CouchDB / PouchDB(the JS compatible cousin) make use of a global sequence counter to track which documents have changed between sync.
Each database uses an arbitrary byte string to mark a position in a sequence of updates to the database. Each document has the sequence counter of when it was created/updated/deleted. This sequence counter only matters to that particular database (it does not need to be mirrored, it's just a local ref. of WHEN the doc was modified in that particular database).
Syncing is then a process of looking up the last read sequence counter from a checkpoint document (i.e. what was the last modification) and passing that sequence counter to the `changes` endpoint to get a list of all documents with a sequence counter AFTER that, and then pulling/pushing those documents to the local/remote database, and saving the new latest sequence counter.
The official docs [1] give some more info. One key point is if I modify a document several times between syncs, it will only show once in the changes feed with the latest sequence counter for that document. Couch's conflict resolution strategy is a topic for another time, but an interesting one.
Each database uses an arbitrary byte string to mark a position in a sequence of updates to the database. Each document has the sequence counter of when it was created/updated/deleted. This sequence counter only matters to that particular database (it does not need to be mirrored, it's just a local ref. of WHEN the doc was modified in that particular database).
Syncing is then a process of looking up the last read sequence counter from a checkpoint document (i.e. what was the last modification) and passing that sequence counter to the `changes` endpoint to get a list of all documents with a sequence counter AFTER that, and then pulling/pushing those documents to the local/remote database, and saving the new latest sequence counter.
The official docs [1] give some more info. One key point is if I modify a document several times between syncs, it will only show once in the changes feed with the latest sequence counter for that document. Couch's conflict resolution strategy is a topic for another time, but an interesting one.
1. https://docs.couchdb.org/en/3.1.1/replication/protocol.html