One of the main challenges related to web development is maintaining state across the client and the server and most web applications have to solve this somehow. Because of this reason, so-called sync engines have become available as they can own a large part of data synchronization.

In his MSc thesis, my student Mikael Siidorow looked into the space to find out the limits of generalized sync. He did his study through multiple methods including literature review, interviews, and a case study. In the end he came up with a taxonomy showing where generalized sync breaks and what you have to keep in mind when implementing these solutions.

This is not to say sync engines are useless, but that there are clear tradeoffs to consider when introducing them to your codebase especially if you have to deal with an offline requirement.

> Generalization breaks down for offline-capable applications. Offline writes require conflict resolution, create authorization edge cases, and demand coordinated schema management across server and client replicas.

> ...These constraints are structural; engineering effort cannot remove them...

> The trade-off analysis shows that three sync engine vendors converged independently on this conclusion from different starting positions.

This is the big irony. That the vendors all converged on the fact that sync engines only really "work" when you remove the offline part. But, at that point they are a complicated/over engineered cache or worse introducing hard distributed computer science problems unnecessarily.