Percolator, a Google internal system, is designed for incremental updates to the search index, reducing page crawl delay by 50% compared to its predecessor. It's built on top of BigTable and uses cross-shard ACID transactions with a two-phase commit protocol. Percolator provides Snapshot Isolation and a monotonically increasing timestamp, but its design is not suitable for OLTP databases due to relaxed latency requirements. Open source databases like TiDB are inspired by Percolator's distributed transaction design. Google Spanner, introduced in 2012, addresses the limitation of Percolator by providing multi-region distributed transactions with low latency, becoming a globally distributed SQL-based RDBMS underpinning most mission-critical services at Google. Spanner's architecture is based on Megastore and Colossus file system, with auto-sharding and replication for global availability and geographic locality.