Accessing keys and arguments in Redis

Introduction

You use keys and arguments to tell Redis exactly which data to work with and what extra information to use.

When you want to get or set a specific value in Redis.

When writing a Redis script that needs to know which keys to access.

When passing extra information to a Redis command or script.

When you want to update multiple keys with some parameters.

When you want to control how a Redis command behaves using arguments.

Syntax

Redis

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

The first argument after the script is the number of keys you will pass.

Keys come first, then any extra arguments after the keys.

Examples

Gets the value of the key named 'mykey'.

Redis

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

Returns the first key and the first argument passed.

Redis

EVAL "return {KEYS[1], ARGV[1]}" 1 key1 value1

Sets 'mykey' to 'newvalue'.

Redis

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

Sample Program

This script returns the first two keys and the first two arguments passed to it.

Redis

EVAL "return {KEYS[1], KEYS[2], ARGV[1], ARGV[2]}" 2 keyA keyB arg1 arg2

Important Notes

Keys are accessed inside the script using KEYS array starting at index 1.

Arguments are accessed inside the script using ARGV array starting at index 1.

Always specify the correct number of keys after the script to avoid errors.

Summary

Keys tell Redis which data to use.

Arguments provide extra information to commands or scripts.

Use KEYS and ARGV arrays inside scripts to access keys and arguments.

Practice

1. In a Redis Lua script, which array is used to access the keys passed to the script?

easy

A. KEYS

B. ARGV

C. KEYSARGS

D. ARGS

Solution

1. Step 1: Understand Redis script inputs

   Redis Lua scripts receive keys and arguments separately.

2. Step 2: Identify the array for keys

   The keys are accessed using the KEYS array inside the script.

3. Final Answer:

   KEYS -> Option A

4. Quick Check:

   Keys = KEYS [OK]

Hint: Keys are always in KEYS array, arguments in ARGV [OK]

Common Mistakes:

• Confusing KEYS with ARGV
• Using ARGS instead of ARGV
• Assuming keys are in ARGV

2. Which of the following is the correct way to access the first argument (not key) inside a Redis Lua script?

easy

A. KEYS[1]

B. ARGV[0]

C. ARGV[1]

D. ARGS[1]

Solution

1. Step 1: Recall Lua array indexing

   Lua arrays start at 1, not 0.

2. Step 2: Identify argument array

   Arguments are accessed via ARGV array, so first argument is ARGV[1].

3. Final Answer:

   ARGV[1] -> Option C

4. Quick Check:

   First argument = ARGV[1] [OK]

Hint: Lua arrays start at 1, so first argument is ARGV[1] [OK]

Common Mistakes:

• Using zero-based index ARGV[0]
• Confusing KEYS with ARGV
• Using ARGS instead of ARGV

3. Consider this Redis Lua script snippet:

local key1 = KEYS[1]
local arg1 = ARGV[1]
return key1 .. '-' .. arg1

If the script is called with keys = ["user:1"] and arguments = ["active"], what is the returned result?

medium

A. "user:1-active"

B. "active-user:1"

C. "user:1active"

D. An error occurs

Solution

1. Step 1: Assign values from KEYS and ARGV

   key1 = "user:1", arg1 = "active" from the inputs.

2. Step 2: Concatenate with separator

   Return value is key1 .. '-' .. arg1 = "user:1-active".

3. Final Answer:

   "user:1-active" -> Option A

4. Quick Check:

   Concatenate KEYS[1] and ARGV[1] with '-' = "user:1-active" [OK]

Hint: Concatenate KEYS and ARGV with .. operator in Lua [OK]

Common Mistakes:

• Swapping KEYS and ARGV values
• Missing separator '-' in concatenation
• Using zero-based indexing

4. You wrote this Redis Lua script:

local key = KEYS[1]
local arg = ARGV[0]
return key .. arg

When running the script with one key and one argument, what will happen?

medium

A. It throws an error due to invalid index

B. It returns nil concatenated with key

C. It returns only the key

D. It returns concatenated key and argument

Solution

1. Step 1: Check Lua indexing rules

   Lua arrays start at 1, so ARGV[0] is invalid but returns nil.

2. Step 2: Understand behavior on invalid index

   Accessing ARGV[0] returns nil, concatenating key .. nil results in key .. "nil" or error depending on Lua version.

3. Final Answer:

   It returns nil concatenated with key -> Option B

4. Quick Check:

   Lua arrays start at 1, ARGV[0] returns nil, concatenation with nil results in string with 'nil' [OK]

Hint: Lua arrays start at 1, never use index 0 [OK]

Common Mistakes:

• Using zero-based index ARGV[0]
• Assuming ARGV[0] returns first argument
• Ignoring Lua indexing rules

5. You want to write a Redis Lua script that takes two keys and one argument. The script should return the value of the first key concatenated with the argument, then the value of the second key. Which is the correct way to access these inside the script?

hard

A. ARGV[1], KEYS[1], KEYS[2]

B. ARGV[0], KEYS[1], ARGV[1]

C. KEYS[0], ARGV[0], KEYS[1]

D. KEYS[1], ARGV[1], KEYS[2]

Solution

1. Step 1: Identify keys and argument positions

   Two keys are KEYS[1] and KEYS[2], one argument is ARGV[1].

2. Step 2: Access in correct order

   Access first key as KEYS[1], argument as ARGV[1], second key as KEYS[2].

3. Final Answer:

   KEYS[1], ARGV[1], KEYS[2] -> Option D

4. Quick Check:

   Keys in KEYS[], args in ARGV[], Lua index starts at 1 [OK]

Hint: Keys in KEYS[], args in ARGV[], index from 1 [OK]

Common Mistakes:

• Using zero-based indexes
• Mixing KEYS and ARGV order
• Using ARGV for keys