MongoDB: phantom records in $unwind results under heavy load - mongodb

Question

I have a simple collection of elements like this
{_id: n, xs: [...]}
I'm trying to count total number of elements in all arrays
db.testRace.aggregate([{ $unwind : "$xs" }, { $group : { _id : null, count : { $sum : 1 } } }])
And it works great unless I start to do massive updates of this collection. Under heavy load of update operations I get wrong total - slightly bigger than it should be.
It can be easily reproduced.
First generate some test data
for(var i = 1; i <= 1000000; i++) {
db.testRace.insert({_id: i, xs: [i]});
}
Then simulate a lot of updates
while(true) {
var id = Math.floor((Math.random() * 1000000) + 1);
var obj = db.testRace.find({_id: id}).next();
obj.some="change";
db.testRace.update({_id: id}, obj);
}
And while it is running do aggregate unwind query.
Without load I get right result - 1000000. But when there are a lot of updates I get bigger numbers, like 1001456.
And if I run query like this
db.testRace.aggregate([{ $unwind : "$xs" }, {$group: {_id:"$xs", count:{$sum: 1}}}, { $sort : { count : -1 } }, { $limit : 2 }]);
I get
"result" : [
{
"_id" : 996972,
"count" : 2
},
{
"_id" : 997789,
"count" : 2
}
],
So it seems aggregate count some records twice.
Is it expected behaviour or maybe I'm doing aggregation wrong?
I tested on local mongodb instance, version - 2.4.9

Answer 1

It's expected behavior due to the way MongoDB handles read isolation. When you have a long running query (and an aggregation that reads every single document is a long running query) with updates to that data during the query it may impact whether or no the updated data is returned in the query - depending on what happens when, you could miss a document, receive it or receive it twice.
From the source code:
Any data inserted, deleted, or modified during a yield that should be
returned by a query may or may not be returned by that query. The
query could return: nothing; the data before; the data after; or both
the data before and the data after.
In short, there is no isolation between a query and an
insert/delete/update. AKA, READ_UNCOMMITTED.
https://github.com/mongodb/mongo/blob/master/src/mongo/db/exec/plan_stage.h
Your aggregation query is yielding mid query, during which some of the data is updated. This impacts the results of the query.