commandhandling-pessimistic-concurrency.puml

title Command Handling, Pessimistic Concurrency

actor User
boundary Handler
boundary Repository
entity BarInstance
database EventStore
autonumber
User -> Handler : Set Foo = 2 on Bar\nwhere version = Y
Handler -> Repository : Get Bar
Repository -> BarInstance : Create Bar instance
activate BarInstance
Repository -> EventStore : Get all Bar\nevents from Bar stream
Repository -> BarInstance : Apply all Bar events\nto Bar instance
Repository -> Handler : Return Bar, version = X

group Pessimistic Version Check
  Handler -> Handler : If Y != X\nthrow Concurrency Exception
  Handler -> User : Concurrency exception
end

Handler -> BarInstance : Invoke method Foo
note right
        0 or more events raised by Bar
end note
Handler -> Repository : Save Bar
Repository -> BarInstance : Get events to append
deactivate BarInstance

group No events to append
  Repository -> Repository : If events.Count = 0, return
  Repository -> Handler: Return
  Handler -> User: Return
end
Repository -> EventStore : Append events to Bar stream
group Concurrency Exception
  EventStore -> EventStore : If stream version != X\nthrow Concurrency Exception
  EventStore -> Repository : Concurrency Exception
  Repository -> Handler : Concurrency Exception
  Handler -> User : Concurrency Exception
end
EventStore -> EventStore : Stream appended
EventStore -> Repository : Return
Repository -> Handler : Return
Handler -> User : Return