If the [tikv,tiflash] engine is used and the tiflash node is down, the query service needs to be able to run on the tikv node

[scsldb]

Description

If the [tikv,tiflash] engine is used and the tiflash node is down, the query service needs to be able to run on the tikv node.
For now, it is recommended that TiFlash uses 2 replicas for mission critical services and by doing so, a single node failure will not stop services. But if users needs further auto-switch to TiKV when isolation mode is not on (engine = tikv, tiflash both), it will provide further HA.

Optional choices to implement:

1. A supervisor services monitoring if TiFlash is available and turn TiFlash engine on and off.
2. When TiDB encounter some certain error type talking to TiFlash (for example, connection refuse), it turn off the TiFlash engine automatically. But this will rely on manual switch back on again.

Category

Feature, Usability

Value

Improve system usability when TiKV and TiFlash both is allowed for TiDB to choose.