PolarDB is Alibaba’s cloud-native SQL database. It has the now-typical architecture of one read-write (RW) node and several read-only (RO) nodes as hot backups, sharing a disaggregated storage layer. Figure 1 from the paper. Each RO has an in-memory cache; to keep this updated, the RO streams the write-ahead log (WAL) from the RW and replays it on the RO’s locally cached data. Nevertheless the RO’s cache is usually a bit out of date, so RO queries may return stale (inconsistent) data. A...
