Avoiding race condition when initializing document values

[Fred Short]

Hi,

  I have a schema defined as:

var dailySchema = new Schema({
  _id: String,
  date: String,
  lastAccessTime: Date,
  hits: Number,
  totalElapsedTime: Number,
  hourly: {},
}, {collection: 'daily'});

The daily collection contains 1 doc for each day. The "hourly" field stores hour/minute aggregate data over the course of the day which gets upserted as data comes into the service. The sub doc looks like this:

hourly: {
  "00": {
    "00": {
      "hits": 10,
      "totalElapsedTime": 35235
    },
......
    "59": {
      "hits": 8,
      "totalElapsedTime": 435
    }
  },
......
  "23": {
    "00": : {
      "hits": 221,
      "totalElapsedTime": 113535
    }
  }
}

If the collection doesn't contain a document for the current day, it should first initialize the hourly sub-document fields to zeros for each hour and minute (0..23 x 0..59) then update the hour/minute aggregate based on the hour/minute contained on the incoming data (i.e. $incr {hourly.01.45.hits: 1}). The logic looks like this:

if (daily.apps.find({date: "20161021"}) == null) {

  initialize hourly sub doc to zeros

  increment specific hour/minute hits and totalElapsedTime in sub doc 

} else {

  increment specific hour/minute hits and totalElapsedTime in sub doc 

}

Problem: How can I avoid a race condition when doing the initialization + update (i.e. the service receives 2 incoming requests in parallel at the start of the day)? I can't just do an upsert since the hourly data in the 2nd update will overwrite the first. Is there a way to lock the collection when I check to see if a doc exists to do the initialization + update?

Thanks,

Fred