@startuml Segment State Machine
title Segment State Machine
state READY
state FREEZE
state MAINTENANCE_RUNNING
state CLOSED
state ERROR

[*] --> FREEZE : open
FREEZE --> MAINTENANCE_RUNNING : maintenance thread\nstarts flush/compact
FREEZE --> READY : swap complete\nor EXCLUSIVE_ACCESS closed\nSegment Initialization
FREEZE --> CLOSED : close completes
READY --> FREEZE : start flush()/compact()\nor openIterator(EXCLUSIVE_ACCESS)/\nclose()
MAINTENANCE_RUNNING --> FREEZE : IO done\nswap starts

ERROR -[hidden]-> READY

note right of ERROR
  Any state can transition to ERROR
  on index or IO failure.
end note

@enduml