Glossary

A

ACID

ACID refers to the four key properties of a transaction: atomicity, consistency, isolation, and durability. Each of these properties is described below.

• Atomicity means that either all the changes of an operation are performed, or none of them are. TiDB ensures the atomicity of the Region that stores the Primary Key to achieve the atomicity of transactions.

• Consistency means that transactions always bring the database from one consistent state to another. In TiDB, data consistency is ensured before writing data to the memory.

• Isolation means that a transaction in process is invisible to other transactions until it completes. This allows concurrent transactions to read and write data without sacrificing consistency. TiDB currently supports the isolation level of REPEATABLE READ.

• Durability means that once a transaction is committed, it remains committed even in the event of a system failure. TiKV uses persistent storage to ensure durability.

L

leader/follower/learner

Leader/Follower/Learner each corresponds to a role in a Raft group of peers. The leader services all client requests and replicates data to the followers. If the group leader fails, one of the followers will be elected as the new leader. Learners are non-voting followers that only serves in the process of replica addition.

O

Old value

The "original value" in the incremental change log output by TiCDC. You can specify whether the incremental change log output by TiCDC contains the "original value".

Operator

An operator is a collection of actions that applies to a Region for scheduling purposes. Operators perform scheduling tasks such as "migrate the leader of Region 2 to Store 5" and "migrate replicas of Region 2 to Store 1, 4, 5".

An operator can be computed and generated by a scheduler, or created by an external API.

Operator step

An operator step is a step in the execution of an operator. An operator normally contains multiple Operator steps.

Currently, available steps generated by PD include:

• TransferLeader: Transfers leadership to a specified member
• AddPeer: Adds peers to a specified store
• RemovePeer: Removes a peer of a Region
• AddLearner: Adds learners to a specified store
• PromoteLearner: Promotes a specified learner to a voting member
• SplitRegion: Splits a specified Region into two

P

pending/down

"Pending" and "down" are two special states of a peer. Pending indicates that the Raft log of followers or learners is vastly different from that of leader. Followers in pending cannot be elected as leader. "Down" refers to a state that a peer ceases to respond to leader for a long time, which usually means the corresponding node is down or isolated from the network.

R

Region/peer/Raft group

Region is the minimal piece of data storage in TiKV, each representing a range of data (96 MiB by default). Each Region has three replicas by default. A replica of a Region is called a peer. Multiple peers of the same Region replicate data via the Raft consensus algorithm, so peers are also members of a Raft instance. TiKV uses Multi-Raft to manage data. That is, for each Region, there is a corresponding, isolated Raft group.

Region split

Regions are generated as data writes increase. The process of splitting is called Region split.

The mechanism of Region split is to use one initial Region to cover the entire key space, and generate new Regions through splitting existing ones every time the size of the Region or the number of keys has reached a threshold.

restore

Restore is the reverse of the backup operation. It is the process of bringing back the system to an earlier state by retrieving data from a prepared backup.

S

scheduler

Schedulers are components in PD that generate scheduling tasks. Each scheduler in PD runs independently and serves different purposes. The commonly used schedulers are:

• balance-leader-scheduler: Balances the distribution of leaders
• balance-region-scheduler: Balances the distribution of peers
• hot-region-scheduler: Balances the distribution of hot Regions
• evict-leader-{store-id}: Evicts all leaders of a node (often used for rolling upgrades)

Store

A store refers to the storage node in the TiKV cluster (an instance of tikv-server). Each store has a corresponding TiKV instance.

T

TSO

Because TiKV is a distributed storage system, it requires a global timing service, Timestamp Oracle (TSO), to assign a monotonically increasing timestamp. In TiKV, such a feature is provided by PD, and in Google Spanner, this feature is provided by multiple atomic clocks and GPS.