Lua script syntax in Redis - Step-by-Step Execution

Concept Flow - Lua script syntax in Redis

Start Script

↓

Define Variables

↓

Use Redis Commands

↓

Return Result

↓

End Script

A Lua script in Redis starts, defines variables, runs Redis commands, returns a result, and ends.

Execution Sample

Redis

local val = redis.call('GET', KEYS[1])
if val == false then
  return 'No value'
end
return val

This script gets a key's value; if missing, returns 'No value', else returns the value.

Execution Table

Step	Action	Evaluation	Result
1	Call redis.call('GET', KEYS[1])	KEYS[1] = 'mykey'	val = 'hello'
2	Check if val == false	val = 'hello'	Condition false
3	Return val	val = 'hello'	Output: 'hello'
4	End script		Script ends

💡 Script ends after returning the value or 'No value' if key missing

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
val	nil	'hello'	'hello'	'hello'

Key Moments - 2 Insights

Why does redis.call return false if the key does not exist?

redis.call returns false when the key is missing, so the script checks val == false to handle missing keys (see execution_table step 2).

What happens if we forget to return a value in the script?

If no return is given, Redis returns nil, which can cause confusion. Always use return to send a result back (see execution_table step 3).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'val' after Step 1?

A nil

B false

C 'hello'

D 'No value'

💡 Hint

Check the 'Result' column in Step 1 of the execution_table.

At which step does the script decide what to return?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' and 'Evaluation' columns in Step 2 of the execution_table.

If the key does not exist, what would the script return?

A 'No value'

B 'hello'

C nil

D false

💡 Hint

Refer to the script code in execution_sample and the condition in execution_table step 2.

Concept Snapshot

Lua scripts in Redis:
- Use redis.call('COMMAND', args) to run Redis commands
- KEYS and ARGV hold input keys and arguments
- Use local variables to store results
- Return a value with return statement
- redis.call returns false if key missing

Full Transcript

This visual execution shows how a Lua script runs in Redis. The script starts by calling redis.call to get a key's value. If the key does not exist, redis.call returns false. The script checks this and returns 'No value' if missing. Otherwise, it returns the actual value. Variables like 'val' store intermediate results. The script ends after returning a value. This step-by-step trace helps beginners see how Lua scripts interact with Redis commands and handle missing keys.

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