Query Compilation

Not Supported

Isolation Levels

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.

System Architecture

Shared-Nothing

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.

Views

Not Supported

Stored Procedures

Not Supported

Query Interface

Custom API

TiKV support queries such as simple Key-Value, transactional Key-Value and push-down. But no matter it’s transactional Key-Value or push-down, it will be transformed to simple Key-Value operations in TiKV.

Concurrency Control

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.

Query Execution

Tuple-at-a-Time Model

Didn't find much information on it.

Joins

Not Supported

Join can be implemented in application level.

Storage Model

Custom

TiKV uses Key-Value model and the Key-Value pairs are ordered according to the Key's binary sequence.

Storage Architecture

Disk-oriented

TiKV does not write data to disk directly, instead it uses RocksDB as it underlying storage engine, where RocksDB can be regard as a standalone Key Value Map.

Data Model

Key/Value

TiKV uses Key-Value model and the Key-Value pairs are ordered according to the Key's binary sequence.