[KAFKA-9277] move all group state transition rules into their states

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: 2.5.0
• Assignee: Deng Ziming
• Reporter: Deng Ziming
• Created: 06/Dec/19 07:38
• Updated: 26/Dec/19 01:37
• Resolved: 23/Dec/19 23:36

Description

Today the `GroupMetadata` maintain a validPreviousStates map of all GroupState:

```

private val validPreviousStates: Map[GroupState, Set[GroupState]] =
Map(Dead -> Set(Stable, PreparingRebalance, CompletingRebalance, Empty, Dead),
CompletingRebalance -> Set(PreparingRebalance),
Stable -> Set(CompletingRebalance),
PreparingRebalance -> Set(Stable, CompletingRebalance, Empty),
Empty -> Set(PreparingRebalance))

```

It would be cleaner to move all state transition rules into their states :

```

private[group] sealed trait GroupState

{ val validPreviousStates: Set[GroupState] }

```