How to send a message to multiple recipients via FCM?

Question

Android

Hello!
Connected the FCM service (Firebase Cloud Messaging).
The documentation describes an example for sending a message to a single user. This is the "to" field. As you can see, it takes one line. Tried to substitute an array of strings there. Not a ride: asks to insert a line.
In the same documentation, there is a "multicast_id" parameter that is incomprehensible to me.
More precisely, the incomprehensible value of the parameter.
How can I send a message to multiple users at once?

Answer 1

When subscribing to receive notifications, subscribe the device to the topic
And when sending a message, you can just specify this topic and the message will reach all devices that have subscribed to this topic

{
  "to" : "/topics/fresh-music-set",
  "data" : {
      "title" : "Вышел новый суперский сборник",
      "text": "Лучшие треки первого века, такого ты еще не слышал",
      "name" : "Музыка для сна"
      "count" : 100
      "id" : 7829,
      "pictureUrl" : "http://cdnimg.zaycev.net/commonImage/packiePresentation/3917/top917.jpg"
  }
}