Drools Grid (version 2) – #2 Drools Grid Services Intro

I’m back with another blog post about how to start using Drools Grid. I will write a set of post with a top down approach. With this I mean that I will write all the articles focused on the User Perspective of the project. That’s why I will start writing about Drools Grid Service (drools-grid-services). This project gives to the user a high level abstraction about how a grid is conformed and how we can start using it. During this blog post I will be showing how you can start using the APIs and the LocalProviders. On following post we will dive into Remote and Distributed Environments.

Starting with Drools Grid Services

First of all we need to start adding the Drools Grid Services project dependency to our project. That’s the easiest part:


Once we get this dependency, we can start using the drools-grid-services API.
The next step, as usual, is to create a simple application with it.

Your First Drools Grid project

As may know, when you want to use drools in your applications you need to be able to create a KnowledgeBuilder that will be used to compile your business assets, a KnowledgeBase that will contain all the knowledge that you want to use in your application and at the end a KnowledgeSession that will represent the runtime status of your knowledge.
Here in Drools Grid we will do exactly the same, but before we need to define the distribution topology that we want to use in our application. In other words, we need to create a high level definition of how our runtime environment will look like (machines/nodes/servers/etc).
We will start simple here, we will define our first grid topology using LocalProviders. This means that our application will run entirely in the same Java Virtual Machine that is running our application. In following post we will see that chaining this definition we can move our application to different environments.

Defining your first Grid Topology

The first that we need in order to define a Grid Topology its a GridTopologyConfiguration that will represent all the configurations that we need to define how our topology looks like.
As I mention before, I will describe here just how to use LocalProviders to describe our node distributions. So, our grid topology configuration will looks like:

GridTopologyConfiguration gtc = new GridTopologyConfiguration("MyTopology");
          new ExecutionEnvironmentConfiguration("MyLocalEnv",
                                                new LocalProvider())
          new DirectoryInstanceConfiguration("MyLocalDir",
                                             new LocalProvider())

Our topology will include two nodes: One ExecutionEnvironment that will let us host knowledge bases and knowledge sessions and a DirectoryInstance that will host information about the topology (where the ksessions are, where the kbases are, etc).

Once we configure our topology, we can build the GridTopology that will represent the running definition that we will use in our applications:

GridTopology grid = GridTopologyFactory.build(gridTopologyConfiguration);

Using a Grid Topology in our applications

Basically we have two aspects of our topology:
1) The static aspects that we configure in the GridTopologyConfiguration. In this example we are only defining two local nodes. But, you will see in following post how this configuration will contain all the necessary information to establish connections with different environments.
2) The dynamic aspects required by our application to interact with the defined environments. When we obtain the GridTopology object we have an access to this dynamic environment.

Let’s use our Grid Topology! As you can see in the configurations we will be able to access to an ExecutionEnvironment and to a DirectoryInstance. Let’s start with a simple ExecutionEnvironment test.

ExecutionNode node = grid.getExecutionEnvironment().getExecutionNode();

With the previous line we access to a running ExecutionEnvironment and we ask for a reference to an ExecutionNode. Remember that one ExecutionEnvironment can contain a set of ExecutionNodes. If we are using LocalProviders, there is no reason to have multiple ExecutionNodes in the same JVM, but if we are running in a highly distributed environment an ExecutionEnvironment could represent more than one JVM instance, more than one virtual machine or more than one physical machine.

Once we get the ExecutionNode we can start creating as usual (with minor differences) our knowledge builders, knowledge bases and knowledge sessions. Please remember, all this code will be running inside the configured providers, in this case the same JVM instance.

String rules = getStringFromRulesFiles("simplerules.drl");

KnowledgeBuilder kbuilder = node.get(KnowledgeBuilderFactoryService.class).newKnowledgeBuilder();
kbuilder.add(ResourceFactory.newByteArrayResource(rules.getBytes()), ResourceType.DRL);
if (kbuilder.hasErrors()) {
  System.out.println("Errors: " + kbuilder.getErrors());
KnowledgeBase kbase = node.get(KnowledgeBaseFactoryService.class).newKnowledgeBase();

StatefulKnowledgeSession ksession = kbase.newStatefulKnowledgeSession();

As you can see, we use the node and some internal services in order to create the knowledge builder and the knowledge base. Nothing strange here, and now you can move the place where the things will run only changing the grid topology configuration.

Once we get this session, we can use it as usual.

StatefulKnowledgeSession ksession = initializeSession(node);

ksession.insert(new Person("",27,Gender.MALE,null));



The grid.dispose() method it’s only used to clean up all the connections and all the reference to the services. If you want to continue using the grid object (GridTopology instance) you can use grid.disconnect() to only clean up the connections.

Using the DirectoryInstance node

As you can see, we can create and use knowledge sessions as usual with ExecutionEnvironments. Now let’s see how we can use the DirectoryInstances in our topology. Because we already configure the DirectoryInstance inside our GridTopology we can ask for it:

DirectoryNode directory = grid.getDirectoryInstance().getDirectoryNode();

Once again, if we are using LocalProviders we will have just one DirectoryNode per DirectoryInstance. So, we go and get it.
But, what can we do with a DirectoryNode? Basically each directory node will contain a registry (map) where we can lookup and register different resources. Right now the basic functionality that a DirectoryNode will give us is:
– register a ksession associated with a name: the directory instance will register the information where the ksession is hosted to be able to locate the session across multiple ExecutionEnvironments.
– register a kbase associated with a name: the directory instance will register where the kbase is hosted to be able to restore it if we need to create a new ksession from a different ExecutionEnvironment.
– lookup a ksession by name: it will retrieve a generic node connector to be able to get the ksession reference.
– lookup a kbase by name: it will retrieve the kbase client to be able to create new ksessions from it.

Take a look at the following code snippet that shows all the interaction:

//We can retrieve the knowledge base using the DirectoryNodeService
KnowledgeBase kbase = directory.get(DirectoryNodeService.class)
//From the directory we get a connector to the environment that contains the
//session that we are looking for
GenericNodeConnector ksessionConnector = directory.get(DirectoryNodeService.class)
//Using the connector we can establish the connection with the ExecutionNode
node = grid.getExecutionEnvironment(ksessionConnector).getExecutionNode();
//When we get the ExecutionNode we can lookup in its internal map of registered sessions.
StatefulKnowledgeSession ksession =  (StatefulKnowledgeSession) node

We can do these lookups because I have do some tricks in the session initialization:
1) If we have one or more directoryInstance node(s) running we can do:

KnowledgeBase kbase = node.get(KnowledgeBaseFactoryService.class)

and the kbase will be registered in all the running DirectoryInstances using the name “simpleValidationKbase”.

2) To register a knowledge session we can use a service that lives inside our ExecutionNodes:

                    .register("validationKsession", ksession);

This will do two important things:
a) It will register internally in the node a reference to the ksession with the name “validationKsession”
b) It will register in all the available DirectoryInstance nodes the name and a reference to the ExecutionNode that is hosting the ksession.

In a highly distributed environment you will need to know at all times where you have all the ksessions and kbases in order to create interactions between them.

In Brief

In this post we have being playing with some of the Drools Grid Services API. You can found here an example project that you can use to play with. On following posts we will be reviewing how to configure remote and distributed environments. The good news is that the APIs will be the same!


16 thoughts on “Drools Grid (version 2) – #2 Drools Grid Services Intro”

  1. If the grid is used to do a parallelized search, then it is realistic to assume some locality WRT nodes. Meaning, facts participating in the RETE branch are local to that process.

    A nested transaction could be used across caches (nodes) as epochs akin to a group-based logical assertion, where the rollback of the transaction is group-based retraction.

    In this way, logically coherent groups can explore the search space using transactions for coherency. The final commit is of course the search result or goal.

    On another note: What happened to “org.drools.runtime.pipeline.*” in 5.1 release?
    I was using that.


  2. Hi Eric, I really appreciate your questions!
    Let me try to explain the vision of Drools Grid. Inside this project we are not trying to duplicate effort for parallel and distributed programming. We are not trying to do parallelized things here. Quite the opposite. We are trying to distribute knowledge across multiple nodes in the grid and then be able to manage that distributed knowledge. We are not splitting RETE itself. Drools Grid is the first step to have autonomous distributed agents that will be able to react to different situations based on their knowledge.

    About Drools Pipiline, it was just an experimental project that was totally replaced by Apache Camel pipelines, so it was deprecated in the new versions.



  3. Okay. I am gathering that is your vision through the study of Drools Grid + Rio (mostly from these few posts and the Rio docs).

    So, the transactions will need to be lifted to participate in the distributed working memory, and probably the Rio directory elements as well, where Nested Tx ~ Directory structure.

    A first approximation could probably be done with a few WM callbacks and some support rules.

    Great work!

    So, for the pipeline, I’m supposed to be using Camel now?


  4. Hi Eric,
    Well don’t take it as it’s my vision only. I’ve start my work based on a virtualization project proposed by MarkP.
    You are welcome to contribute with the ideas that you have.
    I’m not sure to fully understand your idea about transactional behavior between different Working Memories. The idea here is that each Agent will act as a separate entity, and it will hide all the technical details from the outside world. All the transactions will be handled internally from each agent. The directory nodes right now are used as yellow pages, to query who is available. I’m not sure how tx behavior can be mapped there. Probably I’m adopting a higher level vision about how to represent a society of agents, where you can query in a not deterministic way, how is available and who is not.

    PS: yes.. camel provides a mature framework for pipelines and inside drools-camel you can find how we implement a drools endpoint for camel.

    PS: thanks a lot for the feedback I will publish another post today. Hope it make sense.


  5. Hi,

    I am not able to find the drools-grid-services file. What would be the maven repository to download the file.

    Please can you help me in this regard. This is lil urgent.



      1. Hi Salaboy,

        Thanks for your reply. I am looking for version 5.2.0 which is present in this example.



      2. It is little urgent. Please can you let me know from where I can get the dependency jar drools-grid-services file for version 5.2.0.



      1. Thanks for your inputs. Just wanted to check with you if same sample application works for 5.4.0 as well?


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