Concurrency issue

1

Hi everyone, I’ve a concurrency issue in the code below:

Parse.Cloud.define("subscribe", async (request) => {
  .....
  const listId = .....
  const subscriberId = ...
  const SubscribersList = Parse.Object.extend("SubscribersList");
  const query = new Parse.Query(SubscribersList);
  query.equalTo("listId", listId);

  let numberSubscriptions = await query.count({ useMasterKey: true });
  
 // maxSubscribers retrieved from another collection linked to the SubscribersList with the same listId
  const maxSubscribers = ..... get("maxSubscribers");

  if (numberSubscriptions < maxSubscribers) {
      const subscriber = new SubscribersList();
      subscriber.set("subscriberId", subscriberId);
      subscriber.set("subscriptionDate", new Date(Date.now()));
      subscriber.save(null, { useMasterKey: true });       
   }
}, {
  requireUser: true
});

When I add a new subscriber, if there are more concurrent requests, they could exceed the “maxSubscribers” that I have for a specific subscription list.

Any suggestion on how to manage the concurrency in this case?

Thanks

2

MongoDB offers the $min operator for this, but I’m not sure whether Parse Server or the Parse JS SDK support this yet.

3

Is there any kind of mutex / lock that can be implemented in a safe way to manage these behaviors, including the write operation on the same collection field?

Thank you

4

It may be possible to use an aggregation query to do that. Aggregation queries can also perform modifications. So you could in 1 request look for the object with a condition that subscriberCount < x, then in the next pipeline step update the object.

However, I think the proper solution maybe be to implement the findAndUpdate command in Parse Server.

5

Maybe You can do it in JavaScript, “async-mutex” packages.

const { Mutex } = require('async-mutex');
const mutex = new Mutex();

Parse.Cloud.define("subscribe", async (request) => {
  ...
  
  // maxSubscribers retrieved from another collection linked to the SubscribersList with the same listId
  const maxSubscribers = ..... get("maxSubscribers");

  const release = await mutex.acquire();

  try {
    let numberSubscriptions = await query.count({ useMasterKey: true });

    if (numberSubscriptions < maxSubscribers) {
        const subscriber = new SubscribersList();
        subscriber.set("subscriberId", subscriberId);
        subscriber.set("subscriptionDate", new Date(Date.now()));
        await subscriber.save(null, { useMasterKey: true });       
    }
  } finally {
    release();
  }
}, {
  requireUser: true
});

mutex.acquire() and release() .