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The GCM Cloud Connection Server (CCS) is a connection server based on XMPP. CCS allows 3rd-party app servers (which you're responsible for implementing) to communicate with Android devices by establishing a persistent TCP connection with Google servers using the XMPP protocol. This communication is asynchronous and bidirectional.
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 upstream messaging (device-to-cloud) feature of CCS is part of the Google Play services platform. Upstream messaging is available through the {@code 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.
GCM Cloud Connection Server (CCS) is an XMPP endpoint, running on {@code http://gcm.googleapis.com} port 5235.
CCS requires a Transport Layer Security (TLS) connection. That means the XMPP client must initiate a TLS connection. For example in Java, you would call {@code setSocketFactory(SSLSocketFactory)}.
CCS requires a SASL PLAIN authentication mechanism using {@code <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.
You can use most XMPP libraries to interact with CCS.
The following snippets illustrate how to perform authentication in CCS.
<stream:stream to="gcm.googleapis.com"
version="1.0" xmlns="jabber:client"
xmlns:stream="http://etherx.jabber.org/streams"/>
<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>
<auth mechanism="PLAIN" xmlns="urn:ietf:params:xml:ns:xmpp-sasl">MTI2MjAwMzQ3OTMzQHByb2plY3RzLmdjbS5hb mFTeUIzcmNaTmtmbnFLZEZiOW1oekNCaVlwT1JEQTJKV1d0dw==</auth>
<success xmlns="urn:ietf:params:xml:ns:xmpp-sasl"/>
CCS uses normal XMPP <message> stanzas. The body of the message must be:
<gcm xmlns:google:mobile:data>
JSON payload
</gcm>
The JSON payload for server-to-device is similar to what the GCM http endpoint uses, with these exceptions:
message_type = ('ack');
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.
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
}
</gcm>
</message>
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.
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>
A NACK error is a regular XMPP message in which the {@code message_type} status message is "nack". A NACK message contains:
Below are some examples.
Bad registration:
<message>
<data:gcm xmlns:data="google:mobile:data">
{
"error":"BAD_REGISTRATION", // error code
"message_id":"msgId1",
"from":"PA91bHFOtaQGSwupt5l1og",
"message_type":"nack"
}
</data:gcm>
</message>
Invalid "time to live":
<message>
<data:gcm xmlns:data="google:mobile:data">
{
"error":"InvalidJson : INVALID_TTL : Invalid value (-1) for \"time_to_live\": must be between 0 and \"2419200\"\n",
"message_id":"msgId1",
"from":"APA91bHFOtaQGSwupt5l1og",
"message_type":"nack"
}
</data:gcm>
</message>
JSON type error:
<message>
<data:gcm xmlns:data="google:mobile:data">
{
"error":"InvalidJson : JSON_TYPE_ERROR : Field \"delay_while_idle\" must be a JSON java.lang.Boolean: not-boolean-user-supplied-value\n",
"message_id":"msgId1",
"from":"APA91bHFOtaQGSwupt5l1og",
"message_type":"nack"
}
</data:gcm>
</message>
The following table lists some of the more common NACK error codes.
Table 1. NACK error codes.
| Error Code | Description |
|---|---|
| {@code BAD_REGISTRATION} | The device has a registration ID, but it's invalid. |
| {@code DEVICE_UNREGISTERED} | The device is not registered. |
| {@code INTERNAL_SERVER_ERROR} | The server encountered an error while trying to process the request. |
| {@code SERVICE_UNAVAILABLE} | The CCS connection server is temporarily unavailable, try again later (using exponential backoff, etc.). |
| {@code BAD_ACK} | The ACK message is improperly formed. |
| {@code AUTHENTICATION_FAILED} | This is a 401 error indicating that there was an error authenticating the sender account. |
| {@code INVALID_TTL} | There was an error in the supplied "time to live" value. |
| {@code JSON_TYPE_ERROR} | There was an error in the supplied JSON data type. |
You can also get a stanza error in certain cases. A stanza error contains:
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>
Using CCS and the {@code GoogleCloudMessaging} API, you can send messages from a user's device to the cloud.
Here is how you send an upstream message using the {@code 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 4 weeks.
int ttl = [0 seconds, 4 weeks]
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>
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 1000, 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 1000, 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.
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.
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.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.util.StringUtils;
import org.json.simple.JSONValue;
import org.json.simple.parser.ParseException;
import org.xmlpull.v1.XmlPullParser;
import java.util.HashMap;
import java.util.Map;
import java.util.Random;
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.
*
* <p>For illustration purposes only.
*/
public class SmackCcsClient {
Logger logger = Logger.getLogger("SmackCcsClient");
public static final String GCM_SERVER = "gcm.googleapis.com";
public static final int GCM_PORT = 5235;
public static final String GCM_ELEMENT_NAME = "gcm";
public static final String GCM_NAMESPACE = "google:mobile:data";
static Random random = new Random();
XMPPConnection connection;
ConnectionConfiguration config;
/**
* XMPP Packet Extension for GCM Cloud Connection Server.
*/
class GcmPacketExtension extends DefaultPacketExtension {
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, json, GCM_ELEMENT_NAME);
}
@SuppressWarnings("unused")
public Packet toPacket() {
return new Message() {
// Must override toXML() because it includes a <body>
@Override
public String toXML() {
StringBuilder buf = new StringBuilder();
buf.append("<message");
if (getXmlns() != null) {
buf.append(" xmlns=\"").append(getXmlns()).append("\"");
}
if (getLanguage() != null) {
buf.append(" xml:lang=\"").append(getLanguage()).append("\"");
}
if (getPacketID() != null) {
buf.append(" id=\"").append(getPacketID()).append("\"");
}
if (getTo() != null) {
buf.append(" to=\"").append(StringUtils.escapeForXML(getTo())).append("\"");
}
if (getFrom() != null) {
buf.append(" from=\"").append(StringUtils.escapeForXML(getFrom())).append("\"");
}
buf.append(">");
buf.append(GcmPacketExtension.this.toXML());
buf.append("</message>");
return buf.toString();
}
};
}
}
public SmackCcsClient() {
// Add GcmPacketExtension
ProviderManager.getInstance().addExtensionProvider(GCM_ELEMENT_NAME,
GCM_NAMESPACE, new PacketExtensionProvider() {
@Override
public PacketExtension parseExtension(XmlPullParser parser)
throws Exception {
String json = parser.nextText();
GcmPacketExtension packet = new GcmPacketExtension(json);
return packet;
}
});
}
/**
* 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 getRandomMessageId() {
return "m-" + Long.toString(random.nextLong());
}
/**
* Sends a downstream GCM message.
*/
public void send(String jsonRequest) {
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 process an upstream message.
*/
public void handleIncomingDataMessage(Map<String, Object> jsonObject) {
String from = jsonObject.get("from").toString();
// PackageName of the application that sent this message.
String category = jsonObject.get("category").toString();
// Use the packageName as the collapseKey in the echo packet
String collapseKey = "echo:CollapseKey";
@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, getRandomMessageId(), payload, collapseKey, null, false);
send(echo);
}
/**
* Handles an ACK.
*
* <p>By default, it only logs a {@code INFO} message, but subclasses could override it to
* properly handle ACKS.
*/
public void handleAckReceipt(Map<String, Object> jsonObject) {
String messageId = jsonObject.get("message_id").toString();
String from = jsonObject.get("from").toString();
logger.log(Level.INFO, "handleAckReceipt() from: " + from + ", messageId: " + messageId);
}
/**
* Handles a NACK.
*
* <p>By default, it only logs a {@code INFO} message, but subclasses could override it to
* properly handle NACKS.
*/
public void handleNackReceipt(Map<String, Object> jsonObject) {
String messageId = jsonObject.get("message_id").toString();
String from = jsonObject.get("from").toString();
logger.log(Level.INFO, "handleNackReceipt() from: " + from + ", messageId: " + messageId);
}
/**
* 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.
*/
public 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 username GCM_SENDER_ID@gcm.googleapis.com
* @param password API Key
* @throws XMPPException
*/
public void connect(String username, String password) throws XMPPException {
config = new ConnectionConfiguration(GCM_SERVER, GCM_PORT);
config.setSecurityMode(SecurityMode.enabled);
config.setReconnectionAllowed(true);
config.setRosterLoadedAtLogin(false);
config.setSendPresence(false);
config.setSocketFactory(SSLSocketFactory.getDefault());
// NOTE: Set to true to launch a window with information about packets sent and received
config.setDebuggerEnabled(true);
// -Dsmack.debugEnabled=true
XMPPConnection.DEBUG_ENABLED = true;
connection = new XMPPConnection(config);
connection.connect();
connection.addConnectionListener(new ConnectionListener() {
@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.log(Level.INFO, "Connection closed on error.");
}
@Override
public void connectionClosed() {
logger.info("Connection closed.");
}
});
// 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
handleIncomingDataMessage(jsonObject);
// Send ACK to CCS
String messageId = jsonObject.get("message_id").toString();
String from = jsonObject.get("from").toString();
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 {
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, "Couldn't send echo.", 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(username, password);
}
public static void main(String [] args) {
final String userName = "Your GCM Sender Id" + "@gcm.googleapis.com";
final String password = "API Key";
SmackCcsClient ccsClient = new SmackCcsClient();
try {
ccsClient.connect(userName, password);
} catch (XMPPException e) {
e.printStackTrace();
}
// Send a sample hello downstream message to a device.
String toRegId = "RegistrationIdOfTheTargetDevice";
String messageId = ccsClient.getRandomMessageId();
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;
Boolean delayWhileIdle = true;
ccsClient.send(createJsonMessage(toRegId, messageId, payload, collapseKey,
timeToLive, delayWhileIdle));
}
}
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 = 1000
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()