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.