When you say, callback value for the operation, which is the value under consideration.
The callback signature is
function(err, bins, metadata, key)
err - err.code == AEROSPIKE_OK, signifies the success/failure of the operate call.
bins - If there is a read operation specified in the operation array, an example
then the updated value of bin1 is returned. If there’s no read operation specified, then no bins will be retrieved.
metadata - contains ttl and generation of the record. Returned when there is atleast one read operation.
key: Key of the record, returned always.
Hope this extra information helps. But if you can give us more details about which callback value we are talking about, that would be great.