (count)	Approx. Operations
10	10 LPUSH calls + 1 LLEN call
100	100 LPUSH calls + 1 LLEN call
1000	1000 LPUSH calls + 1 LLEN call

Pattern observation: The number of operations grows directly with the input size.

Final Time Complexity

Time Complexity: O(n)

This means the script takes longer roughly in direct proportion to how many items it processes.

Common Mistake

[X] Wrong: "The script runs in constant time no matter how many items are pushed."

[OK] Correct: Each LPUSH call adds work, so more items mean more time.

Interview Connect

Understanding how Lua scripts scale in Redis helps you write efficient scripts and explain their performance clearly.

Self-Check

"What if the script used RPUSH instead of LPUSH? How would the time complexity change?"

Practice

1. What is the correct way to start a Lua script in Redis using the EVAL command?

easy

A. EVAL redis.call('PING') 0

B. EVAL 'return redis.call("PING")' 0

C. EVAL 'PING' 0

D. EVAL return redis.call('PING') 0

Solution

1. Step 1: Understand EVAL syntax

   The EVAL command requires a Lua script as a string, followed by the number of keys and then the keys and arguments.

2. Step 2: Check correct Lua script format

   The Lua script must be a string and use redis.call to execute Redis commands, e.g., return redis.call("PING").

3. Final Answer:

   EVAL 'return redis.call("PING")' 0 -> Option B

4. Quick Check:

   EVAL with Lua string and 0 keys = A [OK]

Hint: EVAL needs Lua code as string, then number of keys [OK]

Common Mistakes:

• Not passing Lua code as a string
• Missing return statement in Lua script
• Incorrect number of keys argument
• Using Redis commands directly without redis.call

2. Which of the following is the correct syntax to call the Redis command SET inside a Lua script?

easy

A. redis.call('SET', 'key', 'value')

B. redis.SET('key', 'value')

C. call.redis('SET', 'key', 'value')

D. redis.call.SET('key', 'value')

Solution

1. Step 1: Recall Redis Lua API

   In Lua scripts for Redis, commands are called using redis.call with the command name as first argument.

2. Step 2: Check syntax correctness

   The correct syntax is redis.call('SET', 'key', 'value'). Other options misuse the API or method calls.

3. Final Answer:

   redis.call('SET', 'key', 'value') -> Option A

4. Quick Check:

   Use redis.call('COMMAND', args...) = B [OK]

Hint: Use redis.call('COMMAND', args) to run Redis commands in Lua [OK]

Common Mistakes:

• Using dot notation like redis.SET
• Swapping order of arguments
• Using call.redis instead of redis.call
• Missing quotes around command name

3. What will the following Lua script return when run with EVAL in Redis?

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

Assuming the key counter does not exist before and the script is called with KEYS[1] = 'counter'.

medium

A. 0

B. Error

C. nil

D. 1

Solution

1. Step 1: Understand INCR behavior on non-existing key

   INCR command creates the key with value 0 then increments it by 1, so the new value is 1.

2. Step 2: Return value of redis.call('INCR', key)

   The call returns the new value after increment, which is 1.

3. Final Answer:

   1 -> Option D

4. Quick Check:

   INCR on new key returns 1 [OK]

Hint: INCR on missing key returns 1 after creating it [OK]

Common Mistakes:

• Assuming INCR returns 0 on new key
• Expecting nil for missing key
• Confusing return value with command success
• Not passing KEYS argument correctly

4. Identify the error in this Lua script for Redis:

local val = redis.call('GET', KEYS[1])
if val == nil then
  redis.call('SET', KEYS[1], '1')
else
  redis.call('INCR', KEYS[1])
end
return redis.call('GET', KEYS[1])

medium

A. Using local variable val is not allowed

B. Missing return statement in Lua script

C. Using redis.call('INCR') on a string value causes error

D. Incorrect use of KEYS array index

Solution

1. Step 1: Review the script logic

   The script fetches the value with GET. If nil, sets to '1'; else calls INCR, then returns the final GET.

2. Step 2: Spot the problem with INCR

   redis.call('INCR', KEYS[1]) will raise an error if the key's existing value is a non-integer string (e.g., 'abc'). The script does not validate the value type before incrementing.

3. Step 3: Rule out other options

   There is a return statement (not B), local variables are allowed in Redis Lua (not C), and KEYS[1] uses correct 1-based indexing (not D).

4. Final Answer:

   Using redis.call('INCR') on a string value causes error -> Option C

5. Quick Check:

   INCR errors on non-integer strings [OK]

Hint: INCR works on strings representing integers only [OK]

Common Mistakes:

• Thinking INCR fails on string '1'
• Confusing local variable usage
• Assuming missing return statement
• Misusing KEYS index

5. You want to atomically increment a counter only if it exists in Redis using a Lua script. Which script correctly implements this?

-- Option A
if redis.call('EXISTS', KEYS[1]) == 1 then
  return redis.call('INCR', KEYS[1])
else
  return nil
end

-- Option B
return redis.call('INCR', KEYS[1])

-- Option C
if redis.call('GET', KEYS[1]) then
  redis.call('INCR', KEYS[1])
end
return redis.call('GET', KEYS[1])

-- Option D
if redis.call('EXISTS', KEYS[1]) == 0 then
  return redis.call('INCR', KEYS[1])
else
  return nil
end

hard

A. Option A

B. Option B

C. Option C

D. Option D

Solution

1. Step 1: Understand requirement

   Increment counter only if it exists. If not, return nil without creating it.

2. Step 2: Analyze the scripts

   The script checking redis.call('EXISTS', KEYS[1]) == 1 then returning redis.call('INCR', KEYS[1]) else nil matches exactly.
   One script calls INCR unconditionally, creating if missing.
   Another checks if redis.call('GET', KEYS[1]) (truthy for any non-nil), calls INCR but returns subsequent GET (may error on non-integers or not return increment directly).
   The last increments if EXISTS == 0, opposite of needed.

3. Final Answer:

   Option A -> Option A

4. Quick Check:

   Check EXISTS before INCR to avoid creating key [OK]

Hint: Check EXISTS == 1 before INCR to avoid creating new key [OK]

Common Mistakes:

• Incrementing without checking key existence
• Using EXISTS == 0 to increment
• Not returning correct value after increment
• Using GET without handling nil properly