# Trimming and Memory Management — Redis Pub/Sub & Streams

Source: https://www.skillbyai.com/en/redis-streams/s-trim

> Cap stream size with MAXLEN and MINID and delete entries safely.

## Streams grow until you trim them

Because reading does not remove entries, streams **grow forever** unless you trim them, and Redis memory is finite. Trim on write with `XADD ... MAXLEN ~ 100000` (keep about the last 100,000 entries) or `MINID ~ <id>` (drop entries older than an ID, which is effectively a time cutoff), or trim separately with **`XTRIM`**. The **`~`** modifier allows **approximate** trimming: Redis removes whole internal nodes only, keeping slightly more entries than requested, which is far cheaper than exact trimming and is the recommended default. **`XDEL`** removes specific entries by ID, but it only marks them deleted inside nodes, so memory is reclaimed later; use it sparingly. Choose retention by need: enough history for consumers to recover from outages (for example, a day's worth), not unlimited. Monitor stream length and memory, and remember that trimming does not care whether consumer groups have processed the entries, so trimming too aggressively can drop unprocessed work.

## Capping streams by length and age

Approximate trimming on every write keeps memory bounded cheaply.

```bash
# keep roughly the latest 100k entries
XADD orders MAXLEN '~' 100000 '*' orderId o-1003 status placed

# drop entries older than 24 hours (MINID uses the ms part of the ID)
# cutoff = now_ms - 86_400_000, computed by the client
XTRIM orders MINID '~' 1727856312345

# inspect size and memory
XLEN orders
MEMORY USAGE orders
XINFO STREAM orders
```

## Never trim below your slowest consumer

If a consumer group is two hours behind and you keep one hour of history, those unread entries are gone. Alert on group lag and size retention to cover realistic outages.

**Quiz:** What does the `~` in `XADD orders MAXLEN ~ 100000 * ...` do?

- [x] Allows approximate, cheaper trimming that may keep slightly more entries
- [ ] Makes trimming exact to the last entry
- [ ] Disables trimming
- [ ] Deletes the whole stream

*Answer:* Allows approximate, cheaper trimming that may keep slightly more entries. Approximate trimming removes whole internal nodes, which is much more efficient.