[ 
https://issues.apache.org/jira/browse/KAFKA-5258?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
 ]

Onur Karaman updated KAFKA-5258:
--------------------------------
    Description: 
Today the PartitionStateMachine and ReplicaStateMachine defines and asserts the 
valid state transitions inline for each state, looking something like:
{code}
private def handleStateChange(...) {
  targetState match {
    case stateA => {
      assertValidPreviousStates(topicAndPartition, List(stateX, stateY, 
stateZ), stateA)
      // actual work
    }
    case stateB => {
      assertValidPreviousStates(topicAndPartition, List(stateD, stateE), stateB)
      // actual work
    }
  }
}
{code}
It would be cleaner to move all partition and replica state transition rules 
into their states and simply do the assertion at the top of the 
handleStateChange method like so:
{code}
private def handleStateChange(...) {
  assertValidTransition(targetState)
  targetState match {
    case stateA => {
      // actual work
    }
    case stateB => {
      // actual work
    }
  }
}

sealed trait State {
  def state: Byte
  def validPreviousStates: Set[State]
}

case object StateA extends State {
  val state: Byte = 1
  val validPreviousStates: Set[State] = Set(StateX)
}

case object StateB extends State {
  val state: Byte = 2
  val validPreviousStates: Set[State] = Set(StateX, StateY, StateZ)
}
{code}

  was:
Today the PartitionStateMachine and ReplicaStateMachine defines and asserts the 
valid state transitions inline for each state, looking something like:
{code}
private def handleStateChange(...) {
  targetState match {
    case stateA => {
      assertValidPreviousStates(topicAndPartition, List(stateX, stateY, 
stateZ), stateA)
      // actual work
    }
    case stateB => {
      assertValidPreviousStates(topicAndPartition, List(stateD, stateE), stateB)
      // actual work
    }
  }
}
{code}
It would be cleaner to move all partition and replica state transition rules 
into their and simply do the assertion at the top of the handleStateChange 
method like so:
{code}
private def handleStateChange(...) {
  assertValidTransition(targetState)
  targetState match {
    case stateA => {
      // actual work
    }
    case stateB => {
      // actual work
    }
  }
}

sealed trait State {
  def state: Byte
  def validPreviousStates: Set[State]
}

case object StateA extends State {
  val state: Byte = 1
  val validPreviousStates: Set[State] = Set(StateX)
}

case object StateB extends State {
  val state: Byte = 2
  val validPreviousStates: Set[State] = Set(StateX, StateY, StateZ)
}
{code}


> move all partition and replica state transition rules into their states
> -----------------------------------------------------------------------
>
>                 Key: KAFKA-5258
>                 URL: https://issues.apache.org/jira/browse/KAFKA-5258
>             Project: Kafka
>          Issue Type: Sub-task
>            Reporter: Onur Karaman
>            Assignee: Onur Karaman
>            Priority: Minor
>
> Today the PartitionStateMachine and ReplicaStateMachine defines and asserts 
> the valid state transitions inline for each state, looking something like:
> {code}
> private def handleStateChange(...) {
>   targetState match {
>     case stateA => {
>       assertValidPreviousStates(topicAndPartition, List(stateX, stateY, 
> stateZ), stateA)
>       // actual work
>     }
>     case stateB => {
>       assertValidPreviousStates(topicAndPartition, List(stateD, stateE), 
> stateB)
>       // actual work
>     }
>   }
> }
> {code}
> It would be cleaner to move all partition and replica state transition rules 
> into their states and simply do the assertion at the top of the 
> handleStateChange method like so:
> {code}
> private def handleStateChange(...) {
>   assertValidTransition(targetState)
>   targetState match {
>     case stateA => {
>       // actual work
>     }
>     case stateB => {
>       // actual work
>     }
>   }
> }
> sealed trait State {
>   def state: Byte
>   def validPreviousStates: Set[State]
> }
> case object StateA extends State {
>   val state: Byte = 1
>   val validPreviousStates: Set[State] = Set(StateX)
> }
> case object StateB extends State {
>   val state: Byte = 2
>   val validPreviousStates: Set[State] = Set(StateX, StateY, StateZ)
> }
> {code}



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