[hw-3] loms service

docs/homework-3/img/cart-cart-item-add.plantuml

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+@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