Once we deploy Parsoid/PHP to production, before we start shifting live traffic to it, as a first production test, it would be useful to simply mirror changeprop reparse event traffic as per T229019: ChangePropagation should mirror reparse events to both Parsoid/PHP and Parsoid/JS. Once problems (load, performance, errors) there are addressed, it would be time to shift live client traffic to Parsoid/PHP.
The strategy is to shift Parsoid clients one at a time. But, even for a single client, it might be useful to start slow by transferring a small fraction of the traffic to Parsoid/PHP and gradually shifting everything over till we hit 100%.
So, RESTBase needs to support this traffic partitioning scheme.