Channels

This page shows you how to use Wheels channels — a pub/sub layer on top of Server-Sent Events. You’ll publish named events from anywhere in your app, stream them to subscribed browsers from a controller action, pick the right adapter for your deployment, and wire up the JavaScript client.

You’ll learn:

• What channels add on top of the low-level SSE API
• Where Wheels stands on WebSockets — and why channels are SSE-only by design
• How to publish with publish() and subscribe with subscribeToChannel()
• When to use the memory adapter versus the database adapter
• How to deliver the WheelsSSE JavaScript client to the browser (it is not auto-served)

Note

You should already know how the low-level SSE API works. See Server-Sent Events.

What channels add

The SSE guide covers the raw primitives: renderSSE() for one-shot events and initSSEStream() / sendSSEEvent() / closeSSEStream() for hand-rolled streams. With those, you write the delivery loop yourself.

Channels layer pub/sub on top. You publish a named event to a named channel from anywhere — a model callback, a background job, another controller — and any browser subscribed to that channel receives it. The framework runs the long-lived connection loop, the event filtering, the heartbeats, and the Last-Event-ID resume handling for you. The low-level SSE API keeps working unchanged; channels are built on it, not a replacement for it.

Three pieces make up the surface:

• publish(channel, event, data) — a global function. Callable from controllers, models, jobs, and anywhere else global helpers are available.
• subscribeToChannel(...) — a controller mixin. Call it from an action to turn that action into a long-lived SSE endpoint streaming the channel’s events.
• channelSSETag(...) — a view helper that emits a small <script> tag opening an EventSource against your subscribe action.

Everything flows one way: server to client.

The WebSocket story

Wheels does not ship WebSocket support. There is no bidirectional channel, no binary framing, no presence tracking, and no cfwebsocket integration. Channels are one-directional — server to client — over SSE, and that is a deliberate design choice, not a stopgap:

• Plain HTTP. SSE rides an ordinary long-lived GET request. Every proxy, load balancer, and firewall that speaks HTTP passes it through; WebSockets need their own upgrade handshake and protocol support at every hop.
• Free reconnection. The browser’s EventSource reconnects automatically and resends Last-Event-ID so you can resume. With WebSockets you write that yourself.
• Auth you already have. Same-origin SSE requests carry your session cookie. No separate token handshake for a second protocol.

AdonisJS made the same call with its Transmit package — SSE-only, no WebSocket server — for the same reasons.

For the client-to-server direction, use what you already have: an ordinary HTTP POST. The action saves whatever it needs to and calls publish() to fan the result out to subscribers. The chat room pattern below is exactly this — and it covers most “I need WebSockets” use cases.

Native WebSocket support is being scoped on the roadmap — follow issue #2962.

Quick start

A subscribe action, a route to it, a publish call, and a view tag.

app/controllers/Notifications.cfc

component extends="Controller" {
    function stream() {
        // Derive the channel server-side — don't trust a client-supplied channel name
        subscribeToChannel(channel = "user.#session.userId#");
    }
}

config/routes.cfm (excerpt)

.get(name = "notificationStream", pattern = "notifications/stream", to = "notifications##stream")

Publish from anywhere — here, a model callback:

app/models/Order.cfc

component extends="Model" {
    function config() {
        afterCreate("notifyUser");
    }

    private function notifyUser() {
        publish(
            channel = "user.#this.userId#",
            event = "message",
            data = SerializeJSON({title: "Order received", orderId: this.id})
        );
    }
}

In the view, channelSSETag() emits a <script> tag that opens an EventSource against the subscribe endpoint:

app/views/orders/index.cfm (excerpt)

#channelSSETag(channel="user.#session.userId#", route="notificationStream")#

The tag sends its channel argument to the endpoint as a channel URL parameter, but the stream() action above deliberately ignores it and derives the channel from the session — a client editing the URL can’t subscribe to another user’s notifications. An endpoint that serves more than one channel can subscribe with subscribeToChannel(channel = params.channel) instead, after validating the subscription — see channelSSETag().

The generated script relays incoming events as wheels:sse CustomEvents on document, with {data, event, id} in e.detail:

illustrative — browser JS

document.addEventListener('wheels:sse', (e) => {
    const payload = JSON.parse(e.detail.data);
    showToast(payload.title);
});

That’s it — publish with the default message event type, render channelSSETag() once per page, listen for wheels:sse on document. Named event types and per-event listeners are covered under channelSSETag() and the JavaScript client below.

Configuration

Set the default adapter in your settings file:

config/settings.cfm (excerpt)

set(channelAdapter = "memory");   // or "database"

When channelAdapter is not set, the default is "memory". Both publish() and subscribeToChannel() also accept a per-call adapter argument that overrides the global setting — useful when one high-traffic channel needs persistence and the rest don’t.

Adapters

Memory (default)

In-process pub/sub backed by a ConcurrentHashMap. Delivery is instant — publish() invokes each subscriber’s callback synchronously on the publishing request, and the subscriber’s streaming loop flushes it to the browser.

• Best for: single-server deployments and development.
• No persistence: an event published while a channel has no connected subscribers is gone. There is no replay, so Last-Event-ID resume cannot recover events missed while disconnected.
• Single-server only: subscribers on server A never see events published on server B.
• Empty channels are pruned automatically when their last subscriber disconnects, so per-entity channel names like user.42 don’t accumulate over the application’s lifetime.

Database

Persists every event to a wheels_events table, which is auto-created on first use (no migration needed). Subscribers poll the table — every pollInterval seconds (default 2) — so events published on any server reach subscribers on every server.

• Best for: multi-server deployments, and anywhere you want resume/replay (events persist, so Last-Event-ID recovery works across reconnects).
• Latency is bounded by the poll interval rather than instant.
• Events are retained for 60 minutes by default.

The table schema:

Column	Type	Description
id	VARCHAR(36), primary key	Event UUID
channel	VARCHAR(255)	Channel name
event	VARCHAR(255)	Event type
data	TEXT / CLOB	Event payload
createdAt	DATETIME / TIMESTAMP	When the event was published

Indexes: (channel, createdAt) and (createdAt).

Expired events are swept opportunistically on the publish path: at most once every 5 minutes (every 15 seconds while a backlog is draining), deleting at most 1,000 of the oldest expired rows per pass so no single publish() call blocks on a large DELETE. On busy multi-server deployments, don’t rely solely on that — run a full sweep from a scheduled task or worker:

illustrative — scheduled task body

var adapter = $getChannelEngine("database");
adapter.cleanup(olderThanMinutes = 60, maxRows = 10000);

cleanup() returns the number of rows deleted. maxRows = 0 (the default) deletes all expired rows in one pass.

Note

$getChannelEngine() is a $-prefixed internal helper — its signature may change across minor versions. If you’d prefer an API not marked internal, instantiate the adapter directly: CreateObject("component", "wheels.channel.DatabaseAdapter").init() — cleanup() works the same on both.

API reference

publish()

Publish an event to a channel. Available anywhere global helpers are — controllers, models, jobs, views.

Parameter	Type	Default	Description
channel	string	required	Channel name (e.g. "user.42", "orders")
event	string	required	Event type (e.g. "notification", "update")
data	string	required	Event payload (typically JSON via SerializeJSON())
adapter	string	""	"memory" or "database"; empty falls back to the channelAdapter setting

The return struct depends on the adapter:

• memory: {id, channel, event, subscriberCount, timestamp} — subscriberCount is how many subscribers the event was delivered to.
• database: {id, channel, event, persisted} — persisted is false if the insert failed (the error is logged to wheels_channels.log, not thrown).

subscribeToChannel()

Call from a controller action to open a long-lived SSE connection streaming a channel’s events until the client disconnects or the timeout elapses. When the connection ends, the browser’s EventSource reconnects automatically and the next request resumes — subscribeToChannel() reads the Last-Event-ID request header on its own when lastEventId isn’t passed.

Parameter	Type	Default	Description
channel	string	required	Channel to subscribe to
events	string	""	Comma-delimited event types to deliver; empty delivers all
lastEventId	string	""	Resume after this event ID; auto-detected from the Last-Event-ID header when empty
adapter	string	""	Per-call adapter override
pollInterval	numeric	2	Seconds between table polls (database adapter only)
timeout	numeric	300	Maximum connection duration in seconds
heartbeatInterval	numeric	15	Seconds between keep-alive comment pings

With the memory adapter, the action subscribes in-process and streams events as they’re published. With the database adapter, it polls wheels_events every pollInterval seconds.

channelSSETag()

Generate a <script> tag that opens an EventSource against a subscribe endpoint and relays events as wheels:sse CustomEvents on document.

Parameter	Type	Default	Description
channel	string	required	Channel name (added as a channel URL parameter)
route	string	""	Named route for the SSE endpoint
controller	string	""	Controller name, used with action when no route is given
action	string	"stream"	Action name
events	string	""	Comma-delimited event types (added as an events URL parameter)

You must pass either route or controller — otherwise it throws Wheels.Channel.MissingEndpoint. Note that the channel and events arrive at your action as URL parameters; your action decides what to do with them (typically subscribeToChannel(channel=params.channel, events=params.events ?: "") after validating the client may subscribe).

Caution

The script channelSSETag() generates wires only the browser’s default onmessage handler, which fires for events whose type is message. Events published with any other event type (like notification) arrive as named SSE events and are not relayed to document. To listen for named event types, either publish with event="message" when using channelSSETag(), or skip the helper and use the JavaScript client or a plain EventSource with addEventListener('notification', ...).

JavaScript client

Wheels ships WheelsSSE, a zero-dependency EventSource wrapper with typed listeners, Last-Event-ID tracking, and exponential-backoff reconnection.

Caution

The file is not served at any URL by the framework. Copy it from vendor/wheels/public/assets/js/wheels-sse.js into your app — for example to public/javascripts/wheels-sse.js — and include it yourself. The plain EventSource API or channelSSETag() work without it.

app/views/layout.cfm (excerpt)

#javaScriptIncludeTag("wheels-sse")#

illustrative — browser JS

const sse = new WheelsSSE('/notifications/stream', {
    channel: 'user.42',
    events: ['notification', 'alert'],
    onMessage: (data, event, id) => console.log(event, data)
});

// Typed listeners for named event types
sse.on('notification', (data, id) => {
    showNotification(data.title, data.body);
});

// Later
sse.off('notification', handler);
sse.close();

// Static factory
const orders = WheelsSSE.subscribe('/orders/stream', {channel: 'orders'});

channel, events, and lastEventId are appended to the URL as query parameters. Event data is JSON.parsed when possible; otherwise handlers receive the raw string.

Constructor options

Option	Type	Default	Description
channel	string	""	Channel name (added as a URL parameter)
events	string[]	[]	Event types to filter (added as a URL parameter)
lastEventId	string	""	Resume from this event ID
reconnectInterval	number	1000	Initial reconnect delay (ms)
maxReconnectInterval	number	30000	Maximum reconnect delay (ms)
reconnectDecay	number	2	Backoff multiplier per attempt
maxRetries	number	0	Maximum reconnect attempts (0 = unlimited)
onOpen	function	null	Called when the connection opens
onError	function	null	Called on connection error
onMessage	function	null	Called for every event: (data, event, id)

Methods

• on(event, callback) — add a typed event listener. Returns this for chaining.
• off(event, callback) — remove a listener. Returns this.
• close() — disconnect and stop reconnecting.
• lastEventId (getter) — the last event ID received.

Reconnect behavior

On connection error the client closes the source and retries: the delay starts at reconnectInterval, multiplies by reconnectDecay each attempt, and caps at maxReconnectInterval. A successful connection resets the backoff. With maxRetries = 0 it retries forever; any other value stops after that many attempts.

Usage patterns

Per-user notifications

One channel per user; publish from wherever the event originates.

app/controllers/Notifications.cfc

component extends="Controller" {
    function stream() {
        subscribeToChannel(channel = "user.#session.userId#");
    }
}

illustrative — publish from a job or callback

publish(
    channel = "user.#user.id#",
    event = "notification",
    data = SerializeJSON({title: "Order shipped", orderId: order.id})
);

Chat room

The client-to-server direction is a plain POST: the send action persists the message and publishes it to everyone subscribed to the room. No WebSocket required.

app/controllers/Chat.cfc

component extends="Controller" {
    function messages() {
        subscribeToChannel(
            channel = "chat.room.#params.roomId#",
            events = "message,typing"
        );
    }

    function send() {
        model("Message").create(
            roomId = params.roomId,
            userId = session.userId,
            text = params.text
        );
        publish(
            channel = "chat.room.#params.roomId#",
            event = "message",
            data = SerializeJSON({user: session.userName, text: params.text})
        );
        renderNothing();
    }
}

Dashboard metrics on the database adapter

A background job publishes metrics; dashboards on every server in the cluster pick them up. A longer poll interval and timeout suit slow-moving data.

app/controllers/Dashboards.cfc

component extends="Controller" {
    function metrics() {
        subscribeToChannel(
            channel = "dashboard.metrics",
            adapter = "database",
            pollInterval = 5,
            timeout = 600
        );
    }
}

illustrative — inside a background job's perform()

publish(
    channel = "dashboard.metrics",
    event = "metrics",
    data = SerializeJSON(calculateMetrics()),
    adapter = "database"
);

Authentication and deployment

A subscribe action is an ordinary controller action — gate it with filters exactly like anything else. Remember that the browser’s EventSource can’t set custom headers, so token-in-header schemes don’t apply; the SSE guide’s authentication section covers the workable alternatives (same-origin session cookies being the default for a Wheels app subscribing to its own channels).

Channel subscriptions are long-lived requests, so the same deployment rules apply as for raw SSE streams: bump the request timeout for endpoints meant to outlive your engine’s default (see Long-lived connections and timeouts) and make sure your reverse proxy doesn’t buffer text/event-stream responses (see Through reverse proxies).

Related guides

Server-Sent Events The transport channels are built on: one-shot events, hand-rolled streams, proxies, and auth.

Background Jobs The natural publisher: jobs do the slow work and publish progress to a channel.

Authorization & Filters Gate subscribe actions with the same filters you use everywhere else.