I’ve been looking for an answer for days without any luck.
I need to know what’s the expected behavior when two machines update a remote database simultaneously.

Update method

async function update () {
  const filters = { orderNumber: 1, paid: false }
  const updates = { paid: true }
  const { modifiedCount } = await OrderModel.updateOne(filters, updates)
} 

Is it guaranteed that modifiedCount will always be 0 for one machine and 1 for another?

>Solution :

According to docs:

MongoDB uses locking and concurrency control to prevent clients from modifying the same data simultaneously.

And:

In addition to a shared (S) locking mode for reads and an exclusive (X) locking mode for write operations, intent shared (IS) and intent exclusive (IX) modes indicate an intent to read or write a resource using a finer granularity lock.

So exists a lock exclusive (X) for write operations.

So yes, only one updating will be doing at the same time.

Also checks What locks are taken by some common client operations?