XRANGE and XREVRANGE in Redis

Introduction

XRANGE and XREVRANGE help you read messages from a Redis stream in order, either from oldest to newest or newest to oldest.

You want to see all messages sent to a chat app in the order they arrived.

You need to get the latest updates from a sensor stream starting from the newest.

You want to process events from a stream starting from the oldest event.

You want to review recent logs in reverse order to find the latest errors quickly.

Syntax

Redis

XRANGE key start end [COUNT count]
XREVRANGE key end start [COUNT count]

XRANGE returns stream entries from the start ID to the end ID in ascending order.

XREVRANGE returns stream entries from the end ID to the start ID in descending order.

Examples

Get all entries from the stream 'mystream' from the oldest (-) to the newest (+).

Redis

XRANGE mystream - +

Get up to 5 entries from 'mystream' between two specific IDs.

Redis

XRANGE mystream 1609459200000-0 1609459300000-0 COUNT 5

Get all entries from 'mystream' from newest (+) to oldest (-).

Redis

XREVRANGE mystream + -

Get up to 3 entries in reverse order between two IDs.

Redis

XREVRANGE mystream 1609459300000-0 1609459200000-0 COUNT 3

Sample Program

First, we add two entries to 'mystream'. Then, XRANGE reads all entries from oldest to newest. Finally, XREVRANGE reads the newest entry only.

Redis

XADD mystream * sensor_id 1 temperature 22.5
XADD mystream * sensor_id 2 temperature 23.0
XRANGE mystream - +
XREVRANGE mystream + - COUNT 1

Important Notes

The IDs in streams are usually timestamps followed by a sequence number.

Use '-' to mean the smallest ID and '+' to mean the largest ID in XRANGE and XREVRANGE.

COUNT limits how many entries you get back, which helps when streams are very long.

Summary

XRANGE reads stream entries from oldest to newest.

XREVRANGE reads stream entries from newest to oldest.

Both commands can limit results with COUNT and specify ID ranges.

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