Lua script syntax in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set a key "name" with value "Alice" in Redis using Lua.

Redis

redis.call('[1]', 'name', 'Alice')

Drag options to blanks, or click blank then click option'

Aget

Bset

Cdel

Dexists

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'get' instead of 'set' will try to retrieve a value, not store it.

Using 'del' deletes keys, which is not what we want here.

The set command stores a value for a key in Redis. Here, it sets key "name" to "Alice".

2fill in blank

medium

Complete the code to increment the integer value stored at key "counter" by 1.

Redis

redis.call('[1]', 'counter')

Drag options to blanks, or click blank then click option'

Aincr

Bdecr

Cappend

Dset

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'decr' will decrease the value instead of increasing it.

Using 'append' is for strings, not numbers.

The incr command increases the integer value of a key by 1.

3fill in blank

hard

Fix the error in the code to get the value of key "user".

Redis

local val = redis.call('[1]', 'user')

Drag options to blanks, or click blank then click option'

Adel

Bset

Cget

Dexists

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' tries to assign a value, not get it.

Using 'del' deletes the key, which is not intended.

The get command retrieves the value of a key in Redis.

4fill in blank

hard

Fill both blanks to check if key "session" exists and delete it if it does.

Redis

if redis.call('[1]', 'session') == 1 then
  redis.call('[2]', 'session')
end

Drag options to blanks, or click blank then click option'

Aexists

Bdel

Cget

Dset

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'get' to check existence returns the value, not a boolean.

Using 'set' instead of 'del' will overwrite the key.

exists checks if the key is present (returns 1 if yes). del deletes the key.

5fill in blank

hard

Fill both blanks to set key "count" to 10 only if it does not exist, then get its value.

Redis

redis.call('[1]', 'count', 10)
local val = redis.call('[2]', 'count')
return val

Drag options to blanks, or click blank then click option'

Asetnx

Bget

Cset

Ddel

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' instead of 'setnx' overwrites the key unconditionally.

Using 'del' deletes the key, which is not intended.

setnx sets the key only if it does not exist. get retrieves the value. set is not used here but included as a distractor.

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