XLEN for stream length in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of XLEN on a non-empty stream?

Given a Redis stream named mystream with 3 entries, what will the command XLEN mystream return?

Redis

XADD mystream * field1 value1
XADD mystream * field2 value2
XADD mystream * field3 value3
XLEN mystream

A 0

B 3

C An error because XLEN requires a key type check

D The ID of the last entry in the stream

💡 Hint

XLEN returns the number of entries in the stream.

XLEN returns the count of entries in the stream. Since 3 entries were added, XLEN mystream returns 3.

❓ query_result

intermediate

What does XLEN return for a non-existent stream?

What will the command XLEN mystream return if mystream does not exist in Redis?

Redis

XLEN mystream

A null

B An error indicating the key does not exist

C 0

D -1

💡 Hint

XLEN returns zero if the stream key does not exist.

If the stream key does not exist, XLEN returns 0 because there are no entries.

📝 Syntax

advanced

Which XLEN command syntax is correct?

Which of the following Redis commands correctly uses XLEN to get the length of a stream named events?

A XLEN -key events

B XLEN(events)

C XLEN events STREAM

D XLEN events

💡 Hint

XLEN takes exactly one argument: the stream key name.

The correct syntax is XLEN key. Options A, B, and C are invalid Redis command syntaxes.

🔧 Debug

advanced

Why does XLEN return an error on this key?

You run XLEN mylist but get an error: WRONGTYPE Operation against a key holding the wrong kind of value. What is the most likely cause?

Redis

RPUSH mylist a b c
XLEN mylist

A mylist is a list, not a stream, so XLEN cannot be used

B The Redis server version does not support XLEN

C XLEN requires a second argument specifying the type

D mylist is empty, so XLEN returns an error

💡 Hint

XLEN only works on stream data types.

XLEN only works on streams. Since mylist is a list, Redis returns a WRONGTYPE error.

🧠 Conceptual

expert

How does XLEN behave under heavy stream trimming?

If a Redis stream logs is trimmed frequently with XTRIM logs MAXLEN ~ 1000, what can be said about the value returned by XLEN logs?

A XLEN returns approximately 1000, but exact count may vary due to approximate trimming

B XLEN always returns exactly 1000 after trimming

C XLEN returns the total number of entries ever added to the stream

D XLEN returns zero if trimming is enabled

💡 Hint

Approximate trimming means the stream length is close to the max length but not exact.

Using ~ in XTRIM causes approximate trimming, so XLEN returns a length close to but not exactly 1000.

Practice

1. What does the Redis command XLEN do when used on a stream key?

easy

A. Deletes the stream key

B. Returns the number of entries in the stream

C. Returns the last entry in the stream

D. Adds a new entry to the stream

Solution

1. Step 1: Understand the purpose of XLEN

   The XLEN command is designed to return the count of entries in a Redis stream.

2. Step 2: Compare with other commands

   Unlike commands that add or delete entries, XLEN only returns the length (number of entries).

3. Final Answer:

   Returns the number of entries in the stream -> Option B

4. Quick Check:

   XLEN = stream length count [OK]

Hint: XLEN always gives stream size count, not data content [OK]

Common Mistakes:

• Confusing XLEN with XADD which adds entries
• Thinking XLEN deletes or modifies the stream
• Assuming XLEN returns stream entries instead of count

2. Which of the following is the correct syntax to get the length of a Redis stream named mystream?

easy

A. XLEN = mystream

B. XLEN(mystream)

C. XLEN mystream

D. XLEN->mystream

Solution

1. Step 1: Recall Redis command syntax

   Redis commands use the format COMMAND key without parentheses or operators.

2. Step 2: Identify correct syntax

   XLEN mystream is the correct way to call the command to get stream length.

3. Final Answer:

   XLEN mystream -> Option C

4. Quick Check:

   Redis commands use space-separated syntax [OK]

Hint: Redis commands never use parentheses or operators [OK]

Common Mistakes:

• Using parentheses like a function call
• Trying to assign XLEN to a variable with =
• Using arrow or other operators in command

3. Given a Redis stream orders with 5 entries, what will the command XLEN orders return?

medium

A. 5

B. 0

C. An error

D. The last entry ID

Solution

1. Step 1: Understand what XLEN returns

   XLEN returns the number of entries in the stream, which is 5 here.

2. Step 2: Confirm no errors expected

   Since the stream exists and has entries, no error or zero is returned.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   XLEN counts entries = 5 [OK]

Hint: XLEN returns count, not content or errors if stream exists [OK]

Common Mistakes:

• Assuming XLEN returns zero if stream has entries
• Expecting XLEN to return entry data or IDs
• Thinking XLEN throws error for existing streams

4. You run XLEN mystream but get (integer) 0. What is the likely cause?

medium

A. Redis server is down

B. XLEN command is misspelled

C. XLEN cannot be used on streams

D. The stream mystream does not exist in Redis

Solution

1. Step 1: Understand XLEN behavior on missing keys

   XLEN returns (integer) 0 if the stream key does not exist or is empty.

2. Step 2: Check command correctness

   If command is correct and server is running, 0 indicates the stream mystream does not exist.

3. Final Answer:

   The stream mystream does not exist in Redis -> Option D

4. Quick Check:

   Missing key returns 0 from XLEN [OK]

Hint: XLEN returns 0 for missing or empty stream keys [OK]

Common Mistakes:

• Assuming XLEN command is wrong
• Ignoring server status or connection issues
• Thinking XLEN can't be used on streams

5. You want to monitor the size of a Redis stream events and alert if it exceeds 1000 entries. Which Redis command helps you check the stream length efficiently?

hard

A. XLEN events

B. XREAD events

C. XTRIM events MAXLEN 1000

D. XDEL events

Solution

1. Step 1: Identify command to get stream length

   XLEN returns the number of entries in a stream efficiently.

2. Step 2: Compare with other commands

   XREAD reads entries, XTRIM trims stream, XDEL deletes entries; none return length.

3. Final Answer:

   XLEN events -> Option A

4. Quick Check:

   Use XLEN to get stream size for monitoring [OK]

Hint: Use XLEN to get stream length quickly for alerts [OK]

Common Mistakes:

• Using XREAD which reads data, not length
• Using XTRIM which modifies stream instead of checking length
• Using XDEL which deletes entries