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Grule Short Tutorial

Tutorial | Rule Engine | GRL | GRL JSON | RETE Algorithm | Functions | FAQ | Benchmark

Preparation

Please note that Grule is using Go 1.13.

To import Grule into your project:

$ go get github.com/tespkg/grule

From your go you can import Grule.

import (
	"github.com/tespkg/grule/ast"
	"github.com/tespkg/grule/builder"
	"github.com/tespkg/grule/engine"
	"github.com/tespkg/grule/pkg"
)

Creating Fact Structure

A fact in grule is a pointer to an instance of a struct. The struct may also contain properties just as any normal Golang struct, including any method you wish to define, provided it adheres to the requirements for methods defined below. As an example:

type MyFact struct {
    IntAttribute       int64
    StringAttribute    string
    BooleanAttribute   bool
    FloatAttribute     float64
    TimeAttribute      time.Time
    WhatToSay          string
}

As with normal Golang conventions, Grule is only able to access those visible attributes and methods exposed with an initial capital letter.

func (mf *MyFact) GetWhatToSay(sentence string) string {
    return fmt.Sprintf("Let say \"%s\"", sentence)
}

NOTE: Member functions are subject to the following requirements:

  • The member function must be visible; it's name must start with a capital letter.
  • The member function must return 0 or 1 values. More than one return value is not supported.
  • All numerical argument and return types must be their 64 bit variant. i.e. int64, uint64, float64.
  • The member function should not change the Fact's internal state. The algorithm cannot automatically detect these changes, things become more difficult to reason about, and bugs can creep in. If you MUST change some internal state of the Fact, then you can notify Grule using Changed(varname string) built-in function.

Add Fact Into DataContext

To add a fact into DataContext you have to create an instance of your fact

myFact := &MyFact{
    IntAttribute: 123,
    StringAttribute: "Some string value",
    BooleanAttribute: true,
    FloatAttribute: 1.234,
    TimeAttribute: time.Now(),
}

You can create as many facts as you wish.

After the fact(s) have been created, you can then add those instances into the DataContext:

dataCtx := ast.NewDataContext()
err := dataCtx.Add("MF", myFact)
if err != nil {
    panic(err)
}

Creating a Fact from JSON

JSON data can also be used to describe facts in Grule as of version 1.8.0. For more detail, see JSON as a Fact.

Creating a KnowledgeLibrary and Adding Rules Into It

A KnowledgeLibrary is a collection of KnowledgeBase blue prints and a KnowledgeBase is a collection of many rules sourced from rule definitions loaded from multiple sources. We use RuleBuilder to build KnowledgeBase instances and then add them to the KnowledgeLibrary.

The source form of a GRL can be:

  • a raw string
  • contents of a file
  • a document at an HTTP endpoint

Lets use the RuleBuilder to start populating our KnowledgeLibrary.

knowledgeLibrary := ast.NewKnowledgeLibrary()
ruleBuilder := builder.NewRuleBuilder(knowledgeLibrary)

Next we can define a basic rule as a raw string in the DSL:

// lets prepare a rule definition
drls := `
rule CheckValues "Check the default values" salience 10 {
    when 
        MF.IntAttribute == 123 && MF.StringAttribute == "Some string value"
    then
        MF.WhatToSay = MF.GetWhatToSay("Hello Grule");
        Retract("CheckValues");
}
`

And finally we can use the builder to add the definition to the knowledgeLibrary from a declared resource:

// Add the rule definition above into the library and name it 'TutorialRules'  version '0.0.1'
bs := pkg.NewBytesResource([]byte(drls))
err := ruleBuilder.BuildRuleFromResource("TutorialRules", "0.0.1", bs)
if err != nil {
    panic(err)
}

The KnowledgeLibrary now contains a KnowledgeBase named TutorialRules with version 0.0.1. To execute this particular rule we must obtain an instance from the KnowledgeLibrary. This will be explained on the next section.

Executing Grule Rule Engine

To execute a KnowledgeBase, we need to get an instance of this KnowledgeBase from KnowledgeLibrary

knowledgeBase := knowledgeLibrary.NewKnowledgeBaseInstance("TutorialRules", "0.0.1")

Each instance you obtain from the knowledgeLibrary is a unique clone from the underlying KnowledgeBase blueprint. Each unique instance also carries its own distinct WorkingMemory. As no instance shares any state with any other instance, you are free to use them in any multithreaded environment provided you aren't executing any single instance from multiple threads simultaneously.

Constructing from the KnowledgeBase blueprint also ensures that we aren't recomputing work every time we want to construct an instance. The computational work is only done once, making the work of cloning the AST extremely efficient.

Now lets execute the KnowledgeBase instance using the prepared DataContext.

engine = engine.NewGruleEngine()
err = engine.Execute(dataCtx, knowledgeBase)
if err != nil {
    panic(err)
}

Obtaining Result

Here's the rule we defined above, just for reference:

rule CheckValues "Check the default values" salience 10 {
    when 
        MF.IntAttribute == 123 && MF.StringAttribute == "Some string value"
    then
        MF.WhatToSay = MF.GetWhatToSay("Hello Grule");
        Retract("CheckValues");
}

Assuming the condition is matched (which it is) the action will modify the MF.WhatToSay attribute. In order to ensure that the rule is not then immediately re-evaluted, the rule is Retracted from the set. In this particular instance, if the rule failed to do this then it would match again on the next cycle, and again, and again. Eventually Grule would terminate with an error, since it would be unable to converge on a terminal result.

In this case, all you have to do in order to obtain the result is just examine your myFact instance for the modification your rule made:

fmt.Println(myFact.WhatToSay)
// this should prints
// Lets Say "Hello Grule"

Resources

GRLs can be stored in external files and there are many ways to obtain and load the contents of those files.

From File

fileRes := pkg.NewFileResource("/path/to/rules.grl")
err := ruleBuilder.BuildRuleFromResource("TutorialRules", "0.0.1", fileRes)
if err != nil {
    panic(err)
}

You can also load multiple files into a bundle with paths and glob patterns:

bundle := pkg.NewFileResourceBundle("/path/to/grls", "/path/to/grls/**/*.grl")
resources := bundle.MustLoad()
for _, res := range resources {
    err := ruleBuilder.BuildRuleFromResource("TutorialRules", "0.0.1", res)
    if err != nil {
        panic(err)
    }
}

From String or ByteArray

bs := pkg.NewBytesResource([]byte(rules))
err := ruleBuilder.BuildRuleFromResource("TutorialRules", "0.0.1", bs)
if err != nil {
    panic(err)
}

From URL

urlRes := pkg.NewUrlResource("http://host.com/path/to/rule.grl")
err := ruleBuilder.BuildRuleFromResource("TutorialRules", "0.0.1", urlRes)
if err != nil {
    panic(err)
}

From GIT

bundle := pkg.NewGITResourceBundle("https://github.com/tespkg/grule.git", "/**/*.grl")
resources := bundle.MustLoad()
for _, res := range resources {
    err := ruleBuilder.BuildRuleFromResource("TutorialRules", "0.0.1", res)
    if err != nil {
        panic(err)
    }
}

From JSON

You can now build rules from JSON! Read how it works

Compile GRL into GRB

If you want to have faster rule set loading performance (e.g. you have very large rule sets and loading GRL is too slow), you can save your rule set into GRB (Grules Rule Binary) file. Read how to store and load GRB