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mongodbchangestream

MongoDB Change Streams events: replace vs update?

I checked the document https://docs.mongodb.com/manual/reference/change-events/

I'm not sure when exactly events "replace" and "update" happen.

For example, when I use MongoDB Compass to change 1 field of a document, I expected that event "update" will fire, but when I logged the event, it's "replace".

So if an expected update is actually a "replace" event, when will the real "update" event fire?

Thanks.

Solution

  • In a nutshell, update event is when you update a document without replacing it. replace event is when you create a new document replacing the old one.

    Simple example:

    replset:PRIMARY> db.test.find()
{
  "_id": 0,
  "txt": "qwerty"
}

    Doing an update:

    replset:PRIMARY> db.test.update({_id:0}, {$set:{txt:'abc'}})

    results in this changestream event:

    { _id:
   ....
  operationType: 'update',
   ....

    Doing a save:

    replset:PRIMARY> db.test.save({_id:0, txt:'qwerty'})

    results in this changestream event:

    { _id:
   ....
  operationType: 'replace',
   ....

    If updating a document in Compass resulted in a replace event, that means that Compass is doing a save operation in the background instead of an update. This would make sense from a GUI point of view, since it's easier to save a changed document instead of going into the trouble of checking the difference between the old document and the new one and crafting a special update query with a specific $set operation, which would be expensive to compute.