Ayende @ Rahien

Hi!
My name is Oren Eini
Founder of Hibernating Rhinos LTD and RavenDB.
You can reach me by phone or email:

ayende@ayende.com

+972 52-548-6969

, @ Q c

Posts: 5,971 | Comments: 44,508

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Ask AyendeWhat about the QA env?


Matthew Bonig asks, with regards to a bug in RavenDB MVC Integration (RavenDB Profiler) that caused major slow down on this blog.:

I'd be very curious to know how this code got published to a production environment without getting caught. I would have thought this problem would have occurred in any testing environment as well as it did here. Ayende, can you comment on where the process broke down and how such an obvious bug was able to slip through?

Well, the answer for that comes in two parts. The first part  is that no process broke down. We use our own assets for final testing of all our software, that means that whenever there is a stable RavenDB release pending (and sometimes just when we feel like it) we move our infrastructure to the latest and greatest.

Why?

Because as hard as you try testing, you will never be able to catch everything. Production is the final test ground, and we have obvious incentives of trying to make sure that everything works.  It is dogfooding, basically. Except that if we get a lemon, that is a very public one.

It means that whenever we make a stable release, we can do that with high degree of confidence that everything is going to work, not just because all the tests are passing, but because our production systems had days to actually see if things are right.

The second part of this answer is that this is neither an obvious bug nor one that is easy to catch. Put simply, things worked. There wasn’t even an infinite loop that would make it obvious that something is wrong, it is just that there was a lot of network traffic that you would notice only if you either had a tracer running, or were trying to figure out why the browser was suddenly so busy.

Here is a challenge, try to devise some form of an automated test that would catch something like this error, but do so without actually testing for this specific issue. After all, it is unlikely that someone would have written a test for this unless they run into the error in the first place. So I would be really interested in seeing what sort of automated approaches would have caught that.

Bug HuntWhat made this blog slow?


A while ago the blog start taking 100% CPU on the client machines. Obviously we were doing something very wrong there, but what exactly was it?

We tracked down the problem to the following code:

image_thumb

image_thumb[1]

As you can probably guess, the problem is that we have what is effective an infinite loop. On any Ajax request, we will generate a new Ajax request. And that applies to our own requests as well.

The fix was pretty obvious when we figured out what was going on, but until then…

image

Northwind Starter Kit ReviewConclusion


This is a review of the Northwind Starter Kit project, this review revision 94815 from Dec 18 2011.

A while ago I said:

Seriously?!  22(!) projects to do a sample application using Northwind?

And people took me up to task about it. The criticism was mostly focused on two parts:

  • I didn’t get that the project wasn’t about Northwind, but about being a sample app for architectural design patterns.
  • I couldn’t actually decide that a project was bad simply by looking at the project structure and some minor code browsing.

I am sad to say that after taking a detailed look at the code, I am even more firmly back at my original conclusion.  I started to do a review of the UI code, but there really is no real need to do so.

The entire project, as I said in the beginning, is supposed to be a sample application for Northwind. Northwind is a CRUD application. Well, not exactly, it is supposed to be an example of an Online Store, which is something much bigger than just Northwind. But it isn’t.

Say what you will, the Northwind Starter Kit is a CRUD application. It does exactly that, and nothing else. It does so in an incredibly complicated fashion, mind, but that is what it does.

Well, it doesn’t do updates, or deletes, or creates. So it is just an R application (I certainly consider the codebase to be R rated, not for impressionable developers).

If you want to have a sample application to show off architectural ideas, make sure that the application can actually, you know, show them. The only thing that NSK does is loading stuff from the database, try as I might, I found no real piece of business logic, no any reason why it is so complicated.

So, to the guys who commented on that, it isn’t a good project. If you like it, I am happy for you, there are also people who loves this guy:

Personally, I would call pest control.

Ask AyendeHandling filtering


With regards to my quests against repositories, Matt asks:

…if my aggregate root query should exclude entities that have, for example, and IsActive = false flag, I also don't want to repeatedly exclude the IsActive = false entities. Using the repository pattern I can expose my Get method where internally it ALWAYS does this.

The problem with this question is that it make a false assumption, then go ahead and follow on that false assumption. The false assumption here is that the only way to handle the IsActive = false in by directly querying that. But that is wrong.

With NHibernate, you can define that with a where condition, or as a filter. With RavenDB, you can define that inside a query listener. You can absolutely set those things up as part of your infrastructure, and you won’t need to create any abstractions for that.

Northwind Starter Kit ReviewThat CQRS thing


This is a review of the Northwind Starter Kit project, this review revision 94815 from Dec 18 2011.

It is obvious from reading the code that there was some attention given to CQRS. Unfortunately, I can’t really figure out what for.

To start with, both the Read Model and the Domain Model are actually sitting on the same physical location. If you are doing that, there is a 95% chance that you don’t need CQRS. If you have that, you are going to waste a lot of time and effort and are very unlikely to get anything from it.

In the case of NSK, here is the domain model vs. the read model for the customer.

imageimage

I marked the difference.

I am sorry, there is nothing that justify a different model here. Just needless complexity.

Remember, our job is to make things simpler, not make it hard to work with the application.

Northwind Starter Kit ReviewIt is all about the services


This is a review of the Northwind Starter Kit project, this review revision 94815 from Dec 18 2011.

Okay, enough about the data access parts. Let us see take a look at a few of the other things that are going on in the application. In particular, this is supposed to be an application with…

Domain logic is implemented by means of a Domain Model, onto a layer of services adds application logic. The model is persisted by a DAL designed around the principles of the "Repository" patterns, which has been implemented in a LINQ-friendly way.

Let us try to figure this out one at a time, okay?

The only method in the domain model that have even a hint of domain logic is the CalculateTotalIncome method. Yes, you got it right, that is a method, as in singular. And that method should be replaced with a query, it has no business being on the domain model.

So let us move to the services, okay? Here are the service definitions in the entire project:

image

Look at the methods carefully. Do you see any action at all? You don’t, the entire thing is just about queries.

And queries should be simple, not abstracted away and made very hard to figure out.

The rule of the thumb is that you try hard to not abstract queries, it is operations that you try to abstract. Operations is where you usually actually find the business logic.

Northwind Starter Kit ReviewFrom start to finishing–tracing a request


This is a review of the Northwind Starter Kit project, this review revision 94815 from Dec 18 2011.

One of the things that I repeatedly call out is the forwarding type of architecture, a simple operation that is hidden away by a large number of abstractions that serves no real purpose.

Instead of a controller, let us look at a web service, just to make things slightly different. We have the following:

image

Okay, let us dig deeper:

image

I really like the fact that this repository actually have have FindById method, which this service promptly ignores in favor of using the IQueryable<Customer> implementation. If you want to know how that is implemented, just look (using the EF Code First repository implementations, the others are fairly similar):

image

 

All in all, the entire thing only serves to make things harder to understand and maintain.

Does anyone really think that this abstraction adds anything? What is the point?!

Ask AyendeLife without repositories, are they worth living?


With regards to my quests against repositories, Matt asks:

For example, you dismiss the repository pattern, but what are the alternatives? For example, in an ASP.NET web application you have controllers. I do NOT want to see this code in my controllers:

var sessionFactory = CreateSessionFactory();

using (var session = sessionFactory.OpenSession()) { using (var transaction = session.BeginTransaction()) { // do a large amount of work

// save entities
session.SaveOrUpdate(myEntity);

transaction.Commit();

} }

That is ugly, repetitive code. I want in my service methods to Get, update, save, and not have to worry about the above.

This is a straw dummy. Set up the alternative as nasty and unattractive as possible, then call out the thing you have just set up as nasty and unattractive. It is a good tactic, except that this isn’t the alternative at all.

If you go with the route that Matt suggested, you are going to get yourself into problems. Serious ones. But that isn’t what I recommend. I talked about this scenario specifically in this post. This is how you are supposed to set things up. In a way that doesn’t get in the way of the application. Everything is wired in the infrastructure, and we can just rely on that to be there. And in your controller, you have a Session property that get the current property, and that is it.

For bonus points, you can move your transaction handling there as well, so you don’t need to handle that either. It makes the code so much easier to work with, because you don’t care about all those external concerns, they are handled elsewhere.

Northwind Starter Kit ReviewIf you won’t respect the database, there will be pain


This is a review of the Northwind Starter Kit project, this review revision 94815 from Dec 18 2011.

The database is usually a pretty important piece in your application, and it likes to remind you that it should be respected. If you don’t take care of that, it will make sure that there will be a lot of pain in your future. Case in point, let us look at this method:

image

It looks nice, it is certainly something that looks like a business service. So let us dig down and see how it works.

image

It seems like a nice thing, the code is clear, and beside the bug where you get 100% discount if you buy enough and the dissonance between the comment and the code, fairly clear. And it seems that we have service logic and entity logic, which is always nice.

Except that this piece of code issues the following queries (let us assume a customer with 50 orders).

1 Query to load the customer, line 34 in this code. And now let us look at line 35… what is actually going on here:

image

Okay, so we have an additional query for loading the customer’s orders. Let us dig deeper.

image

And for each order, we have another query for loading all of that order’s items. Does it gets worse?

image

Phew! I was worried here for a second.

But it turns out that we only have a Select N+2 here, where N is the number of orders that a customer has.

What do you want, calculating the discount for the order is complicated, it is supposed to take a lot of time. Of course, the entire thing can be expressed in SQL as:

SELECT 
  SUM((UnitPrice * Quantity) * (1 - Discount) Income
FROM OrderItems o
WHERE o.OrderID in (
  SELECT Id FROM Orders
  WHERE CustomerId = @CustomerId
)

But go ahead and try putting that optimization in. The architecture for the application will actively fight you on that.

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