docs: rfc of update metadata in single txn (GreptimeTeam#2165)

* docs: rfc of update metadata in single txn * chore: apply suggestion from CR
natomuh · Aug 15, 2023 · f5e44ba · f5e44ba
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+---
+Feature Name: Update Metadata in single transaction
+Tracking Issue: https://github.com/GreptimeTeam/greptimedb/issues/1715
+Date: 2023-08-13
+Author: "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** 
+```rust
+// __table_info/{table_id}
+pub struct TableInfoKey {
+    table_id: TableId,
+}
+
+pub struct TableInfoValue {
+    pub table_info: RawTableInfo,
+    version: u64,
+}
+```
+
+**TableRoute** 
+```rust
+// __table_route/{table_id}
+pub struct NextTableRouteKey {
+    table_id: TableId,
+}
+
+pub struct TableRoute {
+    pub region_routes: Vec<RegionRoute>,
+}
+```
+**DatanodeTable**
+```rust
+// __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**
+```rust
+// __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.