Requirements:

Similar to a message queue, multiple users simultaneously retrieve data from a table for processing, in order to ensure that each user obtains a different number and adds user_id immediately after the number is completed

db.foo.findOneAndUpdate({"user_id":{$exists:false}, {$set:{user_id:"123456"}},{sort:{_id:1}}})

Official documentsIt is not stated in whether the commandThread safetyIf you have the following records

{"_id":1}
 {"_id":2}
 {"_id":3}

Is it possible that all 5 users come to request at the same time_idA record of 1?

Using Python to develop a script to verify what seems to be thread safe but is not sure whether the application code is explicitly locked for safety reasons. Python code is as follows:

def query_and_set_user_id(user_id):
 result = db.test.find_one_and_update({"user_id":{"$exists":False}},{'$set': {'user_id': user_id}}, sort=[("id", 1)])
 print(threading.current_thread(), result)
 return result
 
 max_workers = 5
 pool = ThreadPoolExecutor(max_workers)
 futures = []
 for i in range(max_workers):
 f = pool.submit(query_and_set_user_id, "user_{}".format(i+1))
 futures.append(f)
 
 wait(futures)