MongoDB - Capturing Dropped Objects From A Capped Collection

Quoting from the MongoDB docs on Capped Collections:

Once the space is fully utilized, newly added objects will replace the oldest objects in the collection.

Is there any way to capture a capped collection's "dropped" objects before they are overwritten 开发者_Go百科? What I am interested in doing is implementing a series of rollup collections. eg.

Hourly --> Daily --> Weekly --> Monthly  etc.

so when an object is dropped from the Hourly collections, I want to capture it and aggregate it up to the Daily collection.

Thanks in advance.

//Nicholas

You would have to implement that functionality in code rather than in MongoDB.

I don't think Capped Collections are the right solution for your use-case.

You could insert into a capped collection, and at the same time insert into a "normal" collection, and aggregate them into hourly / daily, weekly, monthly etc... using map reduce.

As per the MongoDB developers, you can't do this:

http://groups.google.com/group/mongodb-user/browse_thread/thread/aec8d0c85f58d89e/d6701df083eb4679?fwc=1

What I am interested in doing is implementing a series of rollup collections.

As alex said, one way to solve this is to use MapReduce. Another way is to have a different collection e.g. per day, for example logs20110414 and have your application manage read/writes to the appropriate collection.