The common pitfalls of ORM frameworks - RBAR

ORM frameworks are a great tool especially for junior developers because they allow bluring the line between the application logic and the data it crunches.
Except that the aforementioned line blurring advantage may become a real production issue if not taken in consideration when writing the code.

Let us consider an example. Let's suppose we're working on a (what else?) e-commerce platform. Somewhere in the depts of that platform there is a IOrderService which exposes the following method:

public interface IOrderService  
{
    void PlaceOrder(Guid customerId, IEnumerable<OrderItem> itemIds)
}

where OrderItem holds the data about an ordered item.

public class OrderItem  
{
    public Guid ItemId { get; set; }

    public int Quantity { get; set; }
}

The PlaceOrder method needs to:

  • lookup the Customer in the database
  • create a new CustomerOrder instance
  • add each Item to the order and decrease stock count
  • save the CustomerOrder in the database

Of course, since we're using an ORM framework, the classes used by the repositories - Customer, CustomerOrder and Item - are mapped to database tables.

Given the above, someone would be tempted to implement the PlaceOrder method like this:

public void PlaceOrder(Guid customerId, IEnumerable<OrderItem> orderItems)  
{
    var customer = _customerRepository.Get(customerId);
    var order = new CustomerOrder(customer);

    foreach(var orderedItem in orderItems)
    {
        var item = _itemRepository.Get(orderedItem.ItemId);
        item.DecreaseStockCount(orderedItem.Quantity);
        order.Add(item);
    }

    _customerOrderRepository.Register(order);
    _unitOfWork.Commit();
}

And why wouldn't they? It seems the most straightforward transposition of the requirements defined above. The code behaves as expected in both Dev and QA environments and afterwards it's promoted to production where lies a database with hundreds of thousands of rows in the Items table. There also, the behavior is as expected until one day an eager customer wants to buy 980 distinct items (because why not?).

What happens with the code above? It still works well but the database command times out and the customer cannot place a significant order.

So what is the problem? Why it times out? Well, because the aforementioned line between application logic and database is blurred enough for the iterative paradigm to creep into the set-based one. In the SQL community this paradigm creep has a name - Row By Agonizing Row.

To put it in the context of the example above - it takes more time to do 980 pairs of SELECT and UPDATE operations than to do one SELECT which returns 980 rows followed by one UPDATE which alters 980 rows.

So, let's switch the paradigm and start working with collections in our code to minimize the number of operations in the database. The first thing to do is to load all items in bulk instead of loading them one by one. This will reduce the number of SELECT operations from 980 to 1 (a whooping 50% of the number of operations). We still need to update the stock counts for each item individually because the ORM framework doesn't know how to translate the changes for each item into a single UPDATE statement but considering that we've halved the total number of operations let's give this approach a try shall we?

public void PlaceOrder(Guid customerId, IEnumerable<OrderItem> orderItems)  
{
    var customer = _customerRepository.Get(customerId);
    var customerOrder = new CustomerOrder();

    var items = itemRepository.Items
        .Join(orderItems,
              item => item.Id,
              orderedItem => orderedItem.ItemId,
              (item, _) => item)
        .ToDictionary(i => i.Id);

    foreach(var orderedItem in orderedItems)
    {
        var item  = items[orderedItem.ItemId];
        item.DecreaseStockCount(orderedItem.Quantity);
        customerOrder.Add(item);
    }

    _customerOrderRepository.Register(order);
    _unitOfWork.Commit();
}

This will solve the problem with the timeout but will create another one - useless load on the system. The code loads 980 rows from the database but only uses two attributes of each row - Id and Barcode. We might say that this can be solved by projecting an Item into a tuple of <Barcode, Id> but this would be a partial solution because we can still place a great burden on system memory by sending an request of 10k items.
Also, there are still 980 UPDATE statements that need to be executed which is still a lot.

The best approach to this is to not load any data at all from the database and to do the processing as close to the actual data as possible.
And how can we do that? Exactly - with stored procedures.

declare procedure CreateCustomerOrder(  
        @customerId uniqueidentifier not null,
        @orderItems udttorderitems readonly)
begin  
    set no_count on

    update sc
    set sc.Count = sc.Count - o.Quantity
    from StockCounts sc
    join Items i on sc.ItemId == i.Id
    join @orderItems 0 on i.Id = o.ItemId

    insert into CustomerOrder(CustomerId, OrderDateTime)
    values (@customerId, GetDate())

    insert into OrderLines(OrderId, ItemId, Quantity)
    select scope_identity(), i.Id, o.Quantity
    from Items i
    join @orderItems o on o.ItemId = i.Id
end  

Now, of course in real life situations there won't be a customer that orders almost 1000 items with a single order and the second approach (bulk load items and iterate the collection) will do just fine.
The important thing to keep in mind in such cases is the need to switch from a procedural mindset to a set-based one thus pruning this phenomenon of paradigm creep which can become a full-blown RBAR processing.