Firebase batch send notification

sendNotification.ts

export const sendNotifications = functions
  .runWith({
    timeoutSeconds: 540,
    memory: '1GB',
  })
  .https.onRequest(async (req, res): Promise<void> => {
    const { fromIndex, toIndex } = req.body;

    const usersDataResponse = await db.users.orderBy('userIndex').startAt(fromIndex).endAt(toIndex).get();
    const userToSendNotifications = usersDataResponse.docs.filter(onlyUserAtTimezone)

    try {
      await PromisePool.for(userToSendNotifications)
        .withConcurrency(100)
        .process(async (user) => {
          try {
            const userData = user.data();
            const tokens = userData.tokens || [];
            const notificationData = prepareNotification(userData);

            if (notificationData && tokens.length > 0) {
              const response = await messaging().sendMulticast({
                tokens: tokens,
                notification: notificationData,
              });

              if (response.failureCount > 0) {
                const failedTokens: string[] = [];
                response.responses.forEach((resp, idx) => {
                  // Cleanup the tokens who are not registered anymore.
                  if (
                    resp.error?.code === 'messaging/invalid-registration-token' ||
                    resp.error?.code === 'messaging/registration-token-not-registered'
                  ) {
                    failedTokens.push(tokens[idx]);
                  }
                });

                // remove failed tokens
                if (failedTokens.length > 0) {
                  await db.users.doc(user.id).update({
                    tokens: firestore.FieldValue.arrayRemove(...failedTokens),
                  });
                }

                console.log('List of tokens that caused failures: ' + failedTokens);
              }
            }
          } catch (error) {
            functions.logger.error('error', error);
          }
        });
      res.send('OK');
    } catch (error) {
      res.sendStatus(500);
    }
  });