Set a specified branch id as default branch. API methods such as SearchService.Search, ProductService.GetProduct, ProductService.ListProducts will treat requests using "default_branch" to the actual branch id set as default. For example, if projects/*/locations/*/catalogs/*/branches/1 is set as default, setting SearchRequest.branch to projects/*/locations/*/catalogs/*/branches/default_branch is equivalent to setting SearchRequest.branch to projects/*/locations/*/catalogs/*/branches/1. Using multiple branches can be useful when developers would like to have a staging branch to test and verify for future usage. When it becomes ready, developers switch on the staging branch using this API while keeping using projects/*/locations/*/catalogs/*/branches/default_branch as SearchRequest.branch to route the traffic to this staging branch. CAUTION: If you have live predict/search traffic, switching the default branch could potentially cause outages if the ID space of the new branch is very different from the old one. More specifically: * PredictionService will only return product IDs from branch {newBranch}. * SearchService will only return product IDs from branch {newBranch} (if branch is not explicitly set). * UserEventService will only join events with products from branch {newBranch}.