EventHub API

The EventHub service provides topic-based pub/sub messaging for real-time communication across channels, Server-Sent Events, and custom transports.

Overview

EventHub enables:

• Topic-based message routing
• Broadcasting to multiple subscribers
• Selective exclusion (send to all except one)
• Stateless and stateful operation modes
• Cross-transport compatibility (WebSockets, SSE, etc.)

Methods

subscribe()

Subscribe a channel/connection to a topic.

await eventHub.subscribe(topic: string, subscriberId: string)

Parameters:

• topic - Topic name (e.g., 'room:lobby', 'user:123')
• subscriberId - Unique identifier for the subscriber (typically channel.channelId)

Example:

// Subscribe channel to a room topic
await eventHub.subscribe(`room:${roomId}`, channel.channelId)

unsubscribe()

Unsubscribe a channel/connection from a topic.

await eventHub.unsubscribe(topic: string, subscriberId: string)

Parameters:

• topic - Topic name to unsubscribe from
• subscriberId - Subscriber identifier to remove

Example:

// Unsubscribe when leaving a room
await eventHub.unsubscribe(`room:${roomId}`, channel.channelId)

Automatic Cleanup

Pikku automatically unsubscribes all topics when a channel disconnects. You only need to manually unsubscribe when changing subscriptions during an active connection.

publish()

Publish a message to all subscribers of a topic.

await eventHub.publish(
  topic: string,
  excludeId: string | null,
  payload: any
)

Parameters:

• topic - Topic to publish to
• excludeId - Subscriber ID to exclude (set to null to broadcast to all)
• payload - Message payload (must be JSON-serializable)

Examples:

Broadcast to all subscribers:

await eventHub.publish(
  `room:${roomId}`,
  null,  // Send to all
  {
    type: 'message',
    text: 'Hello everyone!',
    userId: currentUserId
  }
)

Broadcast to all except sender:

await eventHub.publish(
  `room:${roomId}`,
  channel.channelId,  // Exclude sender
  {
    type: 'user_joined',
    userId: newUserId,
    username: newUserName
  }
)

Usage Patterns

Chat Room Example

// Connection - subscribe to room
export const joinRoom = pikkuChannelConnectionFunc<
  { welcome: string },
  { room: string }
>(async ({ eventHub, channel }) => {
  const room = channel.openingData.room
  await eventHub.subscribe(`room:${room}`, channel.channelId)
  return { welcome: `Welcome to ${room}!` }
})

// Message - broadcast to room (excluding sender)
export const sendMessage = pikkuChannelFunc<
  { message: string },
  void,
  { room: string }
>(async ({ eventHub, channel, userSession }, data) => {
  const room = channel.openingData.room
  await eventHub.publish(
    `room:${room}`,
    channel.channelId, // Don't echo back to sender
    {
      message: data.message,
      userId: userSession.userId,
      timestamp: Date.now()
    }
  )
})

// Disconnect - cleanup (optional, happens automatically)
export const leaveRoom = pikkuChannelDisconnectionFunc<{ room: string }>(
  async ({ eventHub }, data) => {
    await eventHub.unsubscribe(`room:${data.room}`, data.channelId)
  }
)

Presence System

// Publish presence updates
export const updatePresence = pikkuFunc<
  { status: 'online' | 'away' | 'offline' },
  void
>(async ({ eventHub, userSession }, data) => {
  await eventHub.publish('presence:updates', null, {
    userId: userSession.userId,
    status: data.status,
    timestamp: Date.now()
  })
})

// Subscribe to presence on connect
export const watchPresence = pikkuChannelConnectionFunc(
  async ({ eventHub, channel }) => {
    await eventHub.subscribe('presence:updates', channel.channelId)
  }
)

Notification System

// Subscribe user to their personal notification channel
export const connectNotifications = pikkuChannelConnectionFunc<
  void,
  { userId: string }
>(async ({ eventHub, channel }) => {
  const userId = channel.openingData.userId
  await eventHub.subscribe(`notifications:${userId}`, channel.channelId)
})

// Trigger notification from any function
export const sendNotification = pikkuFunc<
  { userId: string; title: string; message: string },
  void
>(async ({ eventHub }, data) => {
  await eventHub.publish(`notifications:${data.userId}`, null, {
    title: data.title,
    message: data.message,
    timestamp: Date.now()
  })
})