DISCARD to abort in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output after DISCARD aborts a transaction?

Consider the following Redis commands executed in sequence:

MULTI
SET key1 value1
DISCARD
GET key1

What will be the result of the GET key1 command?

Redis

MULTI
SET key1 value1
DISCARD
GET key1

A "(empty string)"

B nil (key1 does not exist)

C Error: EXEC without MULTI

D "value1"

💡 Hint

Think about what DISCARD does to the queued commands in a transaction.

DISCARD cancels all commands queued in the current transaction. Since SET key1 value1 was discarded, key1 was never set. Therefore, GET key1 returns nil.

🧠 Conceptual

intermediate

What does DISCARD do in a Redis transaction?

Which statement best describes the effect of the DISCARD command inside a Redis transaction?

A Pauses the transaction without executing or canceling commands

B Executes all queued commands and ends the transaction

C Cancels all queued commands and ends the transaction

D Restarts the transaction from the beginning

💡 Hint

Think about whether DISCARD executes or cancels commands.

DISCARD cancels all commands queued in the current transaction and ends the transaction without executing any commands.

📝 Syntax

advanced

Which Redis command sequence causes a syntax error?

Identify the command sequence that will cause a syntax error related to transaction commands.

A

DISCARD
MULTI
SET key1 value1
EXEC

B

MULTI
SET key1 value1
DISCARD
EXEC

C

MULTI
SET key1 value1
EXEC

D

MULTI
SET key1 value1
EXEC
DISCARD

💡 Hint

Consider the order and validity of DISCARD and EXEC commands.

After EXEC is called, the transaction ends. Calling DISCARD after EXEC without a new MULTI causes a syntax error because there is no active transaction to discard.

❓ optimization

advanced

Why use DISCARD instead of EXEC in some Redis transactions?

In which scenario is using DISCARD preferable to EXEC in a Redis transaction?

A When you want to cancel all queued commands without executing them

B When you want to partially execute queued commands

C When you want to execute all queued commands atomically

D When you want to queue additional commands after execution

💡 Hint

Think about the difference between executing and aborting a transaction.

DISCARD is used to abort a transaction and discard all queued commands without executing them, which is useful if you decide not to proceed.

🔧 Debug

expert

Why does this Redis transaction fail to set a key?

Given the following Redis commands:

MULTI
SET key1 value1
DISCARD
GET key1

Why does GET key1 return nil instead of "value1"?

A Because DISCARD cancels the SET command before execution

B Because key1 was deleted by DISCARD

C Because GET is not allowed after DISCARD

D Because MULTI was not called before SET

💡 Hint

Recall what DISCARD does to queued commands.

DISCARD cancels all commands queued in the transaction, so the SET command never runs. Therefore, key1 is not set and GET returns nil.