Xin Dong

To add more details of the proposed solution for the `count`: - One solution would be using a special index to track the number of the documents in a collection....

We probably want to expand this to profile first and then audit the serialization and deserialization code in general in doc layer.

Just to clarify: by `rebuild`, you really meant `index update` right? Cause rebuild in MongoDB is something like building a new index

Talked offline, this is indeed referring to the rebuild of an index.

Just to clarify: we need to clean the subspaces for index entries and documents all together.

We need to make sure all reply messages comply to the [OP_REPLY message format](https://docs.mongodb.com/v3.0/reference/mongodb-wire-protocol/#wire-op-reply) and the [general header format](https://docs.mongodb.com/v3.0/reference/mongodb-wire-protocol/#wp-message-header)

I agree. The query parsing and validating part can be lifted into the constructor of `ExtOperatorUpdate` and `ExtReplaceUpdate`.

Forgot to say this will resolve #11

What fields does it need in order to be compatible with, I guess existing client drivers?

Yes. This also means that once we have such UUID ready, we need to attach that UUID to the reply message we sent to the user too.