@startuml

actor User as u
collections Cart as c
database CartStorage as cs
collections ProductService as p
collections Order as o

u -> c : POST /user/<user_id>/cart/<sku_id>\n\t- count
activate c
loop for each item in request
  c -> p : /get_product\n\t- sku\n\t- token
  activate p
    p -> c : \nResponse:\n\t- name\n\t- price
  deactivate p
  c -> c : validate product exists

  c -> o : gRPC Loms.StocksInfo\n\t- sku
  activate o
    o -> c : Response:\n\t- count
  deactivate o
  c -> c : validate stocks
  alt stocks enough
    c -> cs : cart.AddItem()
    c -> u : 200 OK
  else
    c -> u : 412 Failed Precondition (insufficient stocks)
  end
end
deactivate c

@enduml