[CouchDB] Finding out what was replicated

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You can do something like this:

@target host
GET /target => {... "update_seq": X, ...}
POST /_replicate {"source": "<source host>/source", "target": "target"} => {...}
GET /target/_changes?since=X => { //here are your replicated changes }

This last request to /target/_changes will also include any writes to the target database that occurred between the initial GET and /_replicate as well as any writes that occurred after the replication but before the /target/_changes request. If you can only make direct requests against the target database then it is impossible to guarantee an exact answer in the presence of concurrent updates to the target.

On the other hand, if you are performing a push replication from a local database or can otherwise afford to request changes from the source after the replication finishes, you can get exactly what you want even in the presence of concurrent updates to the source and/or the target using the response body from the replication request.

For push replication:

@source host
POST /_replicate {"source": "source", "target": "<target host>/target"} =>
  {..., "start_last_seq": A, "end_last_seq": B, ....}
GET /source/_changes?since=A&
limit=X => { //this is what you want }

where X = B - A.

For pull replication:

@target host
POST /_replicate {"source": "<source host>/source", "target": "target"} =>
  {..., "start_last_seq": A, "end_last_seq": B, ....}

@source host
GET /source/_changes?since=A&limit=X => { //this is what you want }

where X = B - A as above.

Both databases residing on the same host makes the push and pull cases essentially identical (specifying source and target as both bare database names [pull] or as both http endpoints [a sort of pull-push with CouchDB acting as a middleman replication proxy])