Receive upstream messages

Caution: Sending messages (including upstream messages) with the FCM XMPP and HTTP legacy APIs was deprecated on June 20, 2023, and will be removed starting June 2024. If you are using the legacy FCM send APIs, we strongly recommend that you migrate to the HTTP v1 API or consider using the Admin SDK to build send requests.

When FCM receives an upstream messaging call from a client app, it generates the necessary XMPP stanza for sending the upstream message. FCM adds the category and from fields, and then sends a stanza like the following to the app server:

<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>

It is recommended to have at least one connection per 500 messages per second. If the app server does not open enough connections to handle all its traffic, FCM drops the excess upstream messages.

Sending an ACK message

In response to an upstream message like the above, the app server must use the same connection to send an ACK message containing the unique message ID. If FCM does not receive an ACK, it may retry sending the message to the app server.

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

See the XMPP Connection Server Reference for more information about upstream message syntax.