greptimedb/docs/rfcs/2023-08-13-metadata-txn.md

Feature Name, Tracking Issue, Date, Author

Feature Name	Tracking Issue	Date	Author
Update Metadata in single transaction	https://github.com/GreptimeTeam/greptimedb/issues/1715	2023-08-13	Feng Yangsen <fengys1996@gmail.com>, Xu Wenkang <wenymedia@gmail.com>

Summary

Update Metadata in single transaction.

Motivation

Currently, multiple transactions are involved during the procedure. This implementation is inefficient, and it's hard to make data consistent. Therefore, We can update multiple metadata in a single transaction.

Details

Now we have the following table metadata keys:

TableInfo

// __table_info/{table_id}
pub struct TableInfoKey {
    table_id: TableId,
}

pub struct TableInfoValue {
    pub table_info: RawTableInfo,
    version: u64,
}

TableRoute

// __table_route/{table_id}
pub struct NextTableRouteKey {
    table_id: TableId,
}

pub struct TableRoute {
    pub region_routes: Vec<RegionRoute>,
}

DatanodeTable

// __table_route/{datanode_id}/{table_id}
pub struct DatanodeTableKey {
    datanode_id: DatanodeId,
    table_id: TableId,
}

pub struct DatanodeTableValue {
    pub table_id: TableId,
    pub regions: Vec<RegionNumber>,
    version: u64,
}

TableNameKey

// __table_name/{CatalogName}/{SchemaName}/{TableName}
pub struct TableNameKey<'a> {
    pub catalog: &'a str,
    pub schema: &'a str,
    pub table: &'a str,
}

pub struct TableNameValue {
    table_id: TableId,
}

These table metadata only updates in the following operations.

Region Failover

It needs to update TableRoute key and DatanodeTable keys. If the TableRoute equals the Snapshot of TableRoute submitting the Failover task, then we can safely update these keys.

After submitting Failover tasks to acquire locks for execution, the TableRoute may be updated by another task. After acquiring the lock, we can get the latest TableRoute again and then execute it if needed.

Create Table DDL

Creates all of the above keys. TableRoute, TableInfo, should be empty.

The TableNameKey's lock will be held by the procedure framework.

Drop Table DDL

TableInfoKey and NextTableRouteKey will be added with __removed- prefix, and the other above keys will be deleted. The transaction will not compare any keys.

Alter Table DDL

1. Rename table, updates TableInfo and TableName. Compares TableInfo, and the new TableNameKey should be empty, and TableInfo should equal the Snapshot when submitting DDL.

The old and new TableNameKey's lock will be held by the procedure framework.

2. Alter table, updates TableInfo. TableInfo should equal the Snapshot when submitting DDL.