> ## Documentation Index
> Fetch the complete documentation index at: https://crosslink.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Scale-out and regions

> Run Redis-backed signaling and quota-enforced relays across regions.

Crosslink scales signaling and relays differently: signaling nodes share short-lived
routing state through Redis, while relay channels stay on the region that allocated
them. Hosts publish ordered regional fallback URLs so clients do not guess where a
channel lives.

## Redis-backed signaling

Set the same Redis URL and namespace for every signaling replica:

```bash theme={null}
CROSSLINK_REDIS_URL=rediss://user:password@redis.internal:6379/0 \
CROSSLINK_REGION=us-east \
CROSSLINK_SIGNALING_TOKEN="$SIGNAL_TOKEN" \
node services/signaling/dist/cli.js
```

Redis stores expiring presence and pairing-code hashes and carries opaque cross-node
routing messages over pub/sub. Identity keys, session keys, pairing plaintext, and
application content are never written to Redis. Use `rediss://`, Redis ACLs, a private
network, eviction monitoring, and a deployment-specific namespace.

## Relay quotas

| Variable                        | Meaning                                       |
| ------------------------------- | --------------------------------------------- |
| `CROSSLINK_RELAY_MAX_CHANNELS`  | Concurrent allocated channels on this process |
| `CROSSLINK_RELAY_MAX_CLIENTS`   | Concurrent clients per channel                |
| `CROSSLINK_RELAY_MAX_BYTES`     | Lifetime bytes in both directions per channel |
| `CROSSLINK_RELAY_BYTES_PER_SEC` | Per-channel token-bucket throughput           |

Quota violations close or reject only the affected channel and increment `quotaDrops`
in authenticated relay statistics. Put infrastructure-level connection and request
limits in front of the process as a second boundary.

## Region catalog

```bash theme={null}
export CROSSLINK_REGION=us-east
export CROSSLINK_RELAY_PUBLIC_URL=https://relay-use1.example.com
export CROSSLINK_RELAY_REGIONS='[
  {"id":"us-east","url":"https://relay-use1.example.com","priority":10},
  {"id":"us-west","url":"https://relay-usw2.example.com","priority":20},
  {"id":"eu-west","url":"https://relay-euw1.example.com","priority":30}
]'
```

Hosts may also pass `relayUrls`. Allocation tries each validated HTTPS URL in order
and reconnects by reallocating when a channel expires. Every region independently
requires host and, when configured, client tokens.

<Warning>
  Do not put a TCP load balancer in front of relay WebSockets unless it preserves
  channel affinity. A channel is process-local by design.
</Warning>