I'm actually working on an isomorphic app. My approach is to simply create a PersistenceService interface, with methods returning promises, and to make two classes implementing it, SqlPersistenceService and AjaxPersistenceService. You inject the correct instance when you create the server and when you create the client. No if needed, and as I'm using Typescript, the compiler ensures both SqlPersistenceService and AjaxPersistenceService will keep following the contract of PersistenceService in the future.
