docs: add region migration RFC (#2703)

feat: add region migration rfc

docs/rfcs/2023-11-07-region-migration.md

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+---
+Feature Name: Region Migration Procedure
+Tracking Issue: https://github.com/GreptimeTeam/greptimedb/issues/2700
+Date: 2023-11-03
+Author: "Xu Wenkang <wenymedia@gmail.com>"
+---
+
+# Summary
+This RFC proposes a way that brings the ability of Meta Server to move regions between the Datanodes.
+
+# Motivation
+Typically, We need this ability in the following scenarios:
+- Migrate hot-spot Regions to idle Datanode
+- Move the failure Regions to an available Datanode
+
+# Details
+
+```mermaid
+flowchart TD
+    style Start fill:#85CB90,color:#fff
+    style End fill:#85CB90,color:#fff
+    style SelectCandidate fill:#F38488,color:#fff
+    style OpenCandidate fill:#F38488,color:#fff
+    style UpdateMetadataDown fill:#F38488,color:#fff
+    style UpdateMetadataUp fill:#F38488,color:#fff
+    style UpdateMetadataRollback fill:#F38488,color:#fff
+    style DowngradeLeader fill:#F38488,color:#fff
+    style UpgradeCandidate fill:#F38488,color:#fff
+
+    Start[Start] 
+    SelectCandidate[Select Candidate] 
+    UpdateMetadataDown["`Update Metadata(Down)
+        1. Downgrade Leader
+    `"]
+    DowngradeLeader["`Downgrade Leader
+    1. Become Follower
+    2. Return **last_entry_id**
+    `"]
+    UpgradeCandidate["`Upgrade Candidate
+    1. Replay to **last_entry_id**
+    2. Become Leader
+    `"]
+    UpdateMetadataUp["`Update Metadata(Up)
+        1. Switch Leader
+        2.1. Remove Old Leader(Opt.)
+        2.2. Move Old Leader to Follower(Opt.)
+    `"]
+    UpdateMetadataRollback["`Update Metadata(Rollback)
+        1. Upgrade old Leader
+    `"]
+    End
+    AnyCandidate{Available?}
+    OpenCandidate["Open Candidate"]
+    CloseOldLeader["Close Old Leader"]
+
+    Start 
+    --> SelectCandidate
+    --> AnyCandidate
+    --> |Yes| UpdateMetadataDown 
+    --> I1["Invalid Frontend Cache"]
+    --> DowngradeLeader 
+    --> UpgradeCandidate
+    --> UpdateMetadataUp
+    --> I2["Invalid Frontend Cache"]
+    --> End
+    
+    UpgradeCandidate
+    --> UpdateMetadataRollback
+    --> I3["Invalid Frontend Cache"]
+    --> End
+
+    I2 
+    --> CloseOldLeader
+    --> End
+
+    AnyCandidate 
+    --> |No| OpenCandidate
+    --> UpdateMetadataDown
+```
+
+**Only the red nodes will persist state after it has succeeded**, and other nodes won't persist state. (excluding the Start and End nodes).
+
+## Steps
+
+**The persistent context:** It's shared in each step and available after recovering. It will only be updated/stored after the Red node has succeeded.
+
+Values: 
+- `region_id`: The target leader region.
+- `peer`: The target datanode.
+- `close_old_leader`: Indicates whether close the region. 
+- `leader_may_unreachable`: It's used to support the failover procedure.
+
+**The Volatile context:** It's shared in each step and available in executing (including retrying). It will be dropped if the procedure runner crashes.
+
+### Select Candidate
+
+The Persistent state: Selected Candidate Region.
+
+### Update Metadata(Down)
+
+**The Persistent context:**
+- The (latest/updated) `version` of `TableRouteValue`, It will be used in the step of `Update Metadata(Up)`.
+
+### Downgrade Leader
+This step sends an instruction via heartbeat and performs:
+1. Downgrades leader region.
+2. Retrieves the `last_entry_id` (if available).
+
+If the target leader region is not found: 
+- Sets `close_old_leader` to true.
+- Sets `leader_may_unreachable` to true.
+
+If the target Datanode is unreachable:
+- Waits for region lease expired.
+- Sets `close_old_leader` to true.
+- Sets `leader_may_unreachable` to true.
+
+**The Persistent context:**
+None
+
+**The Persistent state:** 
+- `last_entry_id`
+
+*Passes to next step.
+
+
+### Upgrade Candidate
+This step sends an instruction via heartbeat and performs:
+1. Replays the WAL to latest(`last_entry_id`).
+2. Upgrades the candidate region.
+
+If the target region is not found: 
+- Rollbacks.
+- Notifies the failover detector if `leader_may_unreachable` == true.
+- Exits procedure.
+
+If the target Datanode is unreachable:
+- Rollbacks.
+- Notifies the failover detector if `leader_may_unreachable` == true.
+- Exits procedure.
+
+**The Persistent context:**
+None
+
+### Update Metadata(Up)
+This step performs
+1. Switches Leader.
+2. Removes Old Leader(Opt.).
+3. Moves Old Leader to follower(Opt.).
+
+The `TableRouteValue` version should equal the `TableRouteValue`'s `version` in Persistent context. Otherwise, verifies whether `TableRouteValue` already updated.
+
+**The Persistent context:**
+None
+
+### Close Old Leader(Opt.)
+This step sends a close region instruction via heartbeat.
+
+If the target leader region is not found: 
+- Ignore.
+
+If the target Datanode is unreachable: 
+- Ignore.
+
+### Open Candidate(Opt.)
+This step sends an open region instruction via heartbeat and waits for conditions to be met (typically, the condition is that the `last_entry_id` of the Candidate Region is very close to that of the Leader Region or the latest).
+
+If the target Datanode is unreachable: 
+- Exits procedure.