Lua script syntax in Redis

Introduction

Lua scripts in Redis let you run multiple commands together safely and quickly without interruptions.

You want to update several keys in Redis at once without other commands interfering.

You need to perform a calculation or check before changing data in Redis.

You want to reduce the number of round trips between your app and Redis server.

You want to ensure a group of commands run as a single atomic operation.

You want to reuse a small program inside Redis for common tasks.

Syntax

Redis

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

script is the Lua code you want to run, given as a string.

numkeys tells Redis how many keys you pass next, so it knows which arguments are keys.

Examples

This script gets the value of 'mykey' from Redis.

Redis

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

This script sets 'mykey' to 'newvalue' and then returns the new value.

Redis

EVAL "redis.call('SET', KEYS[1], ARGV[1]); return redis.call('GET', KEYS[1])" 1 mykey newvalue

This script increments the 'counter' key by 1 and returns the new count.

Redis

EVAL "local val = redis.call('INCR', KEYS[1]); return val" 1 counter

Sample Program

This script checks if the key 'greeting' exists. If not, it sets it to 'Hello' and returns 'Hello'. If it exists, it returns the current value.

Redis

EVAL "local current = redis.call('GET', KEYS[1]); if not current then redis.call('SET', KEYS[1], ARGV[1]); return ARGV[1] else return current end" 1 greeting Hello

Important Notes

KEYS array holds the keys passed after numkeys; ARGV array holds the other arguments.

Lua scripts run atomically in Redis, so no other commands run during the script.

Keep scripts short and simple to avoid blocking Redis for too long.

Summary

Lua scripts let you run multiple Redis commands together safely and quickly.

Use EVAL with your Lua code, number of keys, keys, and arguments.

Scripts can read and write keys, and return results.

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