Channels

Ably Platform service organizes the message traffic within applications into named channels. Channels are the medium through which messages are distributed; clients attach to channels to subscribe to messages, and every message published to a unique channel is broadcast by Ably to all subscribers. This scalable and resilient messaging pattern is commonly called pub/sub.

Getting started

The Ably Realtime client library provides a straightforward API for publishing and subscribing to messages on a channel. If the channel does not exist at the time the client is attached, a channel will be created in the Ably system immediately.

var realtime = new Ably.Realtime('xVLyHw.nnLZ5Q:Oqq38CeAZnA6upZNq_qT6W1E2DG2JvikpSNZrtTId90');
var channel = realtime.channels.get('pal-wet-cub');
channel.subscribe(function(message) {
  alert('Received: ' + message.data);
});
channel.publish('example', 'message data');

var Ably = require('ably');
var realtime = new Ably.Realtime('xVLyHw.nnLZ5Q:Oqq38CeAZnA6upZNq_qT6W1E2DG2JvikpSNZrtTId90');
var channel = realtime.channels.get('pal-wet-cub');
channel.subscribe(function(message) {
  console.log("Received: "  message.data);
});
channel.publish("example", "message data");

realtime = Ably::Realtime.new('xVLyHw.nnLZ5Q:Oqq38CeAZnA6upZNq_qT6W1E2DG2JvikpSNZrtTId90')
channel = realtime.channels.get('pal-wet-cub')
channel.subscribe do |message|
  puts "Received: #{message.data}"
end
channel.publish 'example', 'message data'

AblyRealtime realtime = new AblyRealtime("xVLyHw.nnLZ5Q:Oqq38CeAZnA6upZNq_qT6W1E2DG2JvikpSNZrtTId90");
Channel channel = realtime.channels.get("pal-wet-cub");
channel.subscribe(new MessageListener() {
  @Override
  public void onMessage(Message message) {
    System.out.println("New messages arrived. " + message.name);
  }
});
channel.publish("example", "message data");

AblyRealtime realtime = new AblyRealtime("xVLyHw.nnLZ5Q:Oqq38CeAZnA6upZNq_qT6W1E2DG2JvikpSNZrtTId90");
IRealtimeChannel channel = realtime.Channels.Get("pal-wet-cub");
channel.Subscribe(message => {
  Console.WriteLine($"Message: {message.Name}:{message.Data} received");
});
channel.Publish("example", "message data");

ARTRealtime *realtime = [[ARTRealtime alloc] initWithKey:@"xVLyHw.nnLZ5Q:Oqq38CeAZnA6upZNq_qT6W1E2DG2JvikpSNZrtTId90"];
ARTRealtimeChannel *channel = [realtime.channels get:@"pal-wet-cub"];
[channel subscribe:^(ARTMessage *message) {
    NSLog(@"Received: %@", message.data);
}];
[channel publish:@"example" data:@"message data"];

let realtime = ARTRealtime(key: "xVLyHw.nnLZ5Q:Oqq38CeAZnA6upZNq_qT6W1E2DG2JvikpSNZrtTId90")
let channel = realtime.channels.get("pal-wet-cub")
channel.subscribe { message in
    print("Received: \(message.data)")
}
channel.publish("example", data: "message data")

If you would prefer to just dive into code and see some examples of how to use channels, then we recommend you take a look at our Realtime tutorials.

Channels

In order to publish, subscribe to, or be present on a channel, you must first obtain a channel instance and then attach to that channel. In most instances, as a convenience, it is unnecessary to explicitly attach a channel as it will implicitly attached when performing any operation on the channel such as publishing or subscribing.

Obtaining a channel instance

A Channel object is a reference to a single channel. A channel instance is obtained from the channels collection of the Realtime::ClientRealtimeARTRealtimeAblyRealtime instance, and is uniquely identified by its unicode string name. Find out more about channel naming

var channel = realtime.channels.get('channelName');

var channel = realtime.channels.get('channelName');

Channel channel = realtime.channels.get("channelName");

IRealtimeChannel channel = realtime.Channels.Get("channelName");

channel = realtime.channels.get('channelName')

ARTRealtimeChannel *channel = [realtime.channels get:@"channelName"];

let channel = realtime.channels.get("channelName")

Setting channel options and encryption

A set of “channel options”/api/realtime-sdk/channels#channel-options may also be passed to configure a channel for encryption. Find out more about symmetric message encryption.

Ably.Realtime.Crypto.generateRandomKey(function(err, key) {
  var options = { cipher: { key: key } };
  var channel = realtime.channels.get('channelName', options);
});

Ably.Realtime.Crypto.generateRandomKey(function(err, key) {
  var options = { cipher: { key: key } };
  var channel = realtime.channels.get('channelName', options);
});

CipherParams params = Crypto.getDefaultParams(key);
ChannelOptions options = new ChannelOptions();
options.encrypted = true;
options.cipherParams = params;
Channel channel = realtime.channels.get("channelName", options);

byte[] key = Crypto.GenerateRandomKey();
CipherParams cipherParams = Crypto.GetDefaultParams(key);
ChannelOptions channelOpts = new ChannelOptions(cipherParams);
IRealtimeChannel encryptedChannel = realtime.Channels.Get("channelName", channelOpts);

key = Ably::Util::Crypto.generate_random_key
options = { cipher: { key: key } }
channel = realtime.channels.get('channelName', options)

NSData *key = [ARTCrypto generateRandomKey];
ARTChannelOptions *options = [[ARTChannelOptions alloc] initWithCipherKey:key];
ARTRealtimeChannel *channel = [realtime.channels get:@"channelName" options:options];

let key = ARTCrypto.generateRandomKey()
let options = ARTChannelOptions(cipherKey: key)
let channel = realtime.channels.get("channelName", options: options)

Channel lifecycle

Channels are not pre-configured or provisioned by Ably in advance; they are created on demand when clients attach, and remain active until such time that there are no remaining attached clients. Within the dashboard for your app however, you can pre-configure one or more channel namespaces (i.e. name prefixes), and associate different attributes and access rights with those namespaces. Find out more about channel namespaces.

The following example explicitly attaches to a channel, which results in the channel being provisioned in Ably’s global realtime cluster. This channel will remain available globally until there are no more clients attached to the channel:

realtime.channels.get('chatroom').attach(function(err) {
  console.log('"chatroom" exists and is now available globally in every datacenter');
});

realtime.channels.get('chatroom').attach(function(err) {
  console.log('"chatroom" exists and is now available globally in every datacenter');
});

realtime.channels.get('chatroom').attach do |channel|
  puts "'chatroom' exists and is now available globally in every datacenter"
end

Channel channel = realtime.channels.get("chatroom");
channel.on(new ChannelStateListener() {
  @Override
  public void onChannelStateChanged(ChannelStateChange state) {
    switch (state.current) {
      case attached: {
        System.out.println("'chatroom' exists and is now available globally");
      }
    }
  }
});

IRealtimeChannel channel = realtime.Channels.Get("chatroom");
channel.Attach((success, error) => {
  Console.WriteLine("'chatroom' exists and is now available globally");
});

[[realtime.channels get:@"chatroom" options:options] attach:^(ARTErrorInfo *error) {
  NSLog(@"'chatroom' exists and is now available globally in every datacenter");
}];

realtime.channels.get("chatroom").attach { error in
  print("'chatroom' exists and is now available globally in every datacenter")
}

Clients attach to a channel in order to participate on that channel in any way (either to publish, subscribe, or be present on the channel).

Channel metadata

Ably provides a REST API to query your app for metadata about channels, as well as a realtime API to subscribe to channel lifecycle events. Using the REST API, you can enumerate all active channels, or obtain the status of an individual channel. Using our Realtime API, you can subscribe to channel lifecycle events (such as being created or closed etc), or subscribe to periodic occupancy updates for all active channels (such as how many people are subscribed to a channel).

Implicit attach

Although the attach operation can be initiated explicitly by a client, it is more common for the client to perform a publish or subscribe operation, and the client library will initiate the attach if the channel is not already attached. The client library allows clients to begin publishing messages to a channel as soon as the channel has been created, and messages are queued until such time as the attach has succeeded or failed.

var channel = realtime.channels.get('chatroom');
channel.subscribe('action', function(message) { // implicit attach
  console.log('Message received '' + message.data);
});
channel.publish('action', 'boom!');

var channel = realtime.channels.get('chatroom');
channel.subscribe('action', function(message) { // implicit attach
  console.log('Message received '' + message.data);
});
channel.publish('action', 'boom!');

channel = realtime.channels.get('chatroom')
channel.subscribe('action') do |message| # implicit attach
  puts "Message received: #{message}";
end
channel.publish 'action', 'boom!'

Channel channel = realtime.channels.get("chatroom");
/* Implicit attach when subscribing */
channel.subscribe(new MessageListener() {
  @Override
  public void onMessage(Message message) {
    System.out.println("Message received: " + message.data);
  }
});
channel.publish("action", "boom!");

IRealtimeChannel channel = realtime.Channels.Get("chatroom");
channel.Subscribe(message => Console.WriteLine("Message received: " + message.Data));
channel.Publish("action", "boom");

ARTRealtimeChannel *channel = [realtime.channels get:@"chatroom" options:options];
[channel subscribe:@"action" callback:^(ARTMessage *message) {
    NSLog(@"Message received: %@", message.data);
}]
[channel publish:@"action" data:@"boom!"];

let channel = realtime.channels.get("chatroom")
channel.subscribe("action") { message in
    print("Message received: \(message.data)")
}
channel.publish("action", data: "boom!")

Normally, errors in attaching to a channel are communicated through the attach callback. For implicit attaches (and other cases where a channel is attached or reattached automatically, e.g. following the library reconnecting after a period in the suspended state), there is no callback, so if you want to know what happens, you’ll need to listen for channel state changes.

Channel states

A channel can exist in any of the following states:

initializedInitialized

A Channel object having this state has been initialized but no attach has yet been attempted

attachingAttaching

An attach has been initiated by sending a request to Ably. This is a transient state; it will be followed either by a transition to attached, suspended, or failed

attachedAttached

Attach has succeeded. In the attached state a client may publish and subscribe to messages, or be present

detachingDetaching

A detach has been initiated on the attached Channel by sending a request to Ably. This is a transient state; it will be followed either by a transition to detached or failed

detachedDetached

The Channel, having previously been attached, has been detached by the user

suspendedSuspended

The Channel, having previously been attached, has lost continuity, usually due to the client being disconnected from Ably for more than two minutes. It will automatically attempt to reattach as soon as connectivity is restored

failedFailed

An indefinite failure condition. This state is entered if a Channel error has been received from the Ably service (such as an attempt to attach without the necessary access rights)

Listening for state changes

The Channel object is an EventEmitter and emits an event whose name is the new state whenever there is a channel state change. Listeners are passed a “ChannelStateChange”/api/realtime-sdk/channels#channel-state-change object in the first argument. This contains the current and previous states, a resumed flag which indicated whether message continuity on this channel is preserved (from the last time that the channel was attached), and (sometimes) a reason for the state change. As with all events from an EventEmitter in the Ably library, this within the listener function is a reference to an event object whose event property is the name of the event that fired. This allows a listener to listen for all events with a single registration and know, which event fired when it is entered.

The Channel object can also emit one event that is not a state change: an update event. This happens when there’s a change to channel conditions for which the channel state doesn’t change. For example, a partial loss of message continuity on a channel (typically after a resume) for which the channel state remains attached would lead to an update event being emitted, with both current and previous set to “attached”, and the resumed flag set to false. So if you get such an event, you’ll know there may be messages you’ve missed on the channel, and if necessary you can use the “History”/api/realtime-sdk/channels#history api to retrieve them.

channel.on('attached', function(stateChange) {
  console.log('channel ' + channel.name + ' is now attached');
  console.log('Message continuity on this channel ' + \
    (stateChange.resumed ? 'was' : 'was not') + ' preserved');
});

channel.on('attached', function(stateChange) {
  console.log('channel ' + channel.name + ' is now attached');
  console.log('Message continuity on this channel ' + \
    (stateChange.resumed ? 'was' : 'was not') + ' preserved');
});

channel.on(:attached) do |channel_state_change|
  puts "channel #{channel.name} is now attached"
  puts "Message continuity #{channel_state_change.resumed ? 'was' : 'was not'} preserved"
end

channel.on(ChannelEvent.attached, new ChannelStateListener() {
  @Override
  public void onChannelStateChanged(ChannelStateChange stateChange) {
    System.out.println("channel " + channel.name + " is now attached");
    if (stateChange.resumed) {
      System.out.println("Message continuity was preserved");
    } else {
      System.out.println("Message continuity was not preserved");
    }
  }
});

IRealtimeChannel channel = realtime.Channels.Get("chatroom");
channel.On(ChannelEvent.Attached, stateChange => {
  Console.WriteLine("channel " + channel.Name + " is now attached");
  if (stateChange.resumed) {
    Console.WriteLine("Message continuity was preserved");
  } else {
    Console.WriteLine("Message continuity was not preserved");
  }
});

[channel on:ARTChannelEventAttached callback:^(ARTChannelStateChange *stateChange) {
  NSLog(@"channel %@ is now attached", channel.name);
  if (stateChange.resumed) {
    NSLog(@"Message continuity was preserved");
  } else {
    NSLog(@"Message continuity was not preserved");
  }
}];

channel.on(.attached) { stateChange in
  print("channel \(channel.name) is now attached")
  if (stateChange.resumed) {
    print("Message continuity was preserved")
  } else {
    print("Message continuity was not preserved")
  }
}

Alternatively a listener may be registered so that it receives all state change events.

var myListener = function(stateChange) {
  console.log('channel state is ' + stateChange.current);
  console.log('previous state was ' + stateChange.previous);
  if(stateChange.reason) {
    console.log('the reason for the state change was: ' + stateChange.reason.toString());
  }
});
channel.on(myListener);

var myListener = function(stateChange) {
  console.log('channel state is ' + stateChange.current);
  console.log('previous state was ' + stateChange.previous);
  if(stateChange.reason) {
    console.log('the reason for the state change was: ' + stateChange.reason.toString());
  }
});
channel.on(myListener);

channel.on do |channel_state_change|
  puts "channel state is #{channel_state_change.current}"
end

channel.on(new ChannelStateListener() {
  @Override
  public void onChannelStateChanged(ChannelStateChange stateChange, ErrorInfo reason) {
    System.out.println("channel state is " + ChannelState.values()[stateChange.current]);
  }
});

channel.On(stateChange => Console.WriteLine("channel state is " + stateChange.Current));

ARTEventListener *listener = [channel on:^(ARTChannelStateChange *stateChange) {
    NSLog(@"channel state is %@", stateChange.current);
}];

let listener = channel.on { stateChange in
    print("channel state is \(stateChange.current)")
}

Previously registered listeners can be removed individually or all together.

/* remove the listener registered for a single event */
channel.off('attached', myListener);

/* remove the listener registered for all events */
channel.off(myListener);

/* remove the listener registered for a single event */
channel.off('attached', myListener);

/* remove the listener registered for all events */
channel.off(myListener);

# remove the listener proc registered for a single event
channel.off(:attached, &my_proc)

# remove the listener proc registered for all events
channel.off(&my_proc)

/* remove the listener registered for a single event */
channel.off(ChannelEvent.attached, channelStateListener);

/* remove the listener registered for all events */
channel.off(channelStateListener);

// remove the listener registered for a single event
channel.Off(ChannelEvent.Attached, channelStateListener);

// remove the listener registered for all events
channel.Off(channelStateListener);

// remove the listener registered for a single event
[channel off:ARTChannelEventAttached listener:listener];

// remove the listener registered for all events
[channel off:listener];

// remove the listener registered for a single event
channel.off(.attached, listener: listener)

// remove the listener registered for all events
channel.off(listener)

Handling channel failures

Channel attach and detach operations are asynchronous – after initiating an attach request, the client will wait for a response from Ably that confirms that the channel is established on the service and then trigger a state change event.

The client libraries will attempt to automatically recover from non-fatal error conditions. However, you can handle them yourself if you prefer by subscribing to channel state changes, or using the callbacks availablewaiting for a result when explicitly calling attach.

realtime.channels.get('private:chatroom').attach(function(err) {
  if (err) {
    console.error('Attach failed: ' + err);
  }
});

realtime.channels.get('private:chatroom').attach(function(err) {
  if (err) {
    console.error('Attach failed: ' + err);
  }
});

deferrable = realtime.channels.get('private:chatroom').attach
deferrable.errback do |error|
  puts "Attach failed: #{error}"
end

Channel channel = realtime.channels.get("private:chatroom");
channel.on(new ChannelStateListener() {
  @Override
  public void onChannelStateChanged(ChannelStateChange stateChange, ErrorInfo reason) {
    switch (stateChange.current) {
      case failed: {
        System.out.println("Attach failed: " + reason.message);
      }
    }
  }
});
channel.attach();

IRealtimeChannel privateChannel = realtime.Channels.Get("private:chatroom");
privateChannel.Attach((_, error) => {
    if (error != null)
    {
        Console.WriteLine("Attach failed: " + error.Message);
    }
});

[[realtime.channels get:@"private:chatroom"] attach:^(ARTErrorInfo *error) {
    if (error) {
        NSLog(@"Attach failed: %@", error);
    }
}];

realtime.channels.get("private:chatroom").attach { error in
    if let error = error {
        print("Attach failed: \(error)")
    }
}

Fatal channel errors

Some classes of errors are fatal. These cause the channel to move to the FAILED state. The client library will not attempt any automatic recovery actions. For example: attempting to attach to a channel that the token you’re using does not have the subscribe capability for will cause that channel to enter the FAILED state.

Note that while fatal errors won’t get better on their own, they can be fixable. For example, if a channel goes into the FAILED state due to the client not having the right capabilities to attach to it, that client could use Auth#authorize() to obtain a new token which does have the right capabilities, then call “attach()”/api/realtime-sdk/channels#attach on the channel. While the library will not automatically reattach in the FAILED state, explicit calls to “attach()”/api/realtime-sdk/channels#attach will make the client try again.

Nonfatal errors

Other types of errors are nonfatal. For example, a client may have network connectivity issues, or a channel may experience a loss of strict message continuity. The library will automatically attempt to recover from these events. If channel continuity is lost in the process, the library will notify you though a resumed flag in the ATTACHED or UPDATE event, so you can decide how to handle that failure.

For every channel ATTACHED and UPDATE event, the ChannelStateChange object contains a resumed attribute. When true, there has been no loss of continuity from the last time the channel was attached. When false, there has been a loss of continuity. So for example:

• The first time you attach to a channel on a fresh connection, resumed will be false, as there was nothing to continue from.
• If you successfully recover a connection and reattach to your channels, the resumed flag on the ATTACHED events will tell you whether messages continuity was preserved. Any channel for which it’s true is guaranteed to receive every message it missed while the client was disconnected.
• If you resume or recover a connection unsuccessfully (so you get a fresh connection) — generally because you were disconnected for more than two minutes, which is how long Ably holds connection state for — continuity is lost. If you were resuming, all your channels (which will have gone into the SUSPENDED state after two minutes) will still reattach automatically, and you will get ATTACHED events with resumed false.
• If Ably needs to signal a loss of message continuity on an attached channel (for example, during a partially successful resume where the client was disconnected for less than two minutes), you will get an UPDATE event with resumed false.

Connection state change side effects on channels

• If the connection state becomes CLOSED, all channels will become DETACHED
• If the connection state becomes FAILED, all channels will become FAILED
• If the connection state becomes SUSPENDED, all previously-ATTACHED or ATTACHING channels will become SUSPENDED
• If the connection state becomes CONNECTED, any channels that were SUSPENDED will be automatically reattached

Channel namespaces

One or more channel namespaces, or channel name prefixes, may be configured for an app in your dashboard. When a channel is created whose name is prefixed with one of the namespaces, the channel assumes certain configured attributes associated with that namespace. For example, a channel namespace named “private” would match channels named “private”, “private:chat”, “private:chat:mike”.

Namespace-prefixed channel names are delimited by a single colon :; the first component of the channel name (from the start up to and including the last character before the colon) is the namespace. A channel name may validly contain a colon even if the namespace component does not correspond to a namespace; also, a channel may contain multiple colons and only the component up to the first colon will be matched with a namespace. The only restriction on channel names is that a channel name may not start with a colon :, an open square bracket [ and it may not be empty.

Namespaces are defined and configured via the application dashboard settings. The namespace attributes that can be configured are:

• Persisted messages – If enabled, all messages within this namespace will be stored according to the storage rules for your account. You can access stored messages via the history API
• Require identification – if enabled, clients will not be permitted to subscribe to matching channels unless they are both authenticated and identified (they have an assigned client ID). Anonymous clients are not permitted to join these channels. Find out more about authenticated and identified clients
• Require TLS – if enabled, only clients who have connected to Ably over TLS will be allowed to join the channel

Key or token capabilities can also specify access rights based on channel namespace, find out more about authentication

Presence

Channels expose a presencePresence member which a client can use to obtain channel presence information and to enter and leave the presence channel itself. See the presence documentation for details.

API Reference

View the Channels and Channel API Reference.