Create cloud tasks to send notification

scheduleNotification.ts

const pages = []
const batchSize = 2000;
let currentIndex = 0;
while (currentIndex < totalUsers) {
  pages.push({ fromIndex: currentIndex, toIndex: currentIndex + batchSize });
  currentIndex += batchSize;
}
await PromisePool.for(pages)
      .withConcurrency(10)
      .process(async (page) => {
        await createHttpTaskWithToken({
          projectId,
          queue: 'YOUR-CLOUD-TASK-QUEUE-NAME',
          url: `https://us-central1-${projectId}.cloudfunctions.net/sendNotifications`,
          email: `YOUR-SERVICE-EMAIL@${projectId}.iam.gserviceaccount.com`,
          payload: {
            fromIndex: page.fromIndex,
            toIndex: page.toIndex,
          },
        });
      });