TiKV is an open source distributed Key-Value database which is based on the design of Google Spanner and HBase, but it is much simpler without dependency on any distributed file system. It's has primary features including Geo-Replication, Horizontal scalability, Consistent distributed transactions, Coprocessor support.
Multi-version Concurrency Control (MVCC)
TiKV has a Timestamp Oracle(TSO) to provide globally unique timestamp. The core transaction model of TiKV is called 2-Phase Commit powered by MVCC. There are two stages within each transaction: - PreWrite: - Create a `startTS` timestamp. Select one row as the primary row and others as secondary rows. - Check whether there are locks on this row or whether there are commits after the `startTS`. If conflicts exists, the transaction will be rollback. If not, lock the row. - Repeat the second step on other rows. - Commit: - Write to the `CF_WRITE` with current timestamp `commitTS`. - Release all the locks.
Read Committed Repeatable Read
TiDB/TiKV uses the Percolator transaction model. The default isolation level in TiKV is `Repeatable Read`. When a transaction starts, there will be a global read timestamp; when a transaction commits, there will be a global commit timestamp. The execution order of transactions is confirmed based on the timestamps. The underlying details can be found in the *Concurrency Control* section.
TiKV is built on top of RocksDB, where all data in a TiKV node shares two RocksDB instances. One is for data, and the other is for Raft log. There are some major components in TiKV: - Placement Driver (PD): Manages the metadata about Nodes, Stores, Regions mapping, and makes decisions for data placement and load balancing. - Node: A physical node in the cluster. Each node contains one or more Stores. - Store: Stores data in local disks using RocksDB. Each store contains one or more regions. - Region: The basic unit of Key-Value data movement and corresponds to a data range in a Store. Each Region is replicated to multiple Nodes and form a Raft group. A replica of a Region is called a Peer.