EVAL command for Lua execution in Redis

Introduction

The EVAL command lets you run small Lua scripts inside Redis to do multiple operations at once, making tasks faster and safer.

When you want to update several keys in Redis atomically without interference.

When you need to perform calculations or logic that Redis commands alone can't do.

When you want to reduce network calls by running multiple commands in one script.

When you want to ensure a sequence of commands runs completely or not at all.

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 are passing next, so it knows which arguments are keys.

Examples

This script returns the value of the key 'mykey'.

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

Sample Program

This script gets the current value of 'counter', adds 5 to it, saves it back, and returns the new value.

Redis

EVAL "local current = redis.call('GET', KEYS[1]); if not current then current = 0 else current = tonumber(current) end; local new = current + tonumber(ARGV[1]); redis.call('SET', KEYS[1], new); return new" 1 counter 5

Important Notes

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

Use KEYS[] to access keys and ARGV[] to access additional arguments inside the Lua script.

Always specify the correct number of keys to avoid errors.

Summary

EVAL runs Lua scripts inside Redis for complex or multiple operations.

It helps make operations atomic and reduces network calls.

You pass keys and arguments separately so Redis knows which are keys.

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