XTRIM for stream capping in Redis - Mini Project: Build & Apply

Using XTRIM to Cap Redis Streams

📖 Scenario: You are managing a Redis stream that collects temperature sensor readings from a smart home system. To keep the stream size manageable and avoid using too much memory, you want to limit the stream to only the latest 5 readings.

🎯 Goal: Build a Redis stream capped at 5 entries using the XTRIM command to automatically remove older entries when new ones are added.

📋 What You'll Learn

Create a Redis stream called sensor:temperature with initial entries.

Set a maximum length variable max_len to 5.

Use the XADD command with the MAXLEN option to add new entries while trimming the stream.

Verify that the stream never exceeds 5 entries after adding new data.

💡 Why This Matters

🌍 Real World

IoT devices and real-time data systems often use Redis streams to collect sensor data. Capping streams prevents memory overflow and keeps data manageable.

💼 Career

Understanding how to cap Redis streams is useful for backend developers and system administrators managing real-time data pipelines and ensuring efficient resource use.

1

Create the initial Redis stream with 3 temperature readings

Use the XADD command to create a Redis stream called sensor:temperature and add these exact entries: {"temp": "22.5"}, {"temp": "23.0"}, and {"temp": "22.8"}. Use the * ID to let Redis generate IDs automatically.

Redis

# Use XADD to add three entries to sensor:temperature stream
# Your code here

Hint

Use XADD with * for automatic IDs and specify the field temp with the given values.

2

Set a maximum length variable for the stream

Create a variable called max_len and set it to 5 to represent the maximum number of entries allowed in the stream.

Redis

XADD sensor:temperature * temp 22.5
XADD sensor:temperature * temp 23.0
XADD sensor:temperature * temp 22.8

# Set max_len variable to 5
# Your code here

Hint

Just assign the number 5 to a variable named max_len.

3

Add new entries using XADD with MAXLEN to cap the stream

Use the XADD command with the MAXLEN option set to ~ and the variable max_len to add two new temperature readings: 23.2 and 23.5. Use the * ID for automatic IDs.

Redis

XADD sensor:temperature * temp 22.5
XADD sensor:temperature * temp 23.0
XADD sensor:temperature * temp 22.8

max_len = 5

# Add two new entries with XADD using MAXLEN ~ max_len
# Your code here

Hint

Use XADD sensor:temperature MAXLEN ~ 5 * followed by the field and value to add entries while trimming.

4

Verify the stream is capped at 5 entries

Use the XLEN command on the sensor:temperature stream to check that it contains exactly 5 entries after adding the new data.

Redis

XADD sensor:temperature * temp 22.5
XADD sensor:temperature * temp 23.0
XADD sensor:temperature * temp 22.8

max_len = 5

XADD sensor:temperature MAXLEN ~ 5 * temp 23.2
XADD sensor:temperature MAXLEN ~ 5 * temp 23.5

# Use XLEN to check the stream length
# Your code here

Hint

Use XLEN sensor:temperature to get the number of entries in the stream.

Practice

1. What does the Redis XTRIM command do when used with the MAXLEN option?

easy

A. It renames the stream to a new key.

B. It adds new entries to the stream without removing any.

C. It limits the number of entries in a stream by removing the oldest ones.

D. It deletes the entire stream immediately.

Solution

1. Step 1: Understand the purpose of XTRIM with MAXLEN

   The XTRIM command with MAXLEN is used to keep the stream size limited by removing old entries.

2. Step 2: Identify the effect on the stream

   It removes the oldest entries to keep the stream length within the specified maximum.

3. Final Answer:

   It limits the number of entries in a stream by removing the oldest ones. -> Option C

4. Quick Check:

   XTRIM + MAXLEN = Remove oldest entries [OK]

Hint: XTRIM with MAXLEN trims oldest entries to cap stream size [OK]

Common Mistakes:

• Thinking XTRIM adds entries instead of removing
• Confusing XTRIM with deleting the whole stream
• Assuming XTRIM renames the stream

2. Which of the following is the correct syntax to trim a Redis stream named mystream to keep at most 1000 entries approximately?

easy

A. XTRIM mystream MAXLEN ~1000

B. XTRIM mystream MAXLEN=1000

C. XTRIM mystream MAXLEN ~ 1000

D. XTRIM mystream MAXLEN 1000

Solution

1. Step 1: Recall syntax for approximate trimming

   Approximate trimming uses the tilde (~) before the number in the XTRIM command.

2. Step 2: Match correct syntax

   The correct syntax is XTRIM mystream MAXLEN ~1000 with spaces separating arguments.

3. Final Answer:

   XTRIM mystream MAXLEN ~1000 -> Option A

4. Quick Check:

   Approximate trim syntax = MAXLEN ~ number [OK]

Hint: Use tilde (~) with MAXLEN for approximate trimming [OK]

Common Mistakes:

• Omitting the tilde (~) for approximate trimming
• Using equals sign (=) which is invalid syntax
• Not spacing arguments correctly

3. Given the following commands executed sequentially:

XADD mystream * field1 value1
XADD mystream * field2 value2
XADD mystream * field3 value3
XTRIM mystream MAXLEN 2

How many entries will mystream contain after these commands?

medium

A. 2 entries

B. 3 entries

C. 1 entry

D. 0 entries

Solution

1. Step 1: Count entries added before trimming

   Three entries are added to mystream using XADD.

2. Step 2: Apply XTRIM with MAXLEN 2

   The XTRIM command trims the stream to keep only the latest 2 entries, removing the oldest one.

3. Final Answer:

   2 entries -> Option A

4. Quick Check:

   3 added, trimmed to 2 = 2 entries [OK]

Hint: XTRIM MAXLEN keeps newest entries, removes oldest [OK]

Common Mistakes:

• Assuming no trimming happens after XTRIM
• Counting all added entries without trimming
• Thinking XTRIM deletes all entries

4. You run the command XTRIM mystream MAXLEN ~1000 but notice the stream still has over 2000 entries. What is the most likely reason?

medium

A. The command syntax is incorrect and was ignored.

B. The approximate trim (~) allows the stream to exceed the limit temporarily.

C. XTRIM only works on empty streams.

D. The stream key name is case-sensitive and was misspelled.

Solution

1. Step 1: Understand approximate trimming behavior

   Using the tilde (~) means trimming is approximate and may allow the stream to temporarily exceed the limit.

2. Step 2: Eliminate other causes

   Syntax is correct, XTRIM works on non-empty streams, and key name issues would cause errors, not approximate trimming behavior.

3. Final Answer:

   The approximate trim (~) allows the stream to exceed the limit temporarily. -> Option B

4. Quick Check:

   Approximate trim = possible temporary size excess [OK]

Hint: Approximate trim (~) may exceed limit briefly [OK]

Common Mistakes:

• Assuming syntax error caused no trimming
• Thinking XTRIM only works on empty streams
• Ignoring approximate trimming behavior

5. You want to keep a Redis stream events capped at 500 entries but want to improve trimming speed by allowing approximate trimming. Which command should you use?

hard

A. XTRIM events MAXLEN = 500

B. XTRIM events MAXLEN 500

C. XTRIM events MINLEN ~ 500

D. XTRIM events MAXLEN ~500

Solution

1. Step 1: Identify approximate trimming syntax

   Approximate trimming uses the tilde (~) before the number with MAXLEN.

2. Step 2: Choose correct command

   XTRIM events MAXLEN ~500 caps the stream at about 500 entries with faster trimming.

3. Final Answer:

   XTRIM events MAXLEN ~500 -> Option D

4. Quick Check:

   Approximate trim = MAXLEN ~ number [OK]

Hint: Use MAXLEN ~ number for fast approximate trimming [OK]

Common Mistakes:

• Using MINLEN instead of MAXLEN
• Using equals sign (=) which is invalid
• Omitting the tilde (~) for approximate trimming