Implementing GCM Server

The server side of GCM consists of 2 components:

  • Google-provided GCM Connection Servers take messages from a 3rd-party application server and send them to a GCM-enabled Android application (the "client app") running on a device. For example, Google provides connection servers for HTTP and CCS (XMPP).
  • A 3rd-party application server that you must implement. This application server sends data to a GCM-enabled Android application via the chosen GCM connection server.

Here are the basic steps you follow to implement your 3rd-party app server:

  • Decide which GCM connection server(s) you want to use. Note that if you want to use upstream messaging from your client applications, you must use CCS. For a more detailed discussion of this, see Choosing a GCM Connection Server.
  • Decide how you want to implement your app server. For example:
    • If you decide to use the HTTP connection server, you can use the GCM server helper library and demo app to help in implementing your app server.
    • If you decide to use the XMPP connection server, you can use the provided Python or Java Smack demo apps as a starting point.
    • Note that Google AppEngine does not support connections to CCS.

A full GCM implementation requires both a client implementation and a server implementation. For more information about implementing the client side, see Implementing GCM Client.

Choosing a GCM Connection Server

Currently GCM provides two connection servers: HTTP and CCS (XMPP). You can use them separately or in tandem. CCS messaging differs from GCM HTTP messaging in the following ways:

  • Upstream/Downstream messages
    • GCM HTTP: Downstream only: cloud-to-device.
    • CCS: Upstream and downstream (device-to-cloud, cloud-to-device).
  • Asynchronous messaging
    • GCM HTTP: 3rd-party app servers send messages as HTTP POST requests and wait for a response. This mechanism is synchronous and causes the sender to block before sending another message.
    • CCS: 3rd-party app servers connect to Google infrastructure using a persistent XMPP connection and send/receive messages to/from all their devices at full line speed. CCS sends acknowledgment or failure notifications (in the form of special ACK and NACK JSON-encoded XMPP messages) asynchronously.
  • JSON
    • GCM HTTP: JSON messages sent as HTTP POST.
    • CCS: JSON messages encapsulated in XMPP messages.

Role of the 3rd-party Application Server

Before you can write client Android applications that use the GCM feature, you must have an application server that meets the following criteria:

  • Able to communicate with your client.
  • Able to fire off properly formatted requests to the GCM server.
  • Able to handle requests and resend them as needed, using exponential back-off.
  • Able to store the API key and client registration IDs. The API key is included in the header of POST requests that send messages.
  • Able to generate message IDs to uniquely identify each message it sends. Message IDs should be unique per sender ID.

Sending Messages

Here is the general sequence of events that occurs when a 3rd-party application server sends a message:

  1. The application server sends a message to GCM servers.
  2. Google enqueues and stores the message in case the device is offline.
  3. When the device is online, Google sends the message to the device.
  4. On the device, the system broadcasts the message to the specified Android application via Intent broadcast with proper permissions, so that only the targeted Android application gets the message. This wakes the Android application up. The Android application does not need to be running beforehand to receive the message.
  5. The Android application processes the message.

The following sections describe the basic requirements for sending messages.

Target

Required. When your app server sends a message in GCM, it must specify a target.

For HTTP you must specify the target as one of:

  • registration_ids: For sending to 1 or more devices (up to 1000). When you send a message to multiple registration IDs, that is called a multicast message.
  • notification_key: For sending to multiple devices owned by a single user.

For CCS (XMPP):

  • You must specify the target as the "to" field, where the "to" field may contain a single registration ID or a notification key. CCS does not support multicast messaging.

Payload

Optional. If you are including a payload in the message, you use the data parameter to include the payload. This applies for both HTTP and CCS.

Message parameters

The following table lists the parameters that a 3rd-party app server might include in the JSON messages it sends to a connection server. See the "Where Supported" column for information about which connection servers support that particular parameter.

Table 1. Message parameters.

Field Description Where Supported
to In CCS, used in place of registration_ids to specify the recipient of a message. Its value must be a registration ID. The value is a string. Required. CCS
message_id In CCS, uniquely identifies a message in an XMPP connection. The value is a string that uniquely identifies the associated message. The value is a string. Required. CCS
message_type In CCS, indicates a special status message, typically sent by the system. However, your app server also uses this parameter to send an 'ack' or 'nack' message back to the CCS connection server. For more discussion of this topic, see Cloud Connection Server. The value is a string. Optional. CCS
registration_ids A string array with the list of devices (registration IDs) receiving the message. It must contain at least 1 and at most 1000 registration IDs. To send a multicast message, you must use JSON. For sending a single message to a single device, you could use a JSON object with just 1 registration id, or plain text (see below). A request must include a recipient—this can be either a registration ID, an array of registration IDs, or a notification_key. Required. HTTP
notification_key A string that maps a single user to multiple registration IDs associated with that user. This allows a 3rd-party server to send a single message to multiple app instances (typically on multiple devices) owned by a single user. A 3rd-party server can use notification_key as the target for a message instead of an individual registration ID (or array of registration IDs). The maximum number of members allowed for a notification_key is 10. For more discussion of this topic, see User Notifications. Optional. HTTP. This feature is supported in CCS, but you use it by specifying a notification key in the "to" field.
collapse_key An arbitrary string (such as "Updates Available") that is used to collapse a group of like messages when the device is offline, so that only the last message gets sent to the client. This is intended to avoid sending too many messages to the phone when it comes back online. Note that since there is no guarantee of the order in which messages get sent, the "last" message may not actually be the last message sent by the application server. Collapse keys are also called send-to-sync messages.
Note: GCM allows a maximum of 4 different collapse keys to be used by the GCM server at any given time. In other words, the GCM server can simultaneously store 4 different send-to-sync messages per device, each with a different collapse key. If you exceed this number GCM will only keep 4 collapse keys, with no guarantees about which ones they will be. See Advanced Topics for more discussion of this topic. Optional.		 CCS, HTTP
data A JSON object whose fields represents the key-value pairs of the message's payload data. If present, the payload data it will be included in the Intent as application data, with the key being the extra's name. For instance, "data":{"score":"3x1"} would result in an intent extra named score whose value is the string 3x1. There is no limit on the number of key/value pairs, though there is a limit on the total size of the message (4kb). The values could be any JSON object, but we recommend using strings, since the values will be converted to strings in the GCM server anyway. If you want to include objects or other non-string data types (such as integers or booleans), you have to do the conversion to string yourself. Also note that the key cannot be a reserved word (from or any word starting with google.). To complicate things slightly, there are some reserved words (such as collapse_key) that are technically allowed in payload data. However, if the request also contains the word, the value in the request will overwrite the value in the payload data. Hence using words that are defined as field names in this table is not recommended, even in cases where they are technically allowed. Optional. CCS, HTTP
delay_while_idle If included, indicates that the message should not be sent immediately if the device is idle. The server will wait for the device to become active, and then only the last message for each collapse_key value will be sent. The default value is false, and must be a JSON boolean. Optional. CCS, HTTP
time_to_live How long (in seconds) the message should be kept on GCM storage if the device is offline. Optional (default time-to-live is 4 weeks, and must be set as a JSON number). CCS, HTTP
restricted_package_name A string containing the package name of your application. When set, messages will only be sent to registration IDs that match the package name. Optional. HTTP
dry_run If included, allows developers to test their request without actually sending a message. Optional. The default value is false, and must be a JSON boolean. HTTP

If you want to test your request (either JSON or plain text) without delivering the message to the devices, you can set an optional HTTP or JSON parameter called dry_run with the value true. The result will be almost identical to running the request without this parameter, except that the message will not be delivered to the devices. Consequently, the response will contain fake IDs for the message and multicast fields.

Plain text (HTTP only)

If you are using plain text instead of JSON, the message fields must be set as HTTP parameters sent in the body, and their syntax is slightly different, as described below:

Field Description
registration_id Must contain the registration ID of the single device receiving the message. Required.
collapse_key Same as JSON (see previous table). Optional.
data.<key> Payload data, expressed as parameters prefixed with data. and suffixed as the key. For instance, a parameter of data.score=3x1 would result in an intent extra named score whose value is the string 3x1. There is no limit on the number of key/value parameters, though there is a limit on the total size of the message. Also note that the key cannot be a reserved word (from or any word starting with google.). To complicate things slightly, there are some reserved words (such as collapse_key) that are technically allowed in payload data. However, if the request also contains the word, the value in the request will overwrite the value in the payload data. Hence using words that are defined as field names in this table is not recommended, even in cases where they are technically allowed. Optional.
delay_while_idle Should be represented as 1 or true for true, anything else for false. Optional. The default value is false.
time_to_live Same as JSON (see previous table). Optional.
restricted_package_name Same as JSON (see previous table). Optional.
dry_run Same as JSON (see previous table). Optional.

Receiving Messages

This is the sequence of events that occurs when an Android application installed on a mobile device receives a message:

  1. The system receives the incoming message and extracts the raw key/value pairs from the message payload, if any.
  2. The system passes the key/value pairs to the targeted Android application in a com.google.android.c2dm.intent.RECEIVE Intent as a set of extras.
  3. The Android application extracts the raw data from the com.google.android.c2dm.intent.RECEIVE Intent by key and processes the data.

See the documentation for each connection server for more detail on how it handles responses.