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.

Practice

1. What does the DISCARD command do in a Redis transaction?

easy

A. It starts a new transaction.

B. It cancels all commands queued in the current transaction.

C. It saves the current database state to disk.

D. It executes all commands queued in the current transaction.

Solution

1. Step 1: Understand the role of DISCARD in Redis transactions

   DISCARD is used to cancel all commands queued after MULTI before EXEC is called.

2. Step 2: Compare DISCARD with other transaction commands

   Unlike EXEC which executes queued commands, DISCARD aborts them, so no changes happen.

3. Final Answer:

   It cancels all commands queued in the current transaction. -> Option B

4. Quick Check:

   DISCARD cancels queued commands = D [OK]

Hint: DISCARD always cancels queued commands without executing them [OK]

Common Mistakes:

• Confusing DISCARD with EXEC which executes commands
• Thinking DISCARD saves changes instead of aborting
• Assuming DISCARD starts a transaction

2. Which of the following is the correct syntax to abort a Redis transaction after MULTI?

easy

A. EXEC

B. ABORT

C. ROLLBACK

D. DISCARD

Solution

1. Step 1: Recall Redis transaction commands

   After MULTI, you can either execute with EXEC or abort with DISCARD.

2. Step 2: Identify the correct abort command

   DISCARD is the official command to abort queued commands; ABORT and ROLLBACK do not exist in Redis.

3. Final Answer:

   DISCARD -> Option D

4. Quick Check:

   Abort command after MULTI = DISCARD [OK]

Hint: Use DISCARD to abort, EXEC to execute queued commands [OK]

Common Mistakes:

• Using EXEC instead of DISCARD to abort
• Typing ABORT or ROLLBACK which are invalid Redis commands
• Confusing DISCARD with starting a transaction

3. Consider the following Redis commands executed in order:

MULTI
SET key1 value1
DISCARD
GET key1

What will be the output of the GET key1 command?

medium

A. nil

B. "value1"

C. Error: EXEC without MULTI

D. "key1"

Solution

1. Step 1: Analyze the transaction commands

   After MULTI, SET key1 value1 is queued but not executed yet.

2. Step 2: Effect of DISCARD on queued commands

   DISCARD aborts the transaction, so SET key1 value1 is never applied.

3. Step 3: Result of GET key1

   Since key1 was never set, GET key1 returns nil.

4. Final Answer:

   nil -> Option A

5. Quick Check:

   DISCARD aborts SET, so GET returns nil [OK]

Hint: DISCARD cancels queued SET, so key remains unset [OK]

Common Mistakes:

• Assuming SET runs immediately after MULTI
• Expecting EXEC to run automatically without calling it
• Confusing output with command names

4. You run the following Redis commands:

MULTI
SET key1 value1
DISCARD
EXEC

What will happen when you run EXEC after DISCARD?

medium

A. EXEC returns nil without error

B. EXEC runs and applies SET key1 value1

C. EXEC returns an error because no transaction is active

D. EXEC starts a new transaction

Solution

1. Step 1: Understand DISCARD effect on transaction state

   DISCARD aborts the transaction and clears the queue, ending the transaction state.

2. Step 2: Behavior of EXEC after DISCARD

   Since no transaction is active after DISCARD, calling EXEC causes an error because EXEC requires an active MULTI block.

3. Final Answer:

   EXEC returns an error because no transaction is active -> Option C

4. Quick Check:

   EXEC after DISCARD errors due to no active transaction [OK]

Hint: After DISCARD, no active transaction; EXEC causes error [OK]

Common Mistakes:

• Expecting EXEC to run queued commands after DISCARD
• Thinking EXEC silently returns nil after DISCARD
• Assuming EXEC starts a new transaction

5. You want to safely update multiple keys in Redis but abort if a condition fails. Which sequence correctly uses DISCARD to abort the transaction if the condition is false?

WATCH key1
MULTI
IF condition IS FALSE
DISCARD
ELSE
SET key1 newValue
SET key2 newValue2
EXEC
END

What is the correct way to use DISCARD here?

hard

A. Call DISCARD immediately after MULTI if condition is false, skipping EXEC.

B. Call DISCARD after EXEC to undo changes if condition is false.

C. Call DISCARD before WATCH to abort the transaction.

D. Call DISCARD after SET commands to cancel them.

Solution

1. Step 1: Understand transaction flow with WATCH and MULTI

   WATCH monitors keys, MULTI starts queuing commands, EXEC executes them.

2. Step 2: Use DISCARD to abort before EXEC if condition fails

   If the condition is false, DISCARD cancels queued commands before EXEC runs, preventing changes.

3. Step 3: Why other options are incorrect

   DISCARD after EXEC or after SET commands is invalid; DISCARD before WATCH does not abort a transaction.

4. Final Answer:

   Call DISCARD immediately after MULTI if condition is false, skipping EXEC. -> Option A

5. Quick Check:

   DISCARD aborts queued commands before EXEC [OK]

Hint: Use DISCARD before EXEC to abort if condition fails [OK]

Common Mistakes:

• Calling DISCARD after EXEC which is too late
• Trying to DISCARD before WATCH which does nothing
• Assuming DISCARD cancels commands after they run