Ayende @ Rahien

My name is Oren Eini
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JFHCIConsidering Scale

time to read 5 min | 894 words

There are some really good comments in this post, which I am going to respond to. Before that, I want to emphasis that I am still playing devil's advocate here.

Joe has two very insightful comments, regarding the just hard code everything and don't provide any admin interface for the parameters:

When you're making software that will be used by more than one customer, you can't hardcode these things in.


Again, why not? Why can't Company A use the same software as Company B and A wants to give 30% discount when amount is over $10K whereas B wants to give 20% when it's over $15K?
Why is that so hard to expose? It's just a matter a validation from the admin UI.

There is obviously no technical reason to do that, right?


Well, there is one good reason to avoid this at all costs. It just doesn't work when you have any significant amount of rules. And by that, I am talking anything over two dozens or so.

Let us consider what the implications of such a system would be when talking about a system with any kind of reasonable complexity.

The cost of adding a rule has gone up from a simple "add a class" to creating UI, validating values, saving values, adding to the current UI, loading parameters from persistence storage. This translate to moving from twenty minutes task (including testing) to much more complex endeavor. Consider a rule that should give you discount based on geographical location, you need to provide a way to select that. For that matter, you now need to have multi instance rules, where you need to have the same rule bound to different parameters.

Are you willing to estimate the cost now? It is an order of magnitude or two more complex.

Now, let us say that we have solved the problem in some manner. It is not a hard problem to solve, admittedly. We are still left with operational issues that we have to deal with. How are we going to educate the ops team about all the rules? So far I have chosen a very simple set of rules to show, but real business rules are nothing trivial.

Then, we have UI issues, how are we going to show the ops team the set of rules that they can edit? How are we going to represent hundreds and thousands of rules in a meaningful fashion?

My answer is very simple. Don't even try. Instead of trying so hard to cut developers out of the loop, start from assuming that there will be a developer along the way. Now you need to optimize the hell out of this approach.

Remember, we aren't talking about a system that has a very small set of rules, we are talking about a very large set of them. A competitive advantage is how fast we can go from a business requirement to that behavior running in production.

Take into account that a lot of the changes in the system are not just parameters changes, let us take example of just a few business rules that come up as things that the business wants to do. This is not in design phase. This is when the system is on the air:

  • A customer shopping on his birthday gets a 5% discount
  • A preferred customer that has been with us for over a year gets three [product name] for the price of one for this fall.

Again, pre planning for this might give you a way to deal with those (if you thought about that scenario), but even so, you are going to incur a much higher cost at the system implementation phase. Pushing the changes to code makes the implementation much easier, and the ability to modify the system at a later date is greatly improved, because you already built your system around the concept of dynamically changing environment.

But, as Joe said, this doesn't really work if you have more than one customer. It actually does, in my experience. There are a lot of system out there that have a dedicate person for their care, and giving someone a three hours course in how to do this kind of thing is fairly easy (with the requirement that they can program in C# or VB.Net, which is not inconceivable demand).

If this is still not something that you can do, put a DSL in place. A DSL script is much easier to work with for business oriented stuff, and it incur about the same cost from design and implementation view. However, deployment is ever so much easier, since you are deploying a single script, instead of a dll. You can even provide in app UI to do this, if you really wants.

In short, consider the scenario that I am describing, and oft changing environment with a lot of rules. If you have an opinion on how stupid this is, please provide that opinion within this given context.

More posts in "JFHCI" series:

  1. (27 Aug 2008) Quote of the Day
  2. (22 Aug 2008) Considering Scale
  3. (22 Aug 2008) The evil that is configuration


Cory Foy

Ah, I get it more now. In the Java world, we've used things like JRules (http://www.ilog.com/products/jrules/) before. We've also done the assembly based (well, jar based) rules-in-code way, for a very complex system. For those scenarios, you can't do an admin interface.

For example, our system looked at a set of records and based on business rules and filters dynamically removed or create new virtual records to be passed on to the rest of the system. These rules were insanely complex (If record is from Florida and the title is reported as Michigan and the date was between 6/2003 and 9/2003 and we've seen an earlier record that has a possible event, then change this record type to be from Florida but marked as Baz), and were not easily adjustable from the business.

I think the problem was that the example of discounts is deceiptively simple, and putting it in the context of scale clarifies that.

What we really need is a Rules Engine for .NET (looks like there are a couple out there, I'll have to look more into them)

Ayende Rahien


Rule engines commonly suffer from a few major problems.

One, they tend to be much harder to work with than code for developers.

Two, they requires developers to work in any non trivial scenario.

Three, source control.


Point is that whether you provide an admin tool or a DSL, it is irelevant. You need to make this configurable. Also the support line applies to both cases

Cory Foy


Agreed. It's when you need to have a balance between the two that you would start using one. But you bring up a good point - maybe there's a need to have a rules-engine which is capable of some of the things you mention.

Of course, as you aptly pointed out, for truly complex decision trees, you are going to have to have everyone involved anyway. But still might be fun to poke around with.


Some of the examples you are gving are clearly "you are gonna have to hire a developer for this", but many are "is there a business requirement to be able to change this rule?"

The second for example is ommon in accounting systems, where rules and formulas for things like sales tax are clearly defined and rarely change, but the figures that are used, and the items they apply to changes at least every year if not more often.

A long while back I worked for one of the world's largest accountancy software suppliers, and they literally had to have developers standing by on budget day to rewrite large components of their software the night budget changes were announced, and to ship out disks to their customers the next morning.

This is the result of the scenario you are describing.

A later "evolution" of that software was to make a large number of those calculations depend upon externally defined (via a user administration section) values. They no longer had to have so many devs standing by.

I appreciate YAGNI - but to me that is a chance to call your business owners out ... ask them to justify the requirement for change as a request (user story/whatever) and then give them the cost for letting them change it. They can then make th decision.

As a buiness owner, I wouldn't want to have to have C# developers around just to change things when I wanted a rule changed, and to be honest most DSLs are worse.

Either give the users a simple admin screen to alter these kind of values, or a simple admin screen that can compose formulas or decision rules ... or identify that a developer changing it each time is more cost effective (and for that it would have to be very infrequently changed and horribly complicated to express in an admin screen)

Certainly shy away from XML, but job preservation is something I'm not in favour of ... the sooner computers do get smarter and developers can stop writing code and can start expressing business requirement the better.


I like the hard code approach as there is nothing to stop me from hard coding a rule that talks to storage that is updated by a UI, allowing the rule to be changed when required. Rather than making all the rules editable.

Mike Brown

What are your thoughts on using a Workflow/Business Rules engine to do what you're talking about? It's somewhat the best of both worlds. You don't have to create a bunch of UI elements to support rules changes. And you don't have to go all the way down to code to make the change. In theory a business user could even learn how to use it.

I guess that's the same scenario your DSL script would enable.


Just tell us, when we'll have Rhino.Rules?

Ayende Rahien


You seem to be missing the point I am making. Probably my fault for not choosing a better example.

I am not talking about hard coding stuff like base values for tax calculations. Certainly, for things like that, you need a way to deal with this.

I am talking about hard coding the rules that work on top of that, and not exposing all the rules parameters.

Again, I am talking about using this approach to facilitate change, easily. No, it is not a replacement for configuration and not a replacement for the database.

But it is a great approach to prepare for change.

Joseph Healy

Seems like you're doing a lot of fishing around, and looping back hard onto, the data and code extensibility issues. I will say your choice of examples begs a question I feel all the main vendors have dodged for years - standalone rules engines, i.e. 'MS Rules Server 2009'. We now have the ability to use WWF rules without workflows, but that's not the same as having a full featured VS-like environment for rules development, deployment, and management. Hopefully this will be something that emerges from OSLO and won't be buried deeply in the mix way it has been to-date.

My understanding from folks inside of IBM, MS, and Oracle is that the problem they've had addressing this over the past decade is largely the same at each and primarily revolved around turf issues. MS has OSLO, IBM just picked up ILOG so their direction is clear, not sure what Oracle has had going on of late.

Tyler Burd

Ayende's right on the money, as usual. Hard coding rules and rule logic improves maintainability and reduces time-to-market for those rules. This does not mean that one cannot provide an admin interface for the __values used in rules.

Yes, in order to create new rules developers must be on hand, but this would be necessary for anything but the most trivial of rules anyhow.

Ayende Rahien


Sort of.

What I am saying is actually that you don't want to provide admin interface for the majority of the rules values, because it is a problematic issue

Tyler Burd

True. I often find a need to establish a middle-ground, though, where the simplest and most common values are easily editable by an administrator (or even a standard user, depending on where the rule is applied). For the majority of values, though, JFHCI.

Chris Ortman

I like that I can see how this can become a business decision now.

A higher cost employee that can turn up a new feature in a fraction of the time another one would

I find myself compelled to go research smalltalk now as this scenario seems familiar.

Ayende Rahien


No, I DON'T want to make this configurable. That is the whole point.

There is a huge cost of making things configurable

Lucas Goodwin


One thing not being discussed is that MANY of these rules (notifications in particular) can be removed completely from the system and replaced with pull systems (like Rss Feeds/nService messaged/etc) instead of push systems (email notifications).

I've found some of the most complex business rules in our systems are for notification emails. I don't mean "action" emails like "approve this", but general "someone has done something that you may be interested in knowing about".

Users appreciate the focus shift as well.

So going hand-in-hand with the idea that "Does my customer really ever need to do this" on customizing rules and base values is the idea "Does my customer really need ME to do this".

If people are truely honest about this, they find VERY seldom does all that customization actually need to be built, and usually when it does you're better off writing an extensible system for developers, not for the general users.

Ayende Rahien

Pull system tend to suffer very bad scaling issues

Joseph Healy

I've seen insurance, taxation, and permitting apps with thousands of rules with exceptions to nearly every rule - in these systems configuration, hard coding, and admin forms just don't cut it.

There just gets to be a breaking point where only a [complete] rules environment / system will suffice. The issue seems to be one more of what to do below that breaking point. My own opinion all along has been that rules is a tough problem space and been sort of a 'final frontier' in the software development world such that the breaking point has always remained high with those above it served by ILOG and everyone below it rolling their own [painful] response in some fashion or another.

Lucas Goodwin


Kind of off topic, but how so with the Pull systems not scaling? It seems RSS feeds scale very well and I thought the whole point of nService bus (and others of that ilk) was to allow for better scaling.

Perhaps I'm using the wrong terminology for what these systems are... (wouldn't be the first time)

Ayende Rahien


RSS is distributed polling. What I am talking about is centralized polling.

See Twitter for the problems this can cause


When you beyond the simple, then move to a proper rules/decision engine - don't hard code.

Hard coding says "only a C# developer can ever change your rules" and this is exactly what we should be trying to avoid. I want a system that says "any business analyst can change your rules"

Hard coding is a "cheap" solution - and if that is a business requirement then all well and good - but know the business implications and costs first.

More than a little concered at some of the comments on here which sound very much like job protections schemes.

If you have done a good job on a code base you are then redundant as far as that code base is concerned. A bad job requires you to be involved afterwards.

Ayende Rahien


Traditional rules engines suffers from a very simple problem.

a) they tend to suck

b) they require a dev anyway for anything beyond the truly trivial.

The concept of BA can change the rules is fairly easy to achieve, you can build the DSL for that in an hour.

The problem then becomes the ability of the system to manage that, and that is hugely expensive.

Thomas Eyde

Why is it so important to cut the developer out of the loop?

Joseph Healy

That's the problem in a nutshell and why the breakingpoint / threshold is so high. With ILOG you basically send two programmers off to training and when they get back by and large no one can talk with them anymore and they can't explain what it is they do.

This is where IBM, Oracle, and MS need to step up to the plate and put some effort into making these systems more accessible, usable, and easier to manage and deploy. Absent that the threshold will remain high and lesser solutions will still need to be developed and employed.

It is such a ubiquitous requirement however in systems of any note at all it seems crazy to me that we'd enter a "SaaS" / "PaaS" world without vendors doing some serious investing in critical aspect of any such platform. In the end, a part of enterprise systems of any complexity will always be a nightmare until this imperative is addressed.

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