XRANGE and XREVRANGE in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get all entries from the stream 'mystream' starting from the beginning.

Redis

XRANGE mystream [1] +

Drag options to blanks, or click blank then click option'

A0

B-

C1

D$

Attempts:

3 left

💡 Hint

Common Mistakes

Using '0' instead of '-' as the start ID.

Using '$' which means the latest entry, not the start.

The XRANGE command requires a start and end ID. Using '-' as the start means from the earliest entry.

2fill in blank

medium

Complete the code to get the last 5 entries from the stream 'mystream' in reverse order.

Redis

XREVRANGE mystream + [1] COUNT 5

Drag options to blanks, or click blank then click option'

A-

B0

C1

D$

Attempts:

3 left

💡 Hint

Common Mistakes

Using '0' which is not a valid stream ID.

Using '$' which is the latest entry, not the earliest.

In XREVRANGE, '+' is the latest entry and '-' is the earliest. To get last 5 entries, range from '+' to '-'.

3fill in blank

hard

Fix the error in the command to get entries from 'mystream' starting at ID '1609459200000-0'.

Redis

XRANGE mystream [1] +

Drag options to blanks, or click blank then click option'

A'1609459200000-0'

B1609459200000_0

C1609459200000-0

D1609459200000

Attempts:

3 left

💡 Hint

Common Mistakes

Adding quotes around the ID.

Replacing dash with underscore.

Using incomplete ID without sequence number.

The stream ID must be a string without quotes and with a dash '-' separating timestamp and sequence number.

4fill in blank

hard

Fill both blanks to get entries from 'mystream' between IDs '1609459200000-0' and '1609459300000-0'.

Redis

XRANGE mystream [1] [2]

Drag options to blanks, or click blank then click option'

A1609459200000-0

B1609459300000-0

C-

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping start and end IDs.

Using '-' or '+' instead of actual IDs.

XRANGE requires start and end IDs. Use the given IDs as start and end to get entries in that range.

5fill in blank

hard

Fill all three blanks to get the last 3 entries from 'mystream' in reverse order starting from the latest entry.

Redis

XREVRANGE mystream [1] [2] COUNT [3]

Drag options to blanks, or click blank then click option'

A+

B-

C3

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong start or end IDs.

Using COUNT larger than needed.

To get last 3 entries in reverse, start from '+' (latest) to '-' (earliest) with COUNT 3.

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