MongoDB has support for atomic updates. I.e. I can be sure that when a

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Asked: June 1, 20262026-06-01T16:32:50+00:00 2026-06-01T16:32:50+00:00

MongoDB has support for atomic updates. I.e. I can be sure that when a

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MongoDB has support for atomic updates. I.e. I can be sure that when a document is updated no other update will overwrite my previous change. My question relates to the combination of query and update statement, and is best illustrated by the example shown below.

db.foo.update(
{ state : 1, players: { $size: 2 } } , 
{ $push: { players : { new player document } } }, 
false , true );

In the above example, I only want to push a new player into a collection of players, if the number of players equals 2. Given the above query and update statement, is it possible that two simultaneous updates both push a player onto the same document, because at the time of reading the document its players $size is 2? I.e. does the atomicity span across the query and update part of the update statement or not?

Edit More in-depth sequence of events:

Consider firing the same update twice (U1 and U2) at the same time. Is the following sequence of events possible or not?

U1 finds that document #1 matches the query portion of the update
statement.
U2 finds that document #1 matches the query portion of
the update statement.
U1 pushes a new player in document #1.
U2 pushes a new player in document #1.

The end result is that document #1 contains one more player than expected, because both U1 and U2 were under the impression that document #1 contains only two players.