OpObserverImpl::onTransactionPrepare() defines a side transaction for preparing a multi-document transaction. The MigrationChunkClonerSourceOpObserver also overrides this method for notifications related to chunk migration outside this side transaction. We could look into splitting up this OpObserver method in two - onTransactionPrepare() and postTransactionPrepare() to better define the boundaries of the side transaction currently in place for preparing the multi-doc transaction.