# Limits: Slow Subscribers, Reconnects and Scale — Redis Pub/Sub & Streams

Source: https://www.skillbyai.com/en/redis-streams/p-limits

> Understand output buffer limits, reconnect gaps and Pub/Sub scaling behaviour.

## Where Pub/Sub breaks down

Because Redis pushes messages to subscribers as fast as they are published, a **slow subscriber** builds up an **output buffer** inside Redis. To protect itself, Redis applies `client-output-buffer-limit pubsub` (by default a hard limit of 32 MB, or 8 MB sustained for 60 seconds) and **disconnects** subscribers that exceed it, so they lose messages. Reconnects create **gaps**: any message published between disconnect and re-subscribe is lost, so clients that need continuity should resynchronise state after reconnecting (for example, reload from the database or read recent entries from a stream). Every published message is sent to every subscriber, so cost grows with **subscribers × message rate**, and in a classic Redis Cluster a `PUBLISH` is **broadcast to every node**, which limits scalability; **sharded Pub/Sub** (next section) fixes that. Finally, there is no history: a new subscriber cannot ask what happened before it joined.

## Watching and tuning output buffers

Look for subscribers with large output buffers before Redis disconnects them.

```bash
# list connected clients; check omem (output memory) and flags for subscribers
redis-cli CLIENT LIST TYPE pubsub

# default limit for pub/sub clients: hard 32mb, soft 8mb for 60 s
redis-cli CONFIG GET client-output-buffer-limit

# raising limits only hides a consumer that cannot keep up;
# better: make subscribers fast (hand work to a local queue) or move to Streams
```

## Do slow work off the subscriber loop

A subscriber callback that writes to a database or calls an API for each message will fall behind under load. Push messages into an in-process queue and process them in worker threads, or use Streams with consumer groups.

**Quiz:** What does Redis do to a Pub/Sub subscriber whose output buffer exceeds the configured limit?

- [x] Disconnects the subscriber, which then misses messages
- [ ] Pauses publishers
- [ ] Writes the messages to disk
- [ ] Converts the channel to a stream

*Answer:* Disconnects the subscriber, which then misses messages. Output buffer limits protect Redis memory by disconnecting clients that cannot keep up.