lock-function.js

const functions = require("firebase-functions");
const admin = require("firebase-admin");
const {nanoid} = require('nanoid');

admin.initializeApp();

exports.withLock = functions.pubsub
  .schedule("0 * * * *")
  .onRun((context) => {
    const { eventId, timestamp } = context;

    const executionId = nanoid();
    const lockRef = admin.firestore().collection("locks").doc(eventId);

    const locked = await admin.firestore().runTransaction(async (transaction) => {
      const lockSnapshot = await transaction.get(lockRef);

      if (lockSnapshot.exists) {
        // If the lock exists, check if we locked it
        return lockSnapshot.data().executionId === executionId;
      }
  
      // If it doesn't exist, attempt to create it
      await transaction.set(lockRef, {
        executionId,
        // The timestamp and lockedAt date are just for debugging, why not?
        timestamp,
        lockedAt: new Date(),
      });
  
      // If the write succeeds, we own the lock
      return true;
    });
  
    if (locked !== true) {
      throw new Error("Already locked. This pubsub event has already been serviced.");
    }
  
    console.log("Locked");

    // Do important stuff
    });