Atomic operations with Lua in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the Lua script to increment a Redis key atomically.

Redis

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

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

Adel

Bget

Cincr

Dset

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'get' instead of 'incr' returns the value but does not increment.

Using 'set' overwrites the value instead of incrementing.

The incr command increments the integer value of a key by one atomically.

2fill in blank

medium

Complete the Lua script to set a key only if it does not exist.

Redis

redis.call('[1]', KEYS[1], ARGV[1])

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

Asetnx

Bgetset

Cset

Ddel

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' overwrites the key regardless of existence.

Using 'getset' replaces the value and returns the old one.

The setnx command sets the key only if it does not already exist, ensuring atomicity.

3fill in blank

hard

Fix the error in the Lua script to decrement a key's value atomically.

Redis

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

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

Aincr

Bdecr

Cdel

Dexpire

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'incr' increases the value instead of decreasing.

Using 'del' deletes the key instead of decrementing.

The decr command atomically decrements the integer value of a key by one.

4fill in blank

hard

Fill both blanks to atomically add a member to a set and set an expiration time.

Redis

redis.call('[1]', KEYS[1], ARGV[1])
redis.call('[2]', KEYS[1], tonumber(ARGV[2]))

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

Asadd

Bexpire

Cset

Ddel

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' instead of 'sadd' overwrites the key.

Using 'del' removes the key instead of setting expiration.

sadd adds a member to a set atomically, and expire sets the key's time to live.

5fill in blank

hard

Fill all three blanks to atomically increment a hash field, check its value, and delete the key if zero.

Redis

local newVal = redis.call('[1]', KEYS[1], ARGV[1], 1)
if newVal == tonumber('[2]') then
  redis.call('[3]', KEYS[1])
end

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

Ahincrby

B0

Cdel

Dhset

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'hset' instead of 'hincrby' does not increment.

Comparing to '1' instead of '0' causes wrong deletion logic.

hincrby increments a hash field atomically, then we check if the value is zero, and del deletes the key if so.

Practice

1. What is the main benefit of using Lua scripts with Redis EVAL command?

easy

A. It ensures multiple commands run atomically without interference.

B. It speeds up Redis server startup time.

C. It allows Redis to store Lua scripts permanently.

D. It automatically backs up Redis data to disk.

Solution

1. Step 1: Understand atomicity in Redis

   Atomicity means commands run fully or not at all, preventing partial updates.

2. Step 2: Role of Lua scripts with EVAL

   Lua scripts run all commands inside them as a single atomic operation in Redis.

3. Final Answer:

   It ensures multiple commands run atomically without interference. -> Option A

4. Quick Check:

   Atomic operations = safe multi-command execution [OK]

Hint: Lua scripts run all commands atomically in Redis [OK]

Common Mistakes:

• Thinking Lua scripts speed up server startup
• Confusing script storage with atomicity
• Assuming Lua scripts backup data automatically

2. Which of the following is the correct syntax to run a Lua script that increments a Redis key counter by 1 using EVAL?

easy

A. EVAL "return redis.call('INCR', 'counter')" 0

B. EVAL "return redis.call('INCR', ARGV[1])" 1 counter

C. EVAL "return redis.call('INCR', KEYS[1])" 0 counter

D. EVAL "return redis.call('INCR', KEYS[1])" 1 counter

Solution

1. Step 1: Understand EVAL syntax

   EVAL script numkeys key1 key2 ... args ...; KEYS[] holds keys, ARGV[] holds arguments.

2. Step 2: Identify correct usage for increment

   Since 'counter' is a key, it should be passed as KEYS[1] with numkeys=1.

3. Final Answer:

   EVAL "return redis.call('INCR', KEYS[1])" 1 counter -> Option D

4. Quick Check:

   Use KEYS for keys, ARGV for arguments [OK]

Hint: Keys go in KEYS[], count keys correctly [OK]

Common Mistakes:

• Passing keys in ARGV instead of KEYS
• Setting numkeys to 0 when keys are used
• Using string literal instead of KEYS[1]

3. Given the Lua script run with EVAL below, what will be the output if the key visits does not exist initially?

EVAL "local v = redis.call('GET', KEYS[1]) if not v then v = 0 end v = v + 1 redis.call('SET', KEYS[1], v) return v" 1 visits

medium

A. nil

B. 1

C. 0

D. Error: attempt to perform arithmetic on a nil value

Solution

1. Step 1: Analyze GET result when key missing

   redis.call('GET', KEYS[1]) returns nil if key doesn't exist.

2. Step 2: Check the if condition

   if not v then v = 0 end executes since v is nil, setting v to 0.

3. Step 3: Perform arithmetic and SET

   v = 0 + 1 = 1; redis.call('SET', KEYS[1], 1); return 1.

4. Final Answer:

   1 -> Option B

5. Quick Check:

   Nil handled by if not v, outputs 1 [OK]

Hint: Script checks nil before math, outputs 1 [OK]

Common Mistakes:

• Overlooking the 'if not v then v = 0 end' check
• Assuming nil causes arithmetic error despite handling
• Thinking GET nil leads to error without reading full script

4. Identify the error in this Lua script used with Redis EVAL:

EVAL "local count = redis.call('GET', KEYS[1]) count = tonumber(count) + 1 redis.call('SET', KEYS[1], count) return count" 1 visits

medium

A. Missing check if count is nil before tonumber conversion.

B. Incorrect number of keys specified in EVAL command.

C. Using redis.call instead of redis.pcall causes failure.

D. The return statement should be outside the script string.

Solution

1. Step 1: Understand tonumber behavior

   tonumber(nil) returns nil, so nil + 1 causes error.

2. Step 2: Check script for nil handling

   Script does not check if count is nil before adding 1, causing runtime error if key missing.

3. Final Answer:

   Missing check if count is nil before tonumber conversion. -> Option A

4. Quick Check:

   Always check nil before tonumber and math [OK]

Hint: Check nil before tonumber and math operations [OK]

Common Mistakes:

• Assuming tonumber(nil) returns 0
• Ignoring nil checks before arithmetic
• Confusing redis.call and redis.pcall usage

5. You want to atomically increment a Redis key page_views only if its current value is less than 100. Which Lua script snippet correctly implements this logic using EVAL?

hard

A. local v = redis.call('GET', KEYS[1]) if v < 100 then redis.call('INCR', KEYS[1]) return v + 1 else return v end

B. local v = tonumber(redis.call('GET', KEYS[1])) if v <= 100 then redis.call('INCR', KEYS[1]) return v + 1 else return v end

C. local v = tonumber(redis.call('GET', KEYS[1]) or '0') if v < 100 then redis.call('INCR', KEYS[1]) return v + 1 else return v end

D. local v = tonumber(redis.call('GET', KEYS[1]) or '0') if v <= 100 then redis.call('INCR', KEYS[1]) return v + 1 else return v end

Solution

1. Step 1: Handle missing key safely

   Using 'or "0"' ensures tonumber gets '0' if key missing, avoiding nil errors.

2. Step 2: Check condition and increment

   Condition 'v < 100' matches requirement to increment only if less than 100.

3. Step 3: Return updated or current value

   Returns v+1 if incremented, else returns current v.

4. Final Answer:

   local v = tonumber(redis.call('GET', KEYS[1]) or '0') if v < 100 then redis.call('INCR', KEYS[1]) return v + 1 else return v end -> Option C

5. Quick Check:

   Safe nil handling + correct condition = local v = tonumber(redis.call('GET', KEYS[1]) or '0') if v < 100 then redis.call('INCR', KEYS[1]) return v + 1 else return v end [OK]

Hint: Use 'or "0"' to avoid nil, check '< 100' strictly [OK]

Common Mistakes:

• Not handling nil key values
• Using <= 100 instead of < 100
• Forgetting to convert GET result to number