XTRIM for stream capping in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the effect of this XTRIM command?

Given a Redis stream named mystream with 1000 entries, what will be the number of entries after running this command?

XTRIM mystream MAXLEN 100

Redis

XTRIM mystream MAXLEN 100

A The stream will have 1000 entries, no change occurs.

B The stream will have exactly 900 entries, the newest entries removed.

C The stream will have exactly 100 entries, the oldest entries removed.

D The command will cause an error because MAXLEN requires a minimum of 1000.

💡 Hint

MAXLEN trims the stream to keep only the newest entries up to the specified count.

XTRIM with MAXLEN 100 keeps only the latest 100 entries by removing older ones.

🧠 Conceptual

intermediate

What does the ~ (approximate) flag do in XTRIM?

Consider the command:
XTRIM mystream MAXLEN ~ 100
What is the purpose of the ~ flag in this context?

Redis

XTRIM mystream MAXLEN ~ 100

A It forces Redis to trim exactly 100 entries, no approximation allowed.

B It allows Redis to trim the stream approximately to 100 entries for better performance.

C It disables trimming and only returns the current length of the stream.

D It causes an error because ~ is not a valid flag for XTRIM.

💡 Hint

The ~ flag is used to improve performance by allowing approximate trimming.

Using ~ lets Redis trim close to the specified length but not exactly, improving speed.

📝 Syntax

advanced

Which XTRIM command syntax is correct to cap a stream to 500 entries approximately?

Choose the syntactically correct Redis command to trim mystream to approximately 500 entries.

A XTRIM mystream MAXLEN ~ 500

B XTRIM mystream MAXLEN 500 ~

C XTRIM mystream MAXLEN ~500

D XTRIM mystream MAXLEN=~ 500

💡 Hint

The approximate flag ~ must come immediately after MAXLEN.

The correct syntax places ~ right after MAXLEN followed by the number.

❓ optimization

advanced

Why use approximate trimming (~) instead of exact trimming in XTRIM?

What is the main advantage of using XTRIM mystream MAXLEN ~ 1000 over XTRIM mystream MAXLEN 1000?

A Approximate trimming disables trimming and only returns the stream length.

B Approximate trimming guarantees the stream length is exactly 1000 entries.

C Approximate trimming causes Redis to trim entries randomly.

D Approximate trimming reduces CPU usage by allowing Redis to trim in batches.

💡 Hint

Think about performance and resource usage when trimming large streams.

Approximate trimming lets Redis trim less frequently and in batches, saving CPU.

🔧 Debug

expert

What error occurs with this XTRIM command?

Given the command:
XTRIM mystream MAXLEN -100
What will Redis respond?

Redis

XTRIM mystream MAXLEN -100

A An error: 'ERR The MAXLEN argument must be a positive integer'

B The stream is trimmed to 100 entries ignoring the negative sign.

C The stream is cleared completely because negative length means zero entries.

D No error, Redis treats -100 as a large positive number.

💡 Hint

MAXLEN must be a positive number; negative values are invalid.

Redis rejects negative MAXLEN values with an error message.

Practice

1. What does the Redis XTRIM command do when used with the MAXLEN option?

easy

A. It renames the stream to a new key.

B. It adds new entries to the stream without removing any.

C. It limits the number of entries in a stream by removing the oldest ones.

D. It deletes the entire stream immediately.

Solution

1. Step 1: Understand the purpose of XTRIM with MAXLEN

   The XTRIM command with MAXLEN is used to keep the stream size limited by removing old entries.

2. Step 2: Identify the effect on the stream

   It removes the oldest entries to keep the stream length within the specified maximum.

3. Final Answer:

   It limits the number of entries in a stream by removing the oldest ones. -> Option C

4. Quick Check:

   XTRIM + MAXLEN = Remove oldest entries [OK]

Hint: XTRIM with MAXLEN trims oldest entries to cap stream size [OK]

Common Mistakes:

• Thinking XTRIM adds entries instead of removing
• Confusing XTRIM with deleting the whole stream
• Assuming XTRIM renames the stream

2. Which of the following is the correct syntax to trim a Redis stream named mystream to keep at most 1000 entries approximately?

easy

A. XTRIM mystream MAXLEN ~1000

B. XTRIM mystream MAXLEN=1000

C. XTRIM mystream MAXLEN ~ 1000

D. XTRIM mystream MAXLEN 1000

Solution

1. Step 1: Recall syntax for approximate trimming

   Approximate trimming uses the tilde (~) before the number in the XTRIM command.

2. Step 2: Match correct syntax

   The correct syntax is XTRIM mystream MAXLEN ~1000 with spaces separating arguments.

3. Final Answer:

   XTRIM mystream MAXLEN ~1000 -> Option A

4. Quick Check:

   Approximate trim syntax = MAXLEN ~ number [OK]

Hint: Use tilde (~) with MAXLEN for approximate trimming [OK]

Common Mistakes:

• Omitting the tilde (~) for approximate trimming
• Using equals sign (=) which is invalid syntax
• Not spacing arguments correctly

3. Given the following commands executed sequentially:

XADD mystream * field1 value1
XADD mystream * field2 value2
XADD mystream * field3 value3
XTRIM mystream MAXLEN 2

How many entries will mystream contain after these commands?

medium

A. 2 entries

B. 3 entries

C. 1 entry

D. 0 entries

Solution

1. Step 1: Count entries added before trimming

   Three entries are added to mystream using XADD.

2. Step 2: Apply XTRIM with MAXLEN 2

   The XTRIM command trims the stream to keep only the latest 2 entries, removing the oldest one.

3. Final Answer:

   2 entries -> Option A

4. Quick Check:

   3 added, trimmed to 2 = 2 entries [OK]

Hint: XTRIM MAXLEN keeps newest entries, removes oldest [OK]

Common Mistakes:

• Assuming no trimming happens after XTRIM
• Counting all added entries without trimming
• Thinking XTRIM deletes all entries

4. You run the command XTRIM mystream MAXLEN ~1000 but notice the stream still has over 2000 entries. What is the most likely reason?

medium

A. The command syntax is incorrect and was ignored.

B. The approximate trim (~) allows the stream to exceed the limit temporarily.

C. XTRIM only works on empty streams.

D. The stream key name is case-sensitive and was misspelled.

Solution

1. Step 1: Understand approximate trimming behavior

   Using the tilde (~) means trimming is approximate and may allow the stream to temporarily exceed the limit.

2. Step 2: Eliminate other causes

   Syntax is correct, XTRIM works on non-empty streams, and key name issues would cause errors, not approximate trimming behavior.

3. Final Answer:

   The approximate trim (~) allows the stream to exceed the limit temporarily. -> Option B

4. Quick Check:

   Approximate trim = possible temporary size excess [OK]

Hint: Approximate trim (~) may exceed limit briefly [OK]

Common Mistakes:

• Assuming syntax error caused no trimming
• Thinking XTRIM only works on empty streams
• Ignoring approximate trimming behavior

5. You want to keep a Redis stream events capped at 500 entries but want to improve trimming speed by allowing approximate trimming. Which command should you use?

hard

A. XTRIM events MAXLEN = 500

B. XTRIM events MAXLEN 500

C. XTRIM events MINLEN ~ 500

D. XTRIM events MAXLEN ~500

Solution

1. Step 1: Identify approximate trimming syntax

   Approximate trimming uses the tilde (~) before the number with MAXLEN.

2. Step 2: Choose correct command

   XTRIM events MAXLEN ~500 caps the stream at about 500 entries with faster trimming.

3. Final Answer:

   XTRIM events MAXLEN ~500 -> Option D

4. Quick Check:

   Approximate trim = MAXLEN ~ number [OK]

Hint: Use MAXLEN ~ number for fast approximate trimming [OK]

Common Mistakes:

• Using MINLEN instead of MAXLEN
• Using equals sign (=) which is invalid
• Omitting the tilde (~) for approximate trimming