Using the Workflow Foundation Rule Engine without using Workflow Foundation

Daniel van Wyk – 3fifteen – SA Developer.Net – Information Worker Group

My Story – Great Expectations “... The rule engine will take care of

that ...” “... Certainly the rule engine is

flexible enough to add new rules ...” “... Ahhh ... That’s just a business

rule we’ll add ...” “... We’ll need a complex busines

rule engine to cater for all the scenarios ...”

“ ... We’ll update the rule engine ...”

Typical Examples

Calculate tax, discount, etc Medical claim assessment Add message to account/order Creating gift vouchers Adding a free item based on your

order Fraud checking Error and Message Logging

The essence of a rule

if (condition) then (action1) else (action2)

if person's age is greater than 55 then discount = discount + 10%

if discount is greater than 12% then discount = 12%

What should the “Rule Engine” do? Ability to add rules on the fly

BONUS: easy editor DOUBLE BONUS: a “busines person” can

add rules Save the rules (and load them) Use .net objects used by application Easy to hook into from application /

domain layer Let me code – no rule theory degree

needed

What are the options?

Code the rules in your code (not too flexible)

Create your own scripting language Dependency Injection Rule engines

Open source▪ C# - Drools.Net, NxBRE, SRE▪ Java – Drools, OpenRules, Mandarax,

SweetRules Third party

Use the WF rule engine (thanks Google!)

WF – Workflow Foundation

Using the Rules Engine

Add References System.Workflow.Activities System.Workflow.ComponentModel

Create Instance of RuleSet RuleSet rules = new RuleSet();

Using the Rules Engine (cont.) Load Rules from file

using (XmlTextReader rulesReader = new XmlTextReader(filename))

{ WorkflowMarkupSerializer serializer = new

WorkflowMarkupSerializer(); rules = (RuleSet)serializer.Deserialize(rulesReader); }

Using the Rules Engine (cont.) Apply Rules

RuleValidation validation = new RuleValidation(typeof(Order), null);

RuleExecution execution = new RuleExecution(validation, newOrder);

rules.Execute(execution);

Demo – the basics

Loading rules from file Editing rules Applying rules Saving rules

Chaining

Order of rules Rules impacting each other

A rule might cause another rule to become valid

Controlling chaining Implicit Explicit

Rule Re-evalution options "Always“

Default behaviour for a rule. The rules engine will always re-evaluate a rule with this setting, if the

proper criteria are met. This setting would not override a rule set chaining behaviour of

"Sequential"

"Never“ Turns off re-evaluation.

The rules engine only considers a rule "evaluated" if the rule executes a non-empty action. Consider a rule that has Then actions, but no Else actions. If the rule

is evaluated and its condition returns false, the rule is still a candidate for re-evaluation because the rule did not execute any actions.

Demo – down the rabbit hole ... Rule Re-Evaluation Rule Chaining

Where to next?

Instead of executing the rules from the UI, make the rule execution part of the class

Save rules in a database Create a RuleRepository library

What can you NOT do?

Return a value from a rule Use delegates in the rule action

this.OrderItems.ForEach(delegate(OrderItem i) { i.DiscountAmount = 10; })

References http://odetocode.com/articles/458.aspx

http://blogs.microsoft.co.il/blogs/bursteg/archive/2006/10/11/RuleExecutionWithoutWorkflow.aspx

http://msdn.microsoft.com/en-us/library/aa480193.aspx

http://footheory.com/blogs/bennie/archive/2007/05/12/simple-application-extensibility-with-wf-rules.aspx

http://www.joshlane.net/blog/WindowsWorkflowRulesEngineWithoutWindowsWorkflow.aspx

Questions?