Transactions vs Lua scripts in Redis - Practice Questions

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Redis transaction?

Consider the following Redis transaction commands executed sequentially:

1. MULTI
2. SET key1 100
3. INCR key1
4. EXEC

What will be the value of key1 after the transaction completes?

Redis

MULTI
SET key1 100
INCR key1
EXEC

A Error: key1 is not an integer

B 1

C 100

D 101

💡 Hint

Remember that Redis transactions queue commands and execute them atomically.

The transaction sets key1 to '100' and then increments it by 1, resulting in 101.

❓ query_result

intermediate

What does this Lua script return in Redis?

Given this Lua script run with EVAL:

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

Assuming key1 does not exist before running, what is the output of the script when called with ARGV[1] = 'hello'?

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

A nil

B Error: key does not exist

C "hello"

D false

💡 Hint

Lua scripts in Redis return strings or nil, and redis.call returns nil if key does not exist.

Since key1 does not exist, redis.call('GET', KEYS[1]) returns nil, so the script sets key1 to 'hello' and returns 'hello'.

🧠 Conceptual

advanced

Why might Lua scripts be preferred over Redis transactions?

Which of the following is the best reason to use Lua scripts instead of Redis transactions?

A Lua scripts cannot access Redis keys, so they are safer.

B Lua scripts guarantee atomic execution and can perform complex logic in one call.

C Transactions are faster because they do not support atomicity.

D Transactions allow rollback on error, Lua scripts do not.

💡 Hint

Think about atomicity and complexity of operations.

Lua scripts run atomically and can execute complex logic in one call, unlike transactions which queue commands but cannot perform conditional logic inside.

🔧 Debug

advanced

Identify the error in this Redis transaction usage

A developer writes this Redis transaction:

1. MULTI
2. GET key1
3. INCR key1
4. EXEC

But the GET command returns the old value of key1, not the incremented one. Why?

A Commands in a transaction are queued and executed only after EXEC, so GET sees the old value.

B GET command is invalid inside MULTI block.

C INCR does not change the value immediately, so GET returns nil.

D EXEC discards all commands except the last one.

💡 Hint

Think about when commands inside MULTI are executed.

Inside MULTI, commands are queued but not executed until EXEC. So GET returns the old value because it is queued, not executed immediately.

❓ optimization

expert

Optimizing atomic updates: Lua script vs transaction

You want to atomically increment a counter only if it is less than 10. Which approach is more efficient and why?

Options:
1. Use a Redis transaction with WATCH, GET, INCR, and EXEC.
2. Use a Lua script that checks the value and increments if less than 10.

Choose the best option.

A Lua script is more efficient because it runs atomically on the server without retries.

B Neither is efficient; use client-side locking instead.

C Both are equally efficient because Redis handles atomicity internally.

D Transaction with WATCH is more efficient because it retries automatically.

💡 Hint

Consider network round trips and atomicity guarantees.

Lua scripts run atomically on the server and avoid the overhead of retries and multiple round trips required by WATCH transactions.

Practice

1. What is the main difference between Redis transactions and Lua scripts?

easy

A. Transactions run commands one by one, Lua scripts run commands outside Redis.

B. Transactions group commands to run atomically, Lua scripts run complex logic atomically inside Redis.

C. Transactions can only read data, Lua scripts can only write data.

D. Transactions are faster than Lua scripts in all cases.

Solution

1. Step 1: Understand Redis transactions

   Transactions group multiple commands to run atomically, meaning all commands succeed or none do.

2. Step 2: Understand Lua scripts in Redis

   Lua scripts run inside Redis and can include complex logic, running atomically as a single operation.

3. Final Answer:

   Transactions group commands to run atomically, Lua scripts run complex logic atomically inside Redis. -> Option B

4. Quick Check:

   Transactions vs Lua scripts = Grouped commands vs complex logic [OK]

Hint: Transactions group commands; Lua scripts add logic inside Redis [OK]

Common Mistakes:

• Thinking Lua scripts run outside Redis
• Believing transactions can only read or write
• Assuming transactions are always faster

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

easy

A. EXEC

B. START TRANSACTION

C. BEGIN

D. MULTI

Solution

1. Step 1: Recall Redis transaction commands

   Redis uses the command MULTI to start a transaction block.

2. Step 2: Identify the correct command

   START TRANSACTION and BEGIN are SQL commands, not Redis. EXEC ends the transaction.

3. Final Answer:

   MULTI -> Option D

4. Quick Check:

   Start Redis transaction = MULTI [OK]

Hint: Use MULTI to start Redis transactions, EXEC to finish [OK]

Common Mistakes:

• Using SQL commands like START TRANSACTION
• Confusing EXEC as start command
• Using BEGIN which is not Redis syntax

3. Given this Lua script in Redis:

local current = redis.call('GET', KEYS[1])
if not current then
  redis.call('SET', KEYS[1], ARGV[1])
  return ARGV[1]
else
  return current
end

What will be the output if the key does not exist and ARGV[1] is "100"?

medium

A. Error

B. nil

C. "100"

D. "0"

Solution

1. Step 1: Analyze the Lua script logic

   The script checks if the key exists. If not, it sets the key to ARGV[1] and returns ARGV[1].

2. Step 2: Apply the input values

   Since the key does not exist, it sets the key to "100" and returns "100".

3. Final Answer:

   "100" -> Option C

4. Quick Check:

   Key missing sets and returns ARGV[1] = "100" [OK]

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

Common Mistakes:

• Assuming nil is returned when key missing
• Expecting an error from redis.call
• Confusing return values with zero

4. You wrote this Redis transaction:

MULTI
SET key1 foo
INCR key1
EXEC

But the INCR command fails with an error. What is the likely cause?

medium

A. SET sets key1 as a string, INCR expects an integer value

B. INCR cannot be used inside MULTI blocks

C. EXEC was missing to run the transaction

D. MULTI command syntax is incorrect

Solution

1. Step 1: Understand SET and INCR behavior

   SET stores the value as a string. INCR expects the key to hold an integer value.

2. Step 2: Identify the error cause

   Since SET sets "foo" as a string, INCR fails because it cannot increment a non-integer string.

3. Final Answer:

   SET sets key1 as a string, INCR expects an integer value -> Option A

4. Quick Check:

   INCR needs integer, SET stores string [OK]

Hint: INCR needs integer; SET stores string by default [OK]

Common Mistakes:

• Thinking INCR can't be in MULTI
• Forgetting to EXEC the transaction
• Assuming MULTI syntax is wrong

5. You want to atomically check if a key exists and increment it only if it does, otherwise set it to 1. Which approach is best in Redis?

hard

A. Use a Lua script that checks and updates the key atomically

B. Use Redis pipeline to send GET and INCR commands together

C. Run separate GET and INCR commands without transactions

D. Use a Redis transaction with WATCH, GET, and MULTI commands

Solution

1. Step 1: Understand the atomicity requirement

   The operation must be atomic: check existence and update in one step without race conditions.

2. Step 2: Compare approaches

   Transactions with WATCH can be complex and may fail due to race conditions. Separate commands or pipelines are not atomic. Lua scripts run atomically inside Redis and can handle logic safely.

3. Final Answer:

   Use a Lua script that checks and updates the key atomically -> Option A

4. Quick Check:

   Atomic check and update = Lua script [OK]

Hint: Use Lua scripts for atomic logic beyond simple transactions [OK]

Common Mistakes:

• Assuming WATCH transactions are always safe
• Using separate commands risking race conditions
• Confusing pipelines with atomic execution