为什么 CouchDB 在同步一个 _rev 差异高于修订限制的 PouchDB 文档时会产生冲突？

Question

This is a strange behavior. First, we sync CouchDB and PouchDB databases. Second, the PouchDB database goes offline. After many modifications to a document, it goes online and sync with CouchDB. If PouchDB document _rev number is higher than CouchDB _rev number plus the revs limit, CouchDB generates a 409 "Document update conflict". Why? And what can we do to avoid it?

Answer 1

Unfortunately, this is the expected behaviour for the revs_limit option in PouchDB. The documentation says

revs_limit: Specify how many old revisions we keep track (not a copy) of. Specifying a low value means Pouch may not be able to figure out whether a new revision received via replication is related to any it currently has which could result in a conflict. Defaults to 1000.

If your PouchDB revs_limit is too low, it cannot be determined whether your local revision actually has the server revision in its history and therefore throws a conflict.

The straighforward way would be to increase the local revision limit. But if you're generating over 1000 revisions between syncs, you should consider changing your data model, splitting up large JSONs and storing the modifications in a new document and merging the data in the app instead of modifying the full document.

If that's not an option, simply check for conflicts and resolve them by deleting the server version whenever they occur.