This is how to use Faraday and GoogleAuth to connect to your FCM instance and push to topics

Rails FCM HTTP v1 API

# if you want or need to move off the legacy FCM HTTP endpoints this will get you started
# Use googleauth gem to construct JWT token

def construct_payload(topic, data, notification = nil)
  message = {
    'topic': "fcm_test",
    # 'token': token, # OR, if you want to send to a specific device
    'data': {
      payload: data.to_json # note we can no longer embed objects within data
    },
  }

  if notification.present?
    message['notification'] = notification
  end

  { 'message': message }
end

def send_to_topic(topic, data)
  # where fcm-test-a76f2 is your firebase project id
  response = Faraday.post('https://fcm.googleapis.com/v1/projects/fcm-test-a76f2/messages:send') do |req|
    req.headers['Content-Type'] = 'application/json'
    req.headers['Authorization'] = "Bearer #{get_jwt_token}"
    puts construct_payload(topic, data).to_json
    req.body = construct_payload(topic, data).to_json
  end
  puts response.as_json
  response
end

# fcm-test-a76f2-firebase-adminsdk-piqvh-c40188da2a.json is a service key
# see: https://firebase.google.com/docs/cloud-messaging/auth-server#provide-credentials-manually
def get_jwt_token
  scope = 'https://www.googleapis.com/auth/firebase.messaging'
  authorizer = Google::Auth::ServiceAccountCredentials.make_creds(
    json_key_io: File.open(File.join(Rails.root, 'config', 'secrets', 'fcm-test-a76f2-firebase-adminsdk-piqvh-c40188da2a.json')),
    scope: scope)
  token = authorizer.fetch_access_token!
  token['access_token']
end