# Sharded Pub/Sub in Redis Cluster — Redis Pub/Sub & Streams

Source: https://www.skillbyai.com/en/redis-streams/a-sharded

> Scale Pub/Sub in a cluster with SSUBSCRIBE and SPUBLISH.

## Channels that live on one shard

In Redis Cluster, the classic `PUBLISH` is **propagated to every node** so that subscribers connected anywhere receive it. That is simple, but cluster bus traffic grows with the number of nodes, so classic Pub/Sub does not scale horizontally. **Sharded Pub/Sub**, introduced in **Redis 7.0**, assigns each channel to a **hash slot** exactly like a key, so it belongs to one shard (a primary and its replicas). Clients use **`SSUBSCRIBE`** and **`SPUBLISH`**, connecting to the shard that owns the channel; messages travel only within that shard. Throughput now scales with the number of shards, and you can co-locate related channels and keys with **hash tags** such as `{room:42}:chat`. Cluster-aware client libraries route sharded commands automatically and resubscribe when slots move during resharding or failover. Use sharded Pub/Sub for any new cluster deployment with significant message volume.

## Sharded channels

Each channel hashes to one shard, so messages stay within that shard instead of flooding the cluster.

![Three cluster shard boxes; a message arrow enters one shard and fans out only to subscribers attached to that shard, while the other shards stay quiet.](assets/figures/redis-streams/section-3-map.svg) — Figure 3.1 — Sharded Pub/Sub keeps traffic within one shard.

## Sharded Pub/Sub commands

The channel name is hashed like a key; hash tags group related channels.

```bash
# subscriber (cluster-aware client or redis-cli -c)
redis-cli -c SSUBSCRIBE '{room:42}:chat'

# publisher
redis-cli -c SPUBLISH '{room:42}:chat' '{"from":"asha","text":"hi"}'

# which slot (and so which shard) owns the channel
redis-cli -c CLUSTER KEYSLOT '{room:42}:chat'

# list sharded channels on a node
redis-cli PUBSUB SHARDCHANNELS
```

## Local announcements per building

Classic Pub/Sub in a cluster is announcing every message in every building of the campus. Sharded Pub/Sub sends each announcement only to the building where that room's members actually sit.

**Quiz:** Why does sharded Pub/Sub scale better than classic Pub/Sub in Redis Cluster?

- [x] Each channel belongs to one shard, so messages are not broadcast to every node
- [ ] It stores messages on disk
- [ ] It removes the need for subscribers
- [ ] It compresses all messages

*Answer:* Each channel belongs to one shard, so messages are not broadcast to every node. SPUBLISH keeps traffic within the shard that owns the channel's hash slot.