Lua vs transactions comparison in Redis - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a Redis transaction.

Redis

redis.call('[1]')

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

AMULTI

BEXEC

CWATCH

DDISCARD

Attempts:

3 left

💡 Hint

Common Mistakes

Using EXEC instead of MULTI to start the transaction.

Using WATCH which is for monitoring keys, not starting transactions.

The MULTI command starts a Redis transaction block.

2fill in blank

medium

Complete the Lua script to atomically increment a key by 1.

Redis

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

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

AGET

BINCR

CSET

DDEL

Attempts:

3 left

💡 Hint

Common Mistakes

Using GET and adding 1 in Lua which is not atomic.

Using SET which overwrites the value.

The script uses INCR to atomically increment the key by 1.

3fill in blank

hard

Fix the error in the Lua script to ensure atomic increment.

Redis

local val = redis.call('[1]', KEYS[1])
if not val then
  redis.call('[2]', KEYS[1], 1)
  return 1
end
return redis.call('[3]', KEYS[1])

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

AGET

BSET

CINCR

DDEL

Attempts:

3 left

💡 Hint

Common Mistakes

Using DEL which deletes the key instead of setting or incrementing.

Using GET instead of INCR for incrementing.

The script first GETs the value. If it doesn't exist, it SETs it to 1. Otherwise, it INCRs the value.

4fill in blank

hard

Fill both blanks to watch a key and start a transaction.

Redis

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

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

AWATCH

BMULTI

CEXEC

DDISCARD

Attempts:

3 left

💡 Hint

Common Mistakes

Using EXEC before MULTI.

Using DISCARD which cancels the transaction.

You WATCH the key to monitor changes, then start the transaction with MULTI.

5fill in blank

hard

Fill all three blanks to execute a transaction with error handling.

Redis

redis.call('[1]')
local success = redis.call('[2]')
if not success then
  redis.call('[3]')
end

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

AMULTI

BEXEC

CDISCARD

DWATCH

Attempts:

3 left

💡 Hint

Common Mistakes

Using WATCH instead of DISCARD to cancel.

Calling EXEC before MULTI.

You start the transaction with MULTI, execute it with EXEC, and if it fails, cancel with DISCARD.

Practice

1. Which statement best describes the difference between Lua scripts and transactions in Redis?

easy

A. Transactions are faster and more flexible than Lua scripts.

B. Transactions can execute complex logic, Lua scripts only group commands.

C. Lua scripts allow complex logic, while transactions group simple commands atomically.

D. Both Lua scripts and transactions cannot guarantee atomic execution.

Solution

1. Step 1: Understand atomic execution in Redis

   Both Lua scripts and transactions ensure commands run atomically, meaning all commands execute as one unit without interruption.

2. Step 2: Compare complexity and usage

   Lua scripts allow embedding complex logic and multiple commands in one script, while transactions simply group commands without logic.

3. Final Answer:

   Lua scripts allow complex logic, while transactions group simple commands atomically. -> Option C

4. Quick Check:

   Lua = complex logic, transactions = simple groups [OK]

Hint: Lua = complex logic, transactions = simple command groups [OK]

Common Mistakes:

• Thinking transactions can run complex logic
• Believing Lua scripts do not guarantee atomicity
• Confusing speed differences between Lua and transactions

2. Which of the following is the correct way to start a transaction in Redis?

easy

A. MULTI

B. EVAL

C. EXEC

D. SCRIPT

Solution

1. Step 1: Recall Redis transaction commands

   Redis transactions start with the MULTI command to begin queuing commands.

2. Step 2: Identify correct command usage

   EXEC is used to execute queued commands, EVAL runs Lua scripts, SCRIPT manages scripts, so MULTI is the correct start.

3. Final Answer:

   MULTI -> Option A

4. Quick Check:

   Start transaction with MULTI [OK]

Hint: Start transactions with MULTI, execute with EXEC [OK]

Common Mistakes:

• Using EXEC to start a transaction
• Confusing EVAL with transaction commands
• Using SCRIPT to start transactions

3. Given the following Lua script executed in Redis:

local val = redis.call('GET', KEYS[1])
if val == nil then
  redis.call('SET', KEYS[1], ARGV[1])
  return ARGV[1]
else
  return val
end

What will be the output if the key does not exist and ARGV[1] is "hello"?

medium

A. Error

B. nil

C. false

D. "hello"

Solution

1. Step 1: Check key existence with redis.call('GET')

   If the key does not exist, redis.call('GET') returns nil in Lua script context.

2. Step 2: Set key and return ARGV[1]

   Since val == nil, the script sets the key to ARGV[1] which is "hello" and returns "hello".

3. Final Answer:

   "hello" -> Option D

4. Quick Check:

   Key missing sets and returns ARGV[1] = "hello" [OK]

Hint: If key missing, Lua script sets and returns ARGV[1] [OK]

Common Mistakes:

• Thinking GET returns false instead of nil
• Expecting nil or error instead of returned value
• Confusing Lua false with Redis nil

4. You wrote this Redis transaction:

MULTI
GET key1
SET key1 value1
EXEC

But the GET command returns nil instead of "value1". What is the likely cause?

medium

A. GET command syntax is incorrect

B. GET runs before SET executes in transaction

C. EXEC was not called to execute commands

D. Redis transactions do not guarantee atomicity

Solution

1. Step 1: Understand Redis transaction command queuing

   Commands inside MULTI are queued and executed only after EXEC; GET does not see SET effect during queuing.

2. Step 2: Explain why GET returns nil

   GET is queued before SET, so during EXEC it executes first and returns nil because SET has not been applied yet.

3. Final Answer:

   GET runs before SET executes in transaction -> Option B

4. Quick Check:

   Queued commands run after EXEC, GET sees old value [OK]

Hint: Queued commands run after EXEC; GET sees old value during queue [OK]

Common Mistakes:

• Thinking commands run immediately inside MULTI
• Assuming GET syntax error causes nil
• Believing EXEC was not called

5. You want to atomically increment a counter only if its current value is less than 10. Which approach is best in Redis?

hard

A. Use a Lua script to check and increment atomically

B. Use MULTI/EXEC transaction with GET and INCR commands

C. Run GET and INCR commands separately in client code

D. Use WATCH on the key and then INCR without Lua

Solution

1. Step 1: Understand atomic conditional increment need

   Increment only if value < 10 requires checking and updating atomically to avoid race conditions.

2. Step 2: Compare Lua script vs transactions

   Lua scripts run all logic atomically in Redis, while transactions cannot conditionally check and increment atomically without race.

3. Step 3: Evaluate WATCH option

   WATCH can detect changes but requires retry logic and is less clean than Lua script for this logic.

4. Final Answer:

   Use a Lua script to check and increment atomically -> Option A

5. Quick Check:

   Lua scripts handle complex atomic logic best [OK]

Hint: Use Lua for atomic conditional logic, transactions can't do it alone [OK]

Common Mistakes:

• Assuming transactions can do conditional increments atomically
• Ignoring race conditions with separate GET and INCR
• Overestimating WATCH simplicity without retries