XRANGE and XREVRANGE in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of XRANGE with specific ID range

Given a Redis stream named mystream with entries:

1) ID: 1609459200000-0, fields: {"name": "Alice", "age": "30"}
2) ID: 1609459201000-0, fields: {"name": "Bob", "age": "25"}
3) ID: 1609459202000-0, fields: {"name": "Charlie", "age": "35"}

What is the output of the command:

XRANGE mystream 1609459200000-0 1609459201000-0

Redis

XRANGE mystream 1609459200000-0 1609459201000-0

A [["1609459200000-0", {"name": "Alice", "age": "30"}], ["1609459201000-0", {"name": "Bob", "age": "25"}]]

B [["1609459201000-0", {"name": "Bob", "age": "25"}], ["1609459202000-0", {"name": "Charlie", "age": "35"}]]

C [["1609459200000-0", {"name": "Alice", "age": "30"}], ["1609459202000-0", {"name": "Charlie", "age": "35"}]]

D []

💡 Hint

XRANGE returns entries in the given ID range inclusive, in ascending order.

XRANGE returns all entries with IDs between the start and end IDs inclusive, ordered from smallest to largest ID.

❓ query_result

intermediate

Output of XREVRANGE with specific ID range

Using the same Redis stream mystream as before, what is the output of:

XREVRANGE mystream 1609459202000-0 1609459200000-0

Redis

XREVRANGE mystream 1609459202000-0 1609459200000-0

A [["1609459202000-0", {"name": "Charlie", "age": "35"}], ["1609459201000-0", {"name": "Bob", "age": "25"}], ["1609459200000-0", {"name": "Alice", "age": "30"}]]

B [["1609459200000-0", {"name": "Alice", "age": "30"}], ["1609459201000-0", {"name": "Bob", "age": "25"}]]

C [["1609459201000-0", {"name": "Bob", "age": "25"}], ["1609459202000-0", {"name": "Charlie", "age": "35"}]]

D []

💡 Hint

XREVRANGE returns entries in the given ID range inclusive, in descending order.

XREVRANGE returns all entries with IDs between the start and end IDs inclusive, ordered from largest to smallest ID.

📝 Syntax

advanced

Identify the syntax error in XRANGE usage

Which of the following XRANGE commands will cause a syntax error in Redis?

A XRANGE mystream 1609459200000-0 1609459201000-0

B XRANGE mystream 1609459200000-0

C XRANGE mystream 1609459200000-0 1609459201000-0 COUNT 2

D XRANGE mystream - +

💡 Hint

XRANGE requires both start and end IDs to be specified.

Option B is missing the end ID argument, causing a syntax error.

❓ optimization

advanced

Optimizing XRANGE with COUNT

You want to fetch only the latest 3 entries from a Redis stream mystream. Which command is the most efficient?

A XRANGE mystream - + COUNT 3

B XREVRANGE mystream - + COUNT 3

C XRANGE mystream + - COUNT 3

D XREVRANGE mystream + - COUNT 3

💡 Hint

To get latest entries, use XREVRANGE with descending order and COUNT.

XREVRANGE with + to - and COUNT 3 returns the latest 3 entries efficiently.

🧠 Conceptual

expert

Understanding XRANGE and XREVRANGE ID boundaries

Consider a Redis stream mystream with entries having IDs:
1000-0, 1000-1, 1001-0, 1002-0.

What will be the output of:

XRANGE mystream 1000-1 1002-0

and

XREVRANGE mystream 1002-0 1000-1

respectively?

A XRANGE returns entries with IDs 1000-1, 1001-0; XREVRANGE returns entries 1002-0, 1001-0

B XRANGE returns entries with IDs 1000-0, 1000-1, 1001-0; XREVRANGE returns entries 1002-0, 1001-0, 1000-0

C XRANGE returns entries with IDs 1000-1, 1001-0, 1002-0; XREVRANGE returns entries 1002-0, 1001-0, 1000-1

D Both commands return empty lists because the ranges are invalid

💡 Hint

XRANGE and XREVRANGE include the boundary IDs specified.

Both commands include entries with IDs equal to the start and end IDs. The order differs: XRANGE ascending, XREVRANGE descending.

Practice

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

easy

A. Adds new entries to the stream.

B. Reads stream entries from newest to oldest within a specified range.

C. Deletes entries from the stream within a specified range.

D. Reads stream entries from oldest to newest within a specified range.

Solution

1. Step 1: Understand XRANGE purpose

   XRANGE is used to read entries from a Redis stream in order from the oldest to the newest.

2. Step 2: Compare with other commands

   XREVRANGE reads in reverse order, and other commands like XADD add entries, so XRANGE is for reading oldest to newest.

3. Final Answer:

   Reads stream entries from oldest to newest within a specified range. -> Option D

4. Quick Check:

   XRANGE = oldest to newest read [OK]

Hint: XRANGE always reads forward (oldest to newest) [OK]

Common Mistakes:

• Confusing XRANGE with XREVRANGE order
• Thinking XRANGE adds or deletes entries
• Assuming XRANGE reads newest first

2. Which of the following is the correct syntax to read the last 5 entries from a Redis stream using XREVRANGE?

easy

A. XRANGE mystream + - COUNT 5

B. XREVRANGE mystream + - COUNT 5

C. XREVRANGE mystream - + COUNT 5

D. XREVRANGE mystream 0 - COUNT 5

Solution

1. Step 1: Understand XREVRANGE syntax

   XREVRANGE reads from the newest entry (+) to the oldest (-). To get last 5 entries, use range '+' to '-' with COUNT 5.

2. Step 2: Check options

   XREVRANGE mystream + - COUNT 5 uses correct order '+' to '-' and COUNT 5. XREVRANGE mystream - + COUNT 5 reverses range, XRANGE mystream + - COUNT 5 uses XRANGE, and XREVRANGE mystream 0 - COUNT 5 uses invalid start ID.

3. Final Answer:

   XREVRANGE mystream + - COUNT 5 -> Option B

4. Quick Check:

   XREVRANGE newest to oldest with COUNT [OK]

Hint: XREVRANGE uses '+' then '-' for newest to oldest [OK]

Common Mistakes:

• Swapping '+' and '-' in range
• Using XRANGE instead of XREVRANGE
• Incorrect start or end IDs

3. Given a Redis stream mystream with entries:
1609459200000-0: {"name":"Alice"}
1609459260000-0: {"name":"Bob"}
1609459320000-0: {"name":"Carol"}
What will be the result of XRANGE mystream 1609459200000-0 1609459260000-0?

medium

A. [["1609459320000-0", {"name":"Carol"}]]

B. [["1609459260000-0", {"name":"Bob"}], ["1609459200000-0", {"name":"Alice"}]]

C. [["1609459200000-0", {"name":"Alice"}], ["1609459260000-0", {"name":"Bob"}]]

D. Empty list

Solution

1. Step 1: Understand XRANGE with ID range

   XRANGE returns entries from start ID to end ID in order from oldest to newest.

2. Step 2: Identify matching entries

   Entries with IDs 1609459200000-0 and 1609459260000-0 fall within the range, so both are returned in order.

3. Final Answer:

   [["1609459200000-0", {"name":"Alice"}], ["1609459260000-0", {"name":"Bob"}]] -> Option C

4. Quick Check:

   XRANGE returns oldest to newest in range [OK]

Hint: XRANGE returns entries in ascending ID order [OK]

Common Mistakes:

• Expecting reverse order output
• Including entries outside the range
• Confusing XRANGE with XREVRANGE

4. You run the command XREVRANGE mystream - + COUNT 3 on a stream with more than 3 entries but get an empty list. What is the likely cause?

medium

A. The range start and end IDs are reversed; XREVRANGE expects '+' then '-'.

B. COUNT cannot be used with XREVRANGE.

C. The stream key 'mystream' does not exist.

D. XREVRANGE requires numeric IDs only.

Solution

1. Step 1: Check XREVRANGE range order

   XREVRANGE reads from newest to oldest, so the start ID must be '+' and end ID '-'. Using '-' then '+' reverses the order and results in an empty list.

2. Step 2: Verify other options

   COUNT is valid with XREVRANGE, a missing key returns empty list (but stream exists with >3 entries), and special IDs like -/+ are valid.

3. Final Answer:

   The range start and end IDs are reversed; XREVRANGE expects '+' then '-'. -> Option A

4. Quick Check:

   XREVRANGE range must be '+' to '-' [OK]

Hint: XREVRANGE range must start with '+' and end with '-' [OK]

Common Mistakes:

• Swapping start and end IDs in XREVRANGE
• Assuming COUNT is invalid
• Confusing the empty result with missing key

5. You want to retrieve the 3 newest entries from a stream mystream but only those with IDs greater than 1609459200000-0. Which command achieves this correctly?

hard

A. XREVRANGE mystream + 1609459200000-0 COUNT 3

B. XRANGE mystream 1609459200000-0 + COUNT 3

C. XREVRANGE mystream 1609459200000-0 + COUNT 3

D. XRANGE mystream + 1609459200000-0 COUNT 3

Solution

1. Step 1: Understand the requirement

   We want the 3 newest entries with IDs greater than 1609459200000-0, so we read backwards from newest (+) down to that ID.

2. Step 2: Choose correct command and range

   XREVRANGE reads newest to oldest, so range '+' to '1609459200000-0' with COUNT 3 gets newest entries above that ID.

3. Step 3: Eliminate wrong options

   XRANGE reads oldest to newest, so XRANGE mystream 1609459200000-0 + COUNT 3 and D read forward. XREVRANGE mystream 1609459200000-0 + COUNT 3 reverses range order for XREVRANGE, causing error.

4. Final Answer:

   XREVRANGE mystream + 1609459200000-0 COUNT 3 -> Option A

5. Quick Check:

   Newest entries above ID = XREVRANGE + to ID [OK]

Hint: Use XREVRANGE + to ID for newest entries above ID [OK]

Common Mistakes:

• Using XRANGE instead of XREVRANGE for newest entries
• Reversing range IDs in XREVRANGE
• Omitting COUNT to limit results