Thursday, February 20, 2020

PMML revisited

Hi folks! The beginning of this year brings with it the initiative to re-design the Drools PMML module.
In this post I will describe how we are going to approach it, what's the current status, ideas for future development, etc. etc so... stay tuned!


PMML is a standard whose aim is to "provide a way for analytic applications to describe and exchange predictive models produced by data mining and machine learning algorithms." PMML standard defines a series of models that are managed, and we will refer to them as "Model".
The maybe-not so obvious consequence of this is that, said differently, PMML may be thought as an orchestrator of different predictive models, each of which with different requirements.
Drools has its own PMML implementation. The original design of it was 100% drools-engine based, but in the long term this proved to be not so satisfactory for all the models, so a decision has taken to implement a new version with a different approach. And here the current story begin...


To the bare-bone essence, what a PMML implementation should allow is to:
  1. load a PMML file (xml format)
  2. submit input data to it
  3. returns predicted values
Sounds simple, doesn't it? 


The proposed architecture aims at fulfilling the requirements in a modular way, following “Clean Architecture” principles.
To achieve that, components are defined with clear boundaries and visibility.
General idea is that there are specific tasks strictly related to the core functionality that should be kept agnostic by other “outer” features.
Whoever wanting to deep delve in the matter may read the book "Clean Architecture" by R. C. Martin, but in the essence it is just a matter to apply good-ol' design principles to the overall architecture.
With this target clearly defined, the steps required to achieve it are:
  1. identify the core-logic and the implementation details (model-specific)
  2. implement the core-logic inside "independent" modules
  3. write code for the model-specific modules
We choose to implement a plugin pattern  to bind the core-logic to the model-specific implementations mostly for two reasons:
  1. incremental development and overall code-management: the core module itself does not depend on any of the model-specific implementations, so the latter may be provided/updated/replaced incrementally without any impact on the core
  2. possibility to replace the provided implementation with a custom one
  3. we also foresee the possibility to choose an implementation at runtime, depending on the original PMML structure (e.g. it may make sense to use a different implementation depending on the size of the given PMML)
(I cheated: those are three) 



  1. This is the definition of Kie-representation of the original PMML model.
  2. For every actual model there is a specific implementation, and it may be any kind of object (java map, drools rule, etc).
Could we avoid it? Maybe. We could use the model directly generated by the specification' xsd. But this has been designed to describe all the predictive models, while any of them may use it in different way and with different convention; so this internal view will represent exactly what is needed for each specific model.


We identified the following main functional components:
  1. Compiler
  2. Assembler
  3. Executor


This component read the original PMML file and traslate it to our internal format.
The core-side of it simply unmarshall the xml data into Java object. Then, it uses java SPI to retrieve the model-compiler specific for the given PMML model (if it does not find one, the PMML is simply ignored).
Last, the retrieved model-compiler will “translate” the original PMML model to our model-specific representation (KiePMMLModels).
The core-side part of this component has no direct dependence on any specific Model Compiler implementation and not even with anything drools/kie related - so basically it is a lightweight/standalone library.
This component may be invoked at runtime (i.e. during the execution of the customer project), if its execution is not time-consuming, or during the compilation of the kjar (e.g. for drools-implemented models).


This component stores KiePMMLModels created by the Compiler inside KIE knowledge base. None of the other components should have any dependency/knowledge of this one.
In turns, it must not have any dependency/knowledge/reference on actual Model Compiler implementations.


This component is responsible for actual execution of PMML models. It receives the PMML input data, retrieves the KiePMMLModel specific for the input data and calculates the output.
For each model there will be a specific “executor”, to allow different kinds of execution implementation (drools, external library, etc) depending on the model type.
The core-side of it simply receives the input data and retrieve the model-executor specific for the given PMML model (if it does not find one, the PMML is simply ignored).
Last, the retrieved model-executor will evaluate the prediction based on the input data.
The core-side part of this component has no direct dependence on any specific Model Executor implementation, but of course is strictly dependent on the drool runtime.

Overall Architecture

Model implementations

Drools-based models

Some models will delegate to the drools-engine to allow best performance under heavy load. Here are some details about general scheme for such implementations.
  1. the compiler is invoked at kjar generation (or during runtime for hot-loading of PMML file)
  2. the compiler reads the PMML file and transform it to "descr" object (see BaseDescrDescrFactoryDescrBuilderTest)
  3. regardless of how the model-compiler is invoked, the drools compiler must be invoked soon after it to have java-class generated based on the descr object
  4. the assembler put the generated classes in the kie base
  5. the executor loads the "drools-model" generated and invoke it with the input parameters

DRL details

  • for each field in the DataDictionary, a specific DataType has to be defined
  • for each branch/leaf of the tree, a full-path rule has to be generated (i.e. a rule with the path to get to it - e.g. "sunny", "sunny_temperature", "sunny_temperature_humidity")
  • a "status-holder" object is created and contains the value of the rule fired - changing that value will fire the children branch/leaf rules matching it (e.g. the rule "sunny" will fire "sunny_temperature" that - in turns - will fire "sunny_temperature_humidity")
  • such "status-holder" may contain informations/partial result of evaluation, to be eventually used where combination of results is needed
  • missing value strategy may be implemented inside the status holder or as exploded rules


For each model there will be a set of standard unit tests to mostly verify  individual units of code. Beside that, inside the model-specific module (yes, it is a tongue twister) there will be an integration-test submodule. This latter will verify the overall correct execution of different, more or less complex, PMML files, to simulate as much as possible what may happen in real-world scenarios.


Regression model is the first one to have been implemented. Due to its inherent simplicity, we choose to provide a pure java-based implementation for it. For the moment being it is still under PR, and new full tests are being added.


After evaluating all the pros/cons, we decided that this model could be a good candidate to be implemented with a drools-based approach. Being also a simple model to follow, we choose to use it as first test for drools approach.


This is a list of missing features that are not implemented, yet, and not strictly-related to a specific model. It will be (well, it should be) updated during the development:

Needless to say that any comment (especially nice ones) and suggestion will be greatly appreciated.

Come back in the following days and see what's next! 



  1. How the Drools PMML impl relates to the jPMML impl? I see the actual Drools project dist. has some integration or dependency with jPPML. Its a bit confusing tome if Drools depends or not on jPMML.

    1. Hi,
      there are two jPMML "libraries": the "parser" (pmml-model) and the "evaluator" (pmml-evaluator).
      Drools depends on the pmml-model, but by itself it does not depends on pmml-evaluator.
      pmml-evalutor is used only (but it is not mandatory) for DMN-PMML integration, and it is "embedded" inside kie-dmn-jpmml.
      If the final user add kie-dmn-jpmml (and so pmml-evaluator) to the classpath (dependencies) of his/her project, DMN will use pmml-evaluator for evaluation, otherwise it will use the "Drools" PMML implementation.
      I hope this answer to your question.