Monday, March 25, 2019

drools.js: Towards a Polyglot Drools on GraalVM (with Bonus Tech-Lead Prank)

Image courtesy of Massimiliano Dessì

You can find the full source code for this blog post in the submarine-examples repository.

Different programming languages are better for different purposes. Imagine how hard would it be to query a database using an imperative language: luckily, we use SQL for that. Now, imagine how useless would a rule engine be, if defining rules were not convenient! This is the reason why Drools comes with its own custom language, the DRL. The Drools Rule Language is in a so-called domain-specific language, a special-purpose programming language specifically designed to make interaction with a rule engine easier.

In particular, a rule is made of two main parts, the condition and the consequence.

The condition is a list of logic predicates, usually pattern matches, while the consequence is written using an imperative language, usually Java.

An Abstract Rule Engine

Rules are what really make a rule engine. After all, that's what a rule engine does: processing rules. Thus, it might sound kind of logical for the engine to be a bit entangled with the language for rule definitions. Our engine is not specially tied to the DRL; but it used to.

In the last year or so, we spent a lot of time unbundling the innards of the DRL from the guts of the Drools core. The result of this effort is what we called the Canonical Model; that is, an abstract representation of the components that make up a rule engine, including rule definitions. Incidentally, this also paved the way for supporting GraalVM and the Quarkus framework; but our goal was also different. We wanted to abstract our engine from the rule language.

Internally, the DRL is now translated into the canonical representation; but, as we said previously, this canonical model is described using Java code. While this representation is not currently intended to be hand-coded, it is very possible to do so. The following is a simple rewriting of the previous DRL rule.

As you can see, although the rule definition is now embedded in a Java "host" language, it still shows the main features of a DRL definition, namely, the logic condition and the imperative consequence (introduced by the on...execute pair) In other words, this is a so-called embedded or internal domain-specific language.

A small disclaimer applies: the code above works, but our translator takes extra steps for best performance, such as introducing indexes. In fact, one of the reasons why we do not intend this API for public consumption is that, currently, a naive rewrite like this may produce inefficient rules.

A Polyglot Automation Platform

As part of our journey experimenting with our programming model, we wanted to see whether it was feasible to interact with our engine using different programming languages. DRL aside, the canonical model rule definition API is pure-Java.

But GraalVM is not only a tool to generate native binaries: in fact, this is only one of the capabilities of this terrific project. GraalVM is, first and foremost, the one VM to rule them all: that is, a polyglot runtime, with first-class support for both JVM languages and many other dynamic programming languages, with a state-of-the-art JIT compiler, that easily compares or exceeds the performance of the industry standards. For instance, there is already support for R, Ruby, JavaScript and Python; and, compared to writing a JIT compiler from scratch, the Truffle framework makes it terribly easy to write your own, and fine-tuning it to perfection.

GraalVM gave us a great occasion to show how easy could it be to make Drools polyglot, and, above all, to play an awful practical joke on our beloved, hard-working, conference-speaking, JavaScript-hating, resident Java Champion and tech lead Mario!

Enter drools.js:

And here's a picture of Mario screaming in fear at the monster we have created

Jokes aside, this experiment is a window over one of the many possible futures of our platform. The world of application development today is polyglot. We cannot ignore this, and we are trying to understand how to reach a wider audience with our technologies, be it our rule engine, or our workflow orchestration engine; in fact, we are doing the same experiments with other parts of the platform, such as jBPM.

jBPM provides its own DSL for workflow definition. Although this is, again, work in progress, it shows a lot of promise as well. Behold: jbpm.js!


The DRL has served its purpose for a very long time, but we are already providing different ways to interact with our powerful engine, such as DMN and PMML; but power users will always want to reach for finer tuning and write their own rules.

The canonical model API is still a work-in-progress, and, above all, an internal API that is not intended for human consumption; but, if there is enough interest, we do plan to work further to provide a more convenient embedded DSL for rule definition. Through the power of GraalVM, we will be able to realize an embedded DSL that is just as writable in Java as any other language that GraalVM supports.

And this includes JavaScript; sorry Mario!


Thursday, March 14, 2019

Quarking Drools: How we turned a 13-year-old Java project into a first-class serverless component

“The question of whether a computer can think is no more interesting
than the question of whether a submarine can swim.”
- Edsger W. Dijkstra

Rule-based artificial intelligence (AI) is often overlooked, possibly because people think it’s only useful in heavyweight enterprise software products. However, that’s not necessarily true. Simply put, a rule engine is just a piece of software that allows you to separate domain and business-specific constraint from the main application flow. We are part of the team developing and maintaining Drools—the world’s most popular open source rule engine and part of Red Hat—and, in this article, we will describe how we are changing Drools to make it part of the cloud and serverless revolution.

Technical overview

Our main goal was to make the core of the rule engine lighter, isolated, easily portable across different platforms, and well-suited to run in a container. The software development landscape has changed a lot in the past 20 years. We are moving more and more toward a polyglot world, which is one reason why we are working to make our technology work across a lot of different platforms. This is also why we started looking into GraalVM, the new Oracle Labs polyglot virtual machine (VM) ecosystem, consisting of:
  • A polyglot VM runtime, alternative to the Java virtual machine (JVM) with a just-in-time (JIT) compiler that improves efficiency and speed of applications over traditional HotSpot. This is also the “proper” GraalVM.
  • A framework to write efficient dynamic programming languages (e.g., JavaScript, Python, and R) and to mix and match them (Truffle).
  • A tool to compile programs ahead-of-time (AOT) into a native executable.
Meanwhile at Red Hat, another team was already experimenting with GraalVM and native binary generation for application development. This effort has been realized in a new project you may have heard of called Quarkus. The Quarkus project is a best-of-breed Java stack that works on good old JVM but is also especially tailored for GraalVM, native binary compilation, and cloud-native application development.

GraalVM is an amazing tool, but it also comes with some (understandable) limitations. Thus, Quarkus is designed to integrate seamlessly with GraalVM and native image generation, as well as provide useful utilities to overcome any related limitations. In particular, Drools used to make extensive use of dynamic class generation, class-loading, and quite a bit of reflection. To produce fast, efficient, and small native executables, Graal performs aggressive inlining and dead-code elimination, and it operates under a closed-world assumption: that is, the compiler removes any references to class and methods that cannot be statically reachable in the code. In other words, unrestricted reflective calls and dynamic class loading are a no-go. Although this may at first sound like a showstopper, here we will document in detail how we modified the core of Drools to overcome such limitations, and we will explain why such limitations are not evil and can be liberating.

The Executable Model

In a rule engine, facts are inserted into a working memory. Rules describe actions to take when certain constraints over the facts that are inserted into the working memory become true. For instance, the sentence “when the sun goes down : turn on the lights” expresses a rule over the sun. The fact is that the sun is going down. The action is to turn on the lights. In a rule engine, we insert the “sun is going down” fact inside the working memory. When we fire the rules, the action of turning on the lights will execute.

A rule definition has the form


The constraints part, also called the left-hand side of the rule, describes the constraints that activate the rule and make it ready to fire; the consequence part, also called the right-hand side of the rule, contains the action that rule will take when the rule is fired.
In Drools, a rule is written using the Drools Rule Language (in short, DRL), and it has the form:

rule R1 when
   $r : Result()                               // constraints
   $p : Person( age >= 18 )     
   $r.setValue( $p.getName() + " can drink");  // consequence

Constraints are written using a form of pattern-matching over the data (Java objects) that is inserted into the working memory. Actions are basically a block of Java code with a few Drools-specific extensions.

Historically, the DRL used to be a dynamic language that was interpreted at runtime by the Drools engine. In particular, the pattern matching syntax had a major drawback: it made extensive use of reflection unless the engine detected a constraint was “hot” enough for further optimization; that is, if it had evaluated a certain number of times; in that case the engine would compile it into bytecode on-the-fly.

About one year ago, for performance reasons, we decided to go away with runtime reflection and dynamic code generation and completed the implementation of what we called the Drools Executable Model, providing a pure Java-based representation of a rule set, together with a convenient Java DSL to programmatically define such model.

To give an idea of how this Java API looks, like let’s consider again the simple Drools rule reported above. The rule will fire if the working memory contains any Result instance and any instance of Person where the age field is greater or equal to 18. The consequence is to set the value of the Result object to a String saying that the person can drink. The equivalent rule expressed with the executable model API looks like the following (pretty-printed for readability):

var r = declarationOf(Result.class, "$r");
var p = declarationOf(Person.class, "$p");
var rule =
   rule("com.example", "R1").build(
         pattern(p).expr("e", p -> p.getAge() >= 18),
         alphaIndexedBy(int.class, GREATER_OR_EQUAL, 1, this::getAge, 18),
    on(p, r).execute(($p, $r) ->
         $r.setValue($p.getName() + " can drink")));
As you can see, this representation is more verbose and harder to understand, partly because of the Java syntax, but mostly because it explicitly contains lots of details, such as the specification of how Drools should internally index a given constraint, which was implicit in the corresponding DRL. We did this on purpose because we wanted a totally explicit rule representation that did not require any convoluted inference or reflection sorcery. However, we knew it would be crazy to ask users to be aware of all such intricate details, so we wrote a compiler to translate DRL into the equivalent Java code. We achieved this using JavaParser, a really nice open source library that allows to parse, modify, and generate any Java source code through a convenient API.

In all honesty, when we designed and implemented the executable model, we didn’t have strictly GraalVM in mind. We simply wanted an intermediate and pure Java representation of the rule that could be efficiently interpreted and executed by the engine. Yet, by completely avoiding reflection and dynamic code generation,  the executable model was key to allowing us to support native binary generation with Graal. For instance, because the new model expresses all constraints as lambda predicates, we don’t need to optimize the constraints evaluators through bytecode generation and dynamic classloading, which are totally forbidden in native image generation.

The design and implementation of executable model taught us an important lesson in the process of making Drools compatible with Graal: any limitation can be overcome with a sufficient amount of code generation. We will further discuss this in the next section.

Overcoming other Graal limitations

Having a plain Java model of a Drools rule base was a very good starting point, but more work was needed to make our project compatible with native binary generation.

The executable model makes reflection largely unnecessary; however, our engine still needs reflection for one last feature called property reactivity. Our plans are to get rid of reflection altogether, but, because the change is nontrivial, for this time we resorted to a handy feature of the binary image compiler. This feature does support a form of reflection, provided that we can declare upfront the classes we will need to reflect upon at runtime. This can be supplied by providing a JSON descriptor file to the compiler, or, if you are using Quarkus, you can just annotate the domain classes. For instance, in the rule shown above, our domain classes would be Result and Person. Then we can write:

    "name" : "org.drools.simple.project.Person",
    "allPublicMethods" : true
    "name" : "org.drools.simple.project.Result",
    "allPublicMethods" : true

Then, we can instruct the native binary compiler with the flag
We segregated other redundant reflection trickery to a dynamic module and implemented an alternative static version of the same components that users can choose to import into their project. This approach is especially useful for binary image generation, but it has benefits for regular use cases as well. In particular, avoiding reflection and dynamic loading can result in faster startup time and improved run-time.

At startup time, Drools projects read an XML descriptor called the kmodule, where the user declaratively defines the configuration of the project. Usually, we parse this XML file and load it into memory, but our current XStream-based parser uses a lot of reflection; so, first, we can load the XML with an alternative strategy that avoids reflection. However, we can go further: if we can guarantee that the in-memory representation of the XML will never change across runs, and we can afford to run a quick code-generation phase before repackaging a project for deployment, then we can avoid loading the XML at each boot-up altogether. In fact, we are now able to translate the XML file into a class file that will be loaded at startup time, like any other hand-coded class. Here’s a comparison of the XML with a snippet of the generated code (again, pretty-printed for readability). The generated code is more verbose because it makes explicit all of the configuration defaults.

<kbase name="simpleKB"
  <ksession name="simpleKS" default="true"/>
var m = KieServices.get().newKieModuleModel();
var kb = m.newKieBaseModel("simpleKB");
var ks = kb.newKieSessionModel("simpleKS");

Another issue with startup time is dynamic classpath scanning. Drools supports alternate ways to take decisions other than DRL-based rules, such as decision-tables, the Decision Model and Notation (DMN) or predictive models using the Predictive Model Markup Language (PMML). Such extensions are implemented as dynamically loadable modules, that are hooked into the core engine by scanning the classpath at boot-time. Although this is extremely flexible, it is not essential: even in this case, we can avoid runtime classpath scanning and provide static wiring of the required components either by generating code at build-time, or by providing an explicit API to end users to hook components manually. We resorted to provide a pre-built static module with a minimal core.

private Map<Class<?>, Object> serviceMap = new HashMap<>();
private void wireServices() {
  // … more services here

Note that, although here we are using Class.forName(), the compiler is smart enough to recognize the constant and substitute it with an actual constructor. Of course, it is possible to simplify this further by generating a chain of if statements.
Finally, we tied everything together by getting rid of the last few pre-executable model leftovers: the legacy Drools class-loader. This was the culprit behind the following apparently cryptic error message:

Error: unsupported features in 2 methods
Detailed message:
Unsupported method java.lang.ClassLoader.defineClass(String, byte[], int, int, ProtectionDomain)
is reachable: The declaring class of this element has been substituted, but this element is not
present in the substitution class
To diagnose the issue, you can add the option --report-unsupported-elements-at-runtime. The 
unsupported element is then reported at run time when it is accessed the first time.
       at parsing org.drools.dynamic.common.DynamicComponentsSupplier$DefaultByteArrayClassLoader.defineClass(
Call path from entry point to org.drools.dynamic.common.DynamicComponentsSupplier$DefaultByteArrayClassLoader.defineClass(String, byte[], ProtectionDomain):

Really, however, the message is pretty clear: our custom class-loader is able to dynamically define a class, which is useful when you generate bytecode at run-time. But, if the codebase relies completely on the executable model, we can avoid this altogether, so we isolated the legacy class-loader into the dynamic module.

This is the last step that was necessary to successfully generate a native image of our simple test project, and the results exceeded our expectations, thereby confirming that the time and efforts we spent in this experiment were well invested. Indeed, executing the main class of our test case with a normal JVM takes 43 milliseconds with a occupation of 73M of memory. The corresponding native image generated by Graal lasted is timed at less than 1 millisecond and uses only 21M of memory.

Integrating with Quarkus

Once we had a first version of Drools compatible with Graal native binary generation, the next natural step was to start leveraging the features provided by Quarkus and try to create a simple web service with it. We noticed that Quarkus offers a different and simpler mechanism to let the compiler know that we need reflection on a specific class. In fact, instead of having to declare this in a JSON file as before, you can annotate the class of your domain model as follows:

public class Person { … }
We also decided to go one small step forward with our code generation machinery. In particular, we added one small interface to Drools code

public interface KieRuntimeBuilder {
    KieSession newKieSession();
    KieSession newKieSession(String sessionName);
so that when the Drools compiler creates the executable model from the DRL files it also generates an implementation of this class. This implementation has the purpose of supplying a Drools session automatically configured with the rules and the parameters defined by the user.
After that, we were ready to put both dependency injection and REST support provided by Quarkus to work, and we developed a simple web service exercising the Drools runtime.

public class CanDrinkResource {

    KieRuntimeBuilder runtimeBuilder;

    public String canDrink( @PathParam("name") String name,
                            @PathParam("age") int age ) {

       KieSession ksession = runtimeBuilder.newKieSession();

       Result result = new Result();
       ksession.insert(new Person( name, age ));

       return result.toString();
The example is straightforward enough to not require any further explanation and is fully deployable as a microservice in an OpenShift cluster. Thanks to the extremely low startup time—due to the work we did on Drools and the low overhead of Quarkus—this microservice is fast enough to be deployable in a KNative cloud. You can find the full source code on GitHub.

Introducing Submarine

These days, rule engines are seldom a matter of discussion. This is because they just work. A rule engine is not necessarily antithetical to a cloud environment, but work might be needed to fit the new paradigm. This was the story of our journey. We started with courage and curiosity. In the next few months, we will push this work forward to become more than a simple prototype, to realize a complete suite of business automation tools ready for the cloud. The name of the initiative is Submarine, from the famous Dijkstra quote. So, sit tight, and get ready to dive in.

This article has been originally published on the Red Hat Developer blog here