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.