Introspection & presence

super-line gives you two views of who's connected, plus the ability to reach — and even request — any connection across nodes.

• Local (srv.local) — synchronous, reads only the connections on this node.
• Cluster (srv.cluster) — asynchronous, reads a shared presence registry (in-memory for one node, Redis across many).

Reach for srv.local when you're acting on the current process; reach for srv.cluster when you need the whole fleet.

Connection metadata

Every connection carries identity and liveness:

conn.id           // server-assigned unique id (stable for the connection's life)
conn.role         // its role
conn.connectedAt  // Date.now() at the upgrade
conn.lastPongAt   // last heartbeat pong (liveness) — node-local
conn.lastPingAt   // last heartbeat ping sent

Local introspection (sync)

srv.nodeId                       // this process's stable id
srv.local.connections            // Conn[] on this node
srv.local.rooms                  // room names with members here
srv.local.topics                 // topic names with subscribers here
srv.room('lobby').connections    // members of a room (this node)
srv.room('lobby').size           // member count (this node)

Everything else is plain JavaScript — filter the array:

srv.local.connections.filter((c) => c.role === 'user')
srv.local.connections.find((c) => c.id === someId)
const stale = srv.local.connections.filter((c) => Date.now() - (c.lastPongAt ?? 0) > 60_000)

Heartbeat

One timer pings every connection (default every 30s), updating lastPingAt/lastPongAt. Optionally reap dead sockets:

import { webSocketServerTransport } from '@super-line/transport-websocket'

createSuperLineServer(api, {
  transports: [webSocketServerTransport({ server })],
  authenticate,
  heartbeat: { interval: 30_000, maxMissed: 2 }, // terminate after 2 missed pongs (fires onDisconnect)
})
// heartbeat: false   // disable entirely

A reaped connection is terminate()d and flows through onDisconnect like any other drop.

Cluster introspection (async)

The cluster view reads the presence registry. To make connections identifiable across nodes, give the server an identify hook (a stable user key) and, optionally, a describeConn projector for extra fields. ctx is never auto-serialized.

createSuperLineServer(api, {
  transports: [webSocketServerTransport({ server })],
  authenticate,
  identify: (conn) => conn.ctx.userId,          // powers byUser / isOnline / toUser
  describeConn: (conn) => ({ plan: conn.ctx.plan }), // extra descriptor fields
})

await srv.cluster.count()           // total connections cluster-wide
await srv.cluster.connections()     // ConnDescriptor[] across all nodes
await srv.cluster.byUser('u42')     // that user's connections (any node)
await srv.cluster.room('lobby')     // members of a room across nodes
await srv.cluster.topology()        // [{ nodeId, connections, rooms, alive }]
await srv.isOnline('u42')           // is this user connected anywhere?

A ConnDescriptor is a serializable snapshot (not a live Conn): { id, role, nodeId, connectedAt, userId?, rooms, ...describeConn }. It's taken at connect, so seed any descriptor fields in onConnection (which runs just before the snapshot). Live-updating values like lastPongAt stay node-local and are not in the registry.

Requires a presence-capable adapter

The in-memory and Redis adapters both implement presence. srv.cluster.* throws a clear error on an adapter that doesn't. Liveness is tracked by a per-node key with a TTL refreshed by the heartbeat, so a crashed node's connections drop out of cluster queries automatically; a graceful close() removes them immediately. See Choose an adapter.

Targeted send across nodes

Reach a specific connection or user no matter which node holds them — no registry lookup on the delivery path:

srv.toConn(id).emit('notice', { text: 'hi' })   // one connection, any node
srv.toUser('u42').emit('notice', { text: 'hi' }) // every device of a user, any node

srv.toConn(id).close()       // kick one connection (any node)
srv.toUser('u42').disconnect() // kick all of a user's connections

emit here takes shared events (the vocabulary every role understands), like room.broadcast. This is the cross-node answer to node-local conn.emit.

Ask a client a question — server → client requests

A connection can be asked a typed question. Declare a serverToClient entry with input + output — a third flavor alongside events and topics:

shared: {
  serverToClient: {
    confirm: { input: z.object({ q: z.string() }), output: z.object({ ok: z.boolean() }) },
  },
}

The client answers with implement (throw a SuperLineError for a typed failure):

client.implement({
  confirm: async ({ q }) => ({ ok: q === 'ready?' }),
})

The server awaits the reply — across nodes — with toConn(id).request:

const answer = await srv.toConn(id).request('confirm', { q: 'ready?' }, { timeout: 5_000 })
answer.ok // boolean, typed

request is offered on toConn only (a single, unambiguous target) and is typed to shared server requests — the caller has an id, not a role. If no live node owns the id, or the client doesn't answer in time, it rejects with a TIMEOUT SuperLineError. To request a user, pick a connection first: const [c] = await srv.cluster.byUser(uid); await srv.toConn(c.id).request(...).

Per-connection state

Need mutable scratch state on a connection? Declare a data schema in a role block; conn.data is typed per role and starts {}:

roles: {
  user: {
    data: z.object({ lastSeenMsgId: z.number() }),
    clientToServer: { ack: { input: z.object({ id: z.number() }), output: z.object({}) } },
  },
}

srv.implement({
  user: {
    ack: async ({ id }, _ctx, conn) => {
      conn.data.lastSeenMsgId = id // typed
      return {}
    },
  },
})

Backpressure

Guard nodes against slow consumers:

createSuperLineServer(api, {
  transports: [
    webSocketServerTransport({
      server,
      backpressure: { maxBufferedBytes: 8 * 1024 * 1024, onExceed: 'close' }, // or 'drop'
    }),
  ],
  authenticate,
})

When a connection's ws.bufferedAmount exceeds the limit, 'close' (default) drops it with code 1013; 'drop' skips the frame (logged, never silent).

Next

• Choose an adapter — make srv.cluster.* span more than one node.
• Broadcast across nodes with the cluster event bus — the fan-out counterpart to targeted send.
• Inspect traffic with the Control Center — presence and topology in a live UI.