XTRIM for stream capping in Redis - Step-by-Step Execution

Concept Flow - XTRIM for stream capping

Start with Redis Stream

↓

Add new entries to stream

↓

Check stream length

↓

Is length > max length?

No→Continue adding entries

Yes↓

Trim stream to max length

↓

Stream capped at max length

↓

Wait for next entry to add

XADD adds new entries to a Redis stream and XTRIM trims it to keep the stream length within a maximum limit.

Execution Sample

Redis

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

Add two entries to a stream and then trim it to keep only the latest 2 entries.

Execution Table

Step	Command	Stream Length Before	Action	Stream Length After	Result
1	XADD mystream * field1 value1	0	Add entry	1	Entry added, stream length is 1
2	XADD mystream * field2 value2	1	Add entry	2	Entry added, stream length is 2
3	XTRIM mystream MAXLEN 2	2	Trim stream to max length 2	2	Stream length capped at 2, no entries removed
4	XADD mystream * field3 value3	2	Add entry	3	Entry added, stream length is 3
5	XTRIM mystream MAXLEN 2	3	Trim stream to max length 2	2	Oldest entry removed, stream length back to 2

💡 Stream length is maintained at max length 2 after trimming

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
Stream Length	0	1	2	2	3	2

Key Moments - 2 Insights

Why does the stream length increase to 3 after adding the third entry before trimming?

Because XADD adds the entry first, increasing length to 3, then XTRIM trims it back to max length 2 as shown in steps 4 and 5.

Does XTRIM remove entries if the stream length is already at or below max length?

No, as shown in step 3, when length is 2 and max length is 2, XTRIM does not remove any entries.

Visual Quiz - 3 Questions

Look at the execution table, what is the stream length after step 2?

A 1

B 3

C 2

D 0

💡 Hint

Check the 'Stream Length After' column in row for step 2.

At which step does the stream length first exceed the max length of 2?

A Step 3

B Step 4

C Step 5

D Step 2

💡 Hint

Look at the 'Stream Length After' column and see when it becomes 3.

If we change MAXLEN to 3, what would be the stream length after step 5?

A 3

B 4

C 2

D 1

💡 Hint

With MAXLEN 3, trimming won't remove entries when length is 3, see step 5 logic.

Concept Snapshot

XTRIM trims a Redis stream to a max length.
Syntax: XTRIM key MAXLEN maxlen
Adds entries with XADD.
If stream length > maxlen, oldest entries removed.
Keeps stream size manageable.

Full Transcript

This visual execution shows how Redis XTRIM works to cap stream length. We start with an empty stream. Each XADD command adds an entry, increasing stream length. When the length exceeds the max length set by XTRIM, the oldest entries are removed to keep the stream capped. The execution table tracks each step, showing stream length before and after commands. Key moments clarify that XTRIM only removes entries if length exceeds max length, and that adding entries happens before trimming. The quiz tests understanding of stream length changes at each step. This helps beginners see how XTRIM manages stream size in Redis.

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