XLEN for stream length in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis command XLEN do?

The XLEN command returns the number of entries (length) in a Redis stream.

beginner

How do you use XLEN to find the length of a stream named mystream?

Use the command: XLEN mystream. It returns the count of entries in mystream.

intermediate

What will XLEN return if the stream does not exist?

If the stream does not exist, XLEN returns 0, meaning the stream is empty or missing.

beginner

Why is XLEN useful when working with Redis streams?

XLEN helps you quickly check how many messages or events are stored in a stream, useful for monitoring or processing logic.

intermediate

Can XLEN be used to get the size in bytes of a Redis stream?

No, XLEN only returns the number of entries, not the memory size. Use MEMORY USAGE for size in bytes.

What does the Redis command XLEN mystream return?

A The number of entries in the stream named mystream

B The size in bytes of the stream named mystream

C The last entry ID in the stream named mystream

D The first entry ID in the stream named mystream

XLEN returns the count of entries in the specified stream.

If a Redis stream does not exist, what does XLEN return?

A 1

B Null

C An error

D 0

XLEN returns 0 if the stream does not exist.

Which Redis command would you use to find the number of entries in a stream?

A XLEN

B XREAD

C XADD

D XDEL

XLEN returns the length of a stream.

Does XLEN provide the memory size of a stream?

A Yes, it returns size in bytes

B No, it returns number of entries

C Yes, but only for small streams

D No, it returns the last entry ID

XLEN returns the count of entries, not memory size.

Which of these is a valid use of XLEN?

A To delete entries from a stream

B To add new entries to a stream

C To count how many messages are in a Redis stream

D To read entries from a stream

XLEN counts entries; other commands handle adding, deleting, or reading.

Explain what the Redis command XLEN does and when you might use it.

Think about how you check the size of a list or queue.

Describe the difference between XLEN and MEMORY USAGE commands in Redis streams.

One counts items, the other measures space.

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