DISCARD to abort in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to abort a Redis transaction using the DISCARD command.

Redis

MULTI
SET key1 value1
[1]

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

ADISCARD

BEXEC

CWATCH

DCOMMIT

Attempts:

3 left

💡 Hint

Common Mistakes

Using EXEC instead of DISCARD will execute the transaction instead of aborting it.

Using WATCH or COMMIT are not valid commands to abort a transaction.

The DISCARD command aborts the current transaction in Redis, discarding all queued commands.

2fill in blank

medium

Complete the code to start a transaction and then abort it properly.

Redis

MULTI
INCR counter
[1]

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

AEXEC

BSAVE

CROLLBACK

DDISCARD

Attempts:

3 left

💡 Hint

Common Mistakes

Using ROLLBACK which is not supported in Redis.

Using EXEC which executes the transaction instead of aborting.

After starting a transaction with MULTI, DISCARD aborts it, discarding all queued commands.

3fill in blank

hard

Fix the error in the code to abort the transaction correctly.

Redis

MULTI
SET user:1 name Alice
[1]

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

AEXEC

BSAVE

CDISCARD

DCOMMIT

Attempts:

3 left

💡 Hint

Common Mistakes

Using COMMIT which is not a Redis command.

Using EXEC which executes the transaction instead of aborting.

To abort a Redis transaction, use DISCARD. EXEC or COMMIT will execute the transaction.

4fill in blank

hard

Fill both blanks to start a transaction and then abort it.

Redis

[1]
HSET user:2 age 30
[2]

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

AMULTI

BDISCARD

CEXEC

DWATCH

Attempts:

3 left

💡 Hint

Common Mistakes

Using EXEC instead of DISCARD to abort.

Using WATCH to abort which only watches keys.

Use MULTI to start a transaction and DISCARD to abort it.

5fill in blank

hard

Fill all three blanks to watch a key, start a transaction, and then abort it.

Redis

[1] user:3
[2]
DEL user:3
[3]

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

AWATCH

BMULTI

CDISCARD

DEXEC

Attempts:

3 left

💡 Hint

Common Mistakes

Using EXEC instead of DISCARD to abort.

Not using WATCH before MULTI when monitoring keys.

First, WATCH monitors the key, then MULTI starts the transaction, and DISCARD aborts it.

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