GCM Cloud Connection Server (XMPP)

The Google Cloud Messaging (GCM) Cloud Connection Server (CCS) is an XMPP endpoint that provides a persistent, asynchronous, bidirectional connection to Google servers. The connection can be used to send and receive messages between your server and your users' GCM-connected devices.

Note: The content in this document applies to GCM with Chrome apps as well as Android.

You can continue to use the HTTP request mechanism to send messages to GCM servers, side-by-side with CCS which uses XMPP. Some of the benefits of CCS include:

  • The asynchronous nature of XMPP allows you to send more messages with fewer resources.
  • Communication is bidirectional—not only can your server send messages to the device, but the device can send messages back to your server.
  • The device can send messages back using the same connection used for receiving, thereby improving battery life.

The upstream messaging (device-to-cloud) feature of CCS is part of the Google Play services platform. Upstream messaging is available through the GoogleCloudMessaging APIs. For examples, see Implementing an XMPP-based App Server.

Note: See Implementing GCM Server for a list of all the message parameters and which connection server(s) supports them.

Establishing a Connection

CCS just uses XMPP as an authenticated transport layer, so you can use most XMPP libraries to manage the connection. For an example, see Java sample using the Smack library.

The CCS XMPP endpoint runs at gcm.googleapis.com:5235. When testing functionality (with non-production users), you should instead connect to gcm-preprod.googleapis.com:5236 (note the different port). Regular testing on preprod (a smaller environment where the latest CCS builds run) is beneficial both for isolating real users from test code, as well as for early detection of unexpected behavior changes. Note that a connection receives upstream messages destined for its GCM sender ID, regardless of which environment (gcm or gcm-preprod) it is connected to. Therefore, test code connecting to gcm-preprod.googleapis.com:5236 should use a different GCM sender ID to avoid upstream messages from production traffic being sent over test connections.

The connection has two important requirements:

  • You must initiate a Transport Layer Security (TLS) connection. Note that CCS doesn't currently support the STARTTLS extension.
  • CCS requires a SASL PLAIN authentication mechanism using <your_GCM_Sender_Id>@gcm.googleapis.com (GCM sender ID) and the API key as the password, where the sender ID and API key are the same as described in Getting Started.

If at any point the connection fails, you should immediately reconnect. There is no need to back off after a disconnect that happens after authentication.

Authentication

The following snippets illustrate how to perform authentication in CCS.

Client

<stream:stream to="gcm.googleapis.com" 
        version="1.0" xmlns="jabber:client" 
        xmlns:stream="http://etherx.jabber.org/streams"/>

Server

<str:features xmlns:str="http://etherx.jabber.org/streams">
 <mechanisms xmlns="urn:ietf:params:xml:ns:xmpp-sasl">
   <mechanism>X-OAUTH2</mechanism>
   <mechanism>X-GOOGLE-TOKEN</mechanism>
   <mechanism>PLAIN</mechanism>
 </mechanisms>
</str:features>

Client

<auth mechanism="PLAIN"
xmlns="urn:ietf:params:xml:ns:xmpp-sasl">MTI2MjAwMzQ3OTMzQHByb2plY3RzLmdjbS5hb
mFTeUIzcmNaTmtmbnFLZEZiOW1oekNCaVlwT1JEQTJKV1d0dw==</auth>

Server

<success xmlns="urn:ietf:params:xml:ns:xmpp-sasl"/>

Message Format

Once the XMPP connection is established, CCS and your server use normal XMPP <message> stanzas to send JSON-encoded messages back and forth. The body of the <message> must be:

<gcm xmlns:google:mobile:data>
    JSON payload
</gcm>

The JSON payload for regular GCM messages is similar to what the GCM http endpoint uses, with these exceptions:

  • There is no support for multiple recipients.
  • to is used instead of registration_ids.
  • CCS adds the field message_id, which is required. This ID uniquely identifies the message in an XMPP connection. The ACK or NACK from CCS uses the message_id to identify a message sent from 3rd-party app servers to CCS. Therefore, it's important that this message_id not only be unique (per sender ID), but always present.

In addition to regular GCM messages, control messages are sent, indicated by the message_type field in the JSON object. The value can be either 'ack' or 'nack', or 'control' (see formats below). Any GCM message with an unknown message_type can be ignored by your server.

For each device message your app server receives from CCS, it needs to send an ACK message. It never needs to send a NACK message. If you don't send an ACK for a message, CCS will just resend it.

CCS also sends an ACK or NACK for each server-to-device message. If you do not receive either, it means that the TCP connection was closed in the middle of the operation and your server needs to resend the messages. See Flow Control for details.

Note: See Implementing GCM Server for a list of all the message parameters and which connection server(s) supports them.

Request format

Here is an XMPP stanza containing the JSON message from a 3rd-party app server to CCS:

<message id="">
  <gcm xmlns="google:mobile:data">
  {
      "to":"REGISTRATION_ID",  // "to" replaces "registration_ids"
      "message_id":"m-1366082849205" // new required field
      "data":
      {
          "hello":"world",
      }
      "time_to_live":"600",
      "delay_while_idle": true/false,
      "delivery_receipt_requested": true/false
  }
  </gcm>
</message>

Response format

A CCS response can have 3 possible forms. The first one is a regular 'ack' message. But when the response contains an error, there are 2 different forms the message can take, described below.

ACK message

Here is an XMPP stanza containing the ACK/NACK message from CCS to 3rd-party app server:

<message id="">
  <gcm xmlns="google:mobile:data">
  {
      "from":"REGID",
      "message_id":"m-1366082849205"
      "message_type":"ack"
  }
  </gcm>
</message>

NACK message

A NACK error is a regular XMPP message in which the message_type status message is "nack". A NACK message contains:

  • Nack error code.
  • Nack error description.

Below are some examples.

Bad registration:

<message>
  <gcm xmlns="google:mobile:data">
  {
    "message_type":"nack",
    "message_id":"msgId1",
    "from":"SomeInvalidRegistrationId",
    "error":"BAD_REGISTRATION",
    "error_description":"Invalid token on 'to' field: SomeInvalidRegistrationId"
  }
  </gcm>
</message>

Invalid JSON:

<message>
 <gcm xmlns="google:mobile:data">
 {
   "message_type":"nack",
   "message_id":"msgId1",
   "from":"APA91bHFOtaQGSwupt5l1og",
   "error":"INVALID_JSON",
   "error_description":"InvalidJson: JSON_TYPE_ERROR : Field \"time_to_live\" must be a JSON java.lang.Number: abc"
 }
 </gcm>
</message>

Device Message Rate Exceeded:

<message id="...">
  <gcm xmlns="google:mobile:data">
  {
    "message_type":"nack",
    "message_id":"msgId1",
    "from":"REGID",
    "error":"DEVICE_MESSAGE_RATE_EXCEEDED",
    "error_description":"Downstream message rate exceeded for this registration id"
  }
  </gcm>
</message>

The following table lists NACK error codes. Unless otherwise indicated, a NACKed message should not be retried. Unexpected NACK error codes should be treated the same as INTERNAL_SERVER_ERROR.

Table 1. NACK error codes.

Error Code Description
BAD_ACK The ACK message is improperly formed.
BAD_REGISTRATION The device has a registration ID, but it's invalid or expired.
CONNECTION_DRAINING The message couldn't be processed because the connection is draining. The message should be immediately retried over another connection.
DEVICE_UNREGISTERED The device is not registered.
INTERNAL_SERVER_ERROR The server encountered an error while trying to process the request.
INVALID_JSON The JSON message payload is not valid.
DEVICE_MESSAGE_RATE_EXCEEDED The rate of messages to a particular device is too high. You should reduce the number of messages sent to this device and should not immediately retry sending to this device. This error code is replacing QUOTA_EXCEEDED.
SERVICE_UNAVAILABLE CCS is not currently able to process the message. The message should be retried over the same connection using exponential backoff with an initial delay of 1 second.

Stanza error

You can also get a stanza error in certain cases. A stanza error contains:

  • Stanza error code.
  • Stanza error description (free text).

For example:

<message id="3" type="error" to="123456789@gcm.googleapis.com/ABC">
  <gcm xmlns="google:mobile:data">
     {"random": "text"}
  </gcm>
  <error code="400" type="modify">
    <bad-request xmlns="urn:ietf:params:xml:ns:xmpp-stanzas"/>
    <text xmlns="urn:ietf:params:xml:ns:xmpp-stanzas">
      InvalidJson: JSON_PARSING_ERROR : Missing Required Field: message_id\n
    </text>
  </error>
</message>

Control messages

Periodically, CCS needs to close down a connection to perform load balancing. Before it closes the connection, CCS sends a CONNECTION_DRAINING message to indicate that the connection is being drained and will be closed soon. "Draining" refers to shutting off the flow of messages coming into a connection, but allowing whatever is already in the pipeline to continue. When you receive a CONNECTION_DRAINING message, you should immediately begin sending messages to another CCS connection, opening a new connection if necessary. You should, however, keep the original connection open and continue receiving messages that may come over the connection (and ACKing them)—CCS will handle initiating a connection close when it is ready.

The CONNECTION_DRAINING message looks like this:

<message>
  <data:gcm xmlns:data="google:mobile:data">
  {
    "message_type":"control"
    "control_type":"CONNECTION_DRAINING"
  }
  </data:gcm>
</message>

CONNECTION_DRAINING is currently the only control_type supported.

Upstream Messages

Using CCS and the GoogleCloudMessaging API, you can send messages from a user's device to the cloud.

Here is how you send an upstream message using the GoogleCloudMessaging API. For a complete example, see Implementing GCM Client:

GoogleCloudMessaging gcm = GoogleCloudMessaging.get(context);
String GCM_SENDER_ID = "Your-Sender-ID";
AtomicInteger msgId = new AtomicInteger();
String id = Integer.toString(msgId.incrementAndGet());
Bundle data = new Bundle();
// Bundle data consists of a key-value pair
data.putString("hello", "world");
// "time to live" parameter
// This is optional. It specifies a value in seconds up to 24 hours.
int ttl = [0 seconds, 24 hours]

gcm.send(GCM_SENDER_ID + "@gcm.googleapis.com", id, ttl, data);

This call generates the necessary XMPP stanza for sending the upstream message. The message goes from the app on the device to CCS to the 3rd-party app server. The stanza has the following format:

<message id="">
  <gcm xmlns="google:mobile:data">
  {
      "category":"com.example.yourapp", // to know which app sent it
      "data":
      {
          "hello":"world",
      },
      "message_id":"m-123",
      "from":"REGID"
  }
  </gcm>
</message>

Here is the format of the ACK expected by CCS from 3rd-party app servers in response to the above message:

<message id="">
  <gcm xmlns="google:mobile:data">
  {
      "to":"REGID",
      "message_id":"m-123"
      "message_type":"ack"
  }
  </gcm>
</message>

Receive delivery receipts

You can use upstream messaging to get delivery receipts (sent from CCS to your 3rd party app server) when a device confirms that it received a message sent by CCS.

To enable this feature, the message your 3rd-party app server sends to CCS must include a field called "delivery_receipt_requested". When this field is set to true, CCS sends a delivery receipt when a device confirms that it received a particular message.

Here is an XMPP stanza containing a JSON message with "delivery_receipt_requested" set to true:

<message id="">
  <gcm xmlns="google:mobile:data">
  {
      "to":"REGISTRATION_ID",
      "message_id":"m-1366082849205"
      "data":
      {
          "hello":"world",
      }
      "time_to_live":"600",
      "delay_while_idle": true,
      "delivery_receipt_requested": true
  }
  </gcm>
</message>

Here is an example of the delivery receipt that CCS sends to tell your 3rd-party app server that a device received a message that CCS sent it:

<message id="">
  <gcm xmlns="google:mobile:data">
  {
      "category":"com.example.yourapp", // to know which app sent it
      "data":
      {
         “message_status":"MESSAGE_SENT_TO_DEVICE",
         “original_message_id”:”m-1366082849205”
         “device_registration_id”: “REGISTRATION_ID”
      },
      "message_id":"dr2:m-1366082849205",
      "message_type":"receipt",
      "from":"gcm.googleapis.com"
  }
  </gcm>
</message>

Note the following:

  • The "message_type" is set to "receipt".
  • The "message_status" is set to "MESSAGE_SENT_TO_DEVICE", indicating that the device received the message. Notice that in this case, "message_status" is not a field but rather part of the data payload.
  • The receipt message ID consists of the original message ID, but with a dr2: prefix. Your 3rd-party app server must send an ACK back with this ID, which in this example is dr2:m-1366082849205.
  • The original message ID, the device registration ID, and the status are inside the "data" field.

Flow Control

Every message sent to CCS receives either an ACK or a NACK response. Messages that haven't received one of these responses are considered pending. If the pending message count reaches 100, the 3rd-party app server should stop sending new messages and wait for CCS to acknowledge some of the existing pending messages as illustrated in figure 1:

Figure 1. Message/ack flow.

Conversely, to avoid overloading the 3rd-party app server, CCS will stop sending if there are too many unacknowledged messages. Therefore, the 3rd-party app server should "ACK" upstream messages, received from the client application via CCS, as soon as possible to maintain a constant flow of incoming messages. The aforementioned pending message limit doesn't apply to these ACKs. Even if the pending message count reaches 100, the 3rd-party app server should continue sending ACKs for messages received from CCS to avoid blocking delivery of new upstream messages.

ACKs are only valid within the context of one connection. If the connection is closed before a message can be ACKed, the 3rd-party app server should wait for CCS to resend the upstream message before ACKing it again. Similarly, all pending messages for which an ACK/NACK was not received from CCS before the connection was closed should be sent again.

Implementing an XMPP-based App Server

This section gives examples of implementing an app server that works with CCS. Note that a full GCM implementation requires a client-side implementation, in addition to the server. For more information, see Implementing GCM Client.

Java sample using the Smack library

Here is a sample app server written in Java, using the Smack library.

import org.jivesoftware.smack.ConnectionConfiguration;
import org.jivesoftware.smack.ConnectionConfiguration.SecurityMode;
import org.jivesoftware.smack.ConnectionListener;
import org.jivesoftware.smack.PacketInterceptor;
import org.jivesoftware.smack.PacketListener;
import org.jivesoftware.smack.SmackException;
import org.jivesoftware.smack.SmackException.NotConnectedException;
import org.jivesoftware.smack.XMPPConnection;
import org.jivesoftware.smack.XMPPException;
import org.jivesoftware.smack.filter.PacketTypeFilter;
import org.jivesoftware.smack.packet.DefaultPacketExtension;
import org.jivesoftware.smack.packet.Message;
import org.jivesoftware.smack.packet.Packet;
import org.jivesoftware.smack.packet.PacketExtension;
import org.jivesoftware.smack.provider.PacketExtensionProvider;
import org.jivesoftware.smack.provider.ProviderManager;
import org.jivesoftware.smack.tcp.XMPPTCPConnection;
import org.jivesoftware.smack.util.StringUtils;
import org.json.simple.JSONValue;
import org.json.simple.parser.ParseException;
import org.xmlpull.v1.XmlPullParser;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;
import java.util.UUID;
import java.util.logging.Level;
import java.util.logging.Logger;

import javax.net.ssl.SSLSocketFactory;

/**
 * Sample Smack implementation of a client for GCM Cloud Connection Server. This
 * code can be run as a standalone CCS client.
 *
 * <p>For illustration purposes only.
 */
public class SmackCcsClient {

    private static final Logger logger = Logger.getLogger("SmackCcsClient");

    private static final String GCM_SERVER = "gcm.googleapis.com";
    private static final int GCM_PORT = 5235;

    private static final String GCM_ELEMENT_NAME = "gcm";
    private static final String GCM_NAMESPACE = "google:mobile:data";

    static {

        ProviderManager.addExtensionProvider(GCM_ELEMENT_NAME, GCM_NAMESPACE,
            new PacketExtensionProvider() {
                @Override
                public PacketExtension parseExtension(XmlPullParser parser) throws
                        Exception {
                    String json = parser.nextText();
                    return new GcmPacketExtension(json);
                }
            });
    }

    private XMPPConnection connection;

    /**
     * Indicates whether the connection is in draining state, which means that it
     * will not accept any new downstream messages.
     */
    protected volatile boolean connectionDraining = false;

    /**
     * Sends a downstream message to GCM.
     *
     * @return true if the message has been successfully sent.
     */
    public boolean sendDownstreamMessage(String jsonRequest) throws
            NotConnectedException {
        if (!connectionDraining) {
            send(jsonRequest);
            return true;
        }
        logger.info("Dropping downstream message since the connection is draining");
        return false;
    }

    /**
     * Returns a random message id to uniquely identify a message.
     *
     * <p>Note: This is generated by a pseudo random number generator for
     * illustration purpose, and is not guaranteed to be unique.
     */
    public String nextMessageId() {
        return "m-" + UUID.randomUUID().toString();
    }

    /**
     * Sends a packet with contents provided.
     */
    protected void send(String jsonRequest) throws NotConnectedException {
        Packet request = new GcmPacketExtension(jsonRequest).toPacket();
        connection.sendPacket(request);
    }

    /**
     * Handles an upstream data message from a device application.
     *
     * <p>This sample echo server sends an echo message back to the device.
     * Subclasses should override this method to properly process upstream messages.
     */
    protected void handleUpstreamMessage(Map<String, Object> jsonObject) {
        // PackageName of the application that sent this message.
        String category = (String) jsonObject.get("category");
        String from = (String) jsonObject.get("from");
        @SuppressWarnings("unchecked")
        Map<String, String> payload = (Map<String, String>) jsonObject.get("data");
        payload.put("ECHO", "Application: " + category);

        // Send an ECHO response back
        String echo = createJsonMessage(from, nextMessageId(), payload,
                "echo:CollapseKey", null, false);

        try {
            sendDownstreamMessage(echo);
        } catch (NotConnectedException e) {
            logger.log(Level.WARNING, "Not connected anymore, echo message is
                    not sent", e);
        }
    }

    /**
     * Handles an ACK.
     *
     * <p>Logs a INFO message, but subclasses could override it to
     * properly handle ACKs.
     */
    protected void handleAckReceipt(Map<String, Object> jsonObject) {
        String messageId = (String) jsonObject.get("message_id");
        String from = (String) jsonObject.get("from");
        logger.log(Level.INFO, "handleAckReceipt() from: " + from + ",
                messageId: " + messageId);
    }

    /**
     * Handles a NACK.
     *
     * <p>Logs a INFO message, but subclasses could override it to
     * properly handle NACKs.
     */
    protected void handleNackReceipt(Map<String, Object> jsonObject) {
        String messageId = (String) jsonObject.get("message_id");
        String from = (String) jsonObject.get("from");
        logger.log(Level.INFO, "handleNackReceipt() from: " + from + ",
                messageId: " + messageId);
    }

    protected void handleControlMessage(Map<String, Object> jsonObject) {
        logger.log(Level.INFO, "handleControlMessage(): " + jsonObject);
        String controlType = (String) jsonObject.get("control_type");
        if ("CONNECTION_DRAINING".equals(controlType)) {
            connectionDraining = true;
        } else {
            logger.log(Level.INFO, "Unrecognized control type: %s. This could
                    happen if new features are " + "added to the CCS protocol.",
                    controlType);
        }
    }

    /**
     * Creates a JSON encoded GCM message.
     *
     * @param to RegistrationId of the target device (Required).
     * @param messageId Unique messageId for which CCS will send an
     *         "ack/nack" (Required).
     * @param payload Message content intended for the application. (Optional).
     * @param collapseKey GCM collapse_key parameter (Optional).
     * @param timeToLive GCM time_to_live parameter (Optional).
     * @param delayWhileIdle GCM delay_while_idle parameter (Optional).
     * @return JSON encoded GCM message.
     */
    public static String createJsonMessage(String to, String messageId,
            Map<String, String> payload, String collapseKey, Long timeToLive,
            Boolean delayWhileIdle) {
        Map<String, Object> message = new HashMap<String, Object>();
        message.put("to", to);
        if (collapseKey != null) {
            message.put("collapse_key", collapseKey);
        }
        if (timeToLive != null) {
            message.put("time_to_live", timeToLive);
        }
        if (delayWhileIdle != null && delayWhileIdle) {
            message.put("delay_while_idle", true);
        }
      message.put("message_id", messageId);
      message.put("data", payload);
      return JSONValue.toJSONString(message);
    }

    /**
     * Creates a JSON encoded ACK message for an upstream message received
     * from an application.
     *
     * @param to RegistrationId of the device who sent the upstream message.
     * @param messageId messageId of the upstream message to be acknowledged to CCS.
     * @return JSON encoded ack.
     */
        protected static String createJsonAck(String to, String messageId) {
        Map<String, Object> message = new HashMap<String, Object>();
        message.put("message_type", "ack");
        message.put("to", to);
        message.put("message_id", messageId);
        return JSONValue.toJSONString(message);
    }

    /**
     * Connects to GCM Cloud Connection Server using the supplied credentials.
     *
     * @param senderId Your GCM project number
     * @param apiKey API Key of your project
     */
    public void connect(long senderId, String apiKey)
            throws XMPPException, IOException, SmackException {
        ConnectionConfiguration config =
                new ConnectionConfiguration(GCM_SERVER, GCM_PORT);
        config.setSecurityMode(SecurityMode.enabled);
        config.setReconnectionAllowed(true);
        config.setRosterLoadedAtLogin(false);
        config.setSendPresence(false);
        config.setSocketFactory(SSLSocketFactory.getDefault());

        connection = new XMPPTCPConnection(config);
        connection.connect();

        connection.addConnectionListener(new LoggingConnectionListener());

        // Handle incoming packets
        connection.addPacketListener(new PacketListener() {

            @Override
            public void processPacket(Packet packet) {
                logger.log(Level.INFO, "Received: " + packet.toXML());
                Message incomingMessage = (Message) packet;
                GcmPacketExtension gcmPacket =
                        (GcmPacketExtension) incomingMessage.
                        getExtension(GCM_NAMESPACE);
                String json = gcmPacket.getJson();
                try {
                    @SuppressWarnings("unchecked")
                    Map<String, Object> jsonObject =
                            (Map<String, Object>) JSONValue.
                            parseWithException(json);

                    // present for "ack"/"nack", null otherwise
                    Object messageType = jsonObject.get("message_type");

                    if (messageType == null) {
                        // Normal upstream data message
                        handleUpstreamMessage(jsonObject);

                        // Send ACK to CCS
                        String messageId = (String) jsonObject.get("message_id");
                        String from = (String) jsonObject.get("from");
                        String ack = createJsonAck(from, messageId);
                        send(ack);
                    } else if ("ack".equals(messageType.toString())) {
                          // Process Ack
                          handleAckReceipt(jsonObject);
                    } else if ("nack".equals(messageType.toString())) {
                          // Process Nack
                          handleNackReceipt(jsonObject);
                    } else if ("control".equals(messageType.toString())) {
                          // Process control message
                          handleControlMessage(jsonObject);
                    } else {
                          logger.log(Level.WARNING,
                                  "Unrecognized message type (%s)",
                                  messageType.toString());
                    }
                } catch (ParseException e) {
                    logger.log(Level.SEVERE, "Error parsing JSON " + json, e);
                } catch (Exception e) {
                    logger.log(Level.SEVERE, "Failed to process packet", e);
                }
            }
        }, new PacketTypeFilter(Message.class));

        // Log all outgoing packets
        connection.addPacketInterceptor(new PacketInterceptor() {
            @Override
                public void interceptPacket(Packet packet) {
                    logger.log(Level.INFO, "Sent: {0}", packet.toXML());
                }
            }, new PacketTypeFilter(Message.class));

        connection.login(senderId + "@gcm.googleapis.com", apiKey);
    }

    public static void main(String[] args) throws Exception {
        final long senderId = 1234567890L; // your GCM sender id
        final String password = "Your API key";

        SmackCcsClient ccsClient = new SmackCcsClient();

        ccsClient.connect(senderId, password);

        // Send a sample hello downstream message to a device.
        String toRegId = "RegistrationIdOfTheTargetDevice";
        String messageId = ccsClient.nextMessageId();
        Map<String, String> payload = new HashMap<String, String>();
        payload.put("Hello", "World");
        payload.put("CCS", "Dummy Message");
        payload.put("EmbeddedMessageId", messageId);
        String collapseKey = "sample";
        Long timeToLive = 10000L;
        String message = createJsonMessage(toRegId, messageId, payload,
                collapseKey, timeToLive, true);

        ccsClient.sendDownstreamMessage(message);
    }

    /**
     * XMPP Packet Extension for GCM Cloud Connection Server.
     */
    private static final class GcmPacketExtension extends DefaultPacketExtension {

        private final String json;

        public GcmPacketExtension(String json) {
            super(GCM_ELEMENT_NAME, GCM_NAMESPACE);
            this.json = json;
        }

        public String getJson() {
            return json;
        }

        @Override
        public String toXML() {
            return String.format("<%s xmlns=\"%s\">%s</%s>",
                    GCM_ELEMENT_NAME, GCM_NAMESPACE,
                    StringUtils.escapeForXML(json), GCM_ELEMENT_NAME);
        }

        public Packet toPacket() {
            Message message = new Message();
            message.addExtension(this);
            return message;
        }
    }

    private static final class LoggingConnectionListener
            implements ConnectionListener {

        @Override
        public void connected(XMPPConnection xmppConnection) {
            logger.info("Connected.");
        }

        @Override
        public void authenticated(XMPPConnection xmppConnection) {
            logger.info("Authenticated.");
        }

        @Override
        public void reconnectionSuccessful() {
            logger.info("Reconnecting..");
        }

        @Override
        public void reconnectionFailed(Exception e) {
            logger.log(Level.INFO, "Reconnection failed.. ", e);
        }

        @Override
        public void reconnectingIn(int seconds) {
            logger.log(Level.INFO, "Reconnecting in %d secs", seconds);
        }

        @Override
        public void connectionClosedOnError(Exception e) {
            logger.info("Connection closed on error.");
        }

        @Override
        public void connectionClosed() {
            logger.info("Connection closed.");
        }
    }
}

Python sample

Here is an example of a CCS app server written in Python. This sample echo server sends an initial message, and for every upstream message received, it sends a dummy response back to the application that sent the upstream message. This example illustrates how to connect, send, and receive GCM messages using XMPP. It shouldn't be used as-is on a production deployment.

#!/usr/bin/python
import sys, json, xmpp, random, string

SERVER = 'gcm.googleapis.com'
PORT = 5235
USERNAME = "Your GCM Sender Id"
PASSWORD = "API Key"
REGISTRATION_ID = "Registration Id of the target device"

unacked_messages_quota = 100
send_queue = []

# Return a random alphanumerical id
def random_id():
  rid = ''
  for x in range(8): rid += random.choice(string.ascii_letters + string.digits)
  return rid

def message_callback(session, message):
  global unacked_messages_quota
  gcm = message.getTags('gcm')
  if gcm:
    gcm_json = gcm[0].getData()
    msg = json.loads(gcm_json)
    if not msg.has_key('message_type'):
      # Acknowledge the incoming message immediately.
      send({'to': msg['from'],
            'message_type': 'ack',
            'message_id': msg['message_id']})
      # Queue a response back to the server.
      if msg.has_key('from'):
        # Send a dummy echo response back to the app that sent the upstream message.
        send_queue.append({'to': msg['from'],
                           'message_id': random_id(),
                           'data': {'pong': 1}})
    elif msg['message_type'] == 'ack' or msg['message_type'] == 'nack':
      unacked_messages_quota += 1

def send(json_dict):
  template = ("<message><gcm xmlns='google:mobile:data'>{1}</gcm></message>")
  client.send(xmpp.protocol.Message(
      node=template.format(client.Bind.bound[0], json.dumps(json_dict))))

def flush_queued_messages():
  global unacked_messages_quota
  while len(send_queue) and unacked_messages_quota > 0:
    send(send_queue.pop(0))
    unacked_messages_quota -= 1

client = xmpp.Client('gcm.googleapis.com', debug=['socket'])
client.connect(server=(SERVER,PORT), secure=1, use_srv=False)
auth = client.auth(USERNAME, PASSWORD)
if not auth:
  print 'Authentication failed!'
  sys.exit(1)

client.RegisterHandler('message', message_callback)

send_queue.append({'to': REGISTRATION_ID,
                   'message_id': 'reg_id',
                   'data': {'message_destination': 'RegId',
                            'message_id': random_id()}})

while True:
  client.Process(1)
  flush_queued_messages()