XTRIM for stream capping in Redis

Introduction

XTRIM helps keep a Redis stream from growing too big by removing old entries. This saves space and keeps your data fresh.

You want to keep only the latest 100 messages in a chat app.

You need to limit event logs to the last 1000 entries to save memory.

You want to keep sensor data streams capped to recent readings only.

You want to prevent a stream from growing endlessly in a real-time feed.

You want to keep your Redis storage under control by trimming old stream data.

Syntax

Redis

XTRIM key MAXLEN [~] count

Use MAXLEN to specify the maximum number of entries to keep.

The optional '~' makes trimming approximate for better performance.

Examples

Trim 'mystream' to keep only the latest 100 entries exactly.

Redis

XTRIM mystream MAXLEN 100

Trim 'mystream' approximately to keep about 1000 entries for faster trimming.

Redis

XTRIM mystream MAXLEN ~ 1000

Sample Program

Add three entries to 'mystream', then trim it to keep only the last 2 entries. Finally, read all entries to see the result.

Redis

XADD mystream * sensor-id 1 temperature 22.5
XADD mystream * sensor-id 2 temperature 23.0
XADD mystream * sensor-id 3 temperature 21.8
XTRIM mystream MAXLEN 2
XRANGE mystream - +

Important Notes

Trimming a stream helps control memory but removes old data permanently.

Using '~' for approximate trimming is faster but may keep slightly more entries than the limit.

XTRIM can be used automatically with XADD by adding MAXLEN option there.

Summary

XTRIM limits the size of a Redis stream by removing old entries.

Use MAXLEN to set how many entries to keep.

Approximate trimming (~) improves speed but is less exact.

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