Parameterizing a factory

Estimated time: 5 minutes

Overview

Questions

• How do I give a factory configurable parameters?
• How do I override those parameters from a generator or the command line?
• How do I access services such as geometry from a factory?

Objectives

• Learn how to set parameters on a factory.
• Learn how to override factories via a generator.
• Learn how to override factories via the command line.
• Learn how to access services from a factory.

Setting parameters on a factory

---

Parameters are also handled using registered members. JOmniFactory provides a Parameter class template which can hold its own value, but in EICrecon we prefer to use Config structs. Thus JOmniFactory provides ParameterRef, which stores a reference into the Config object.

CPP

    ParameterRef<double> m_samplingFraction {this, "samplingFraction", config().sampFrac};
    ParameterRef<std::string> m_energyWeight {this, "energyWeight", config().energyWeight};

Parameters are fetched immediately before Configure() is called, so you may access them from any of the callbacks like so:

CPP

    void Process(int32_t run_number, uint64_t event_number) {
        logger()->debug( "Event {}: samplingFraction = {}", event_number, m_samplingFraction() );
    }

Because we are using ParameterRefs, we can also access the field the ref points to directly:

CPP

    void Process(int32_t run_number, uint64_t event_number) {
        logger()->debug( "Event {}: samplingFraction = {}", event_number, config().sampFrac );
    }

Config objects

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We create a plain-old struct to hold our parameters. For now this config struct can live in the same header file as our factory, although eventually it should belong with the algorithm instead.

CPP

struct ReconstructedElectrons_config {
    double threshold = 0.9;
    int bucket_count = 4;
    // ...
}

By passing it in to the JOmniFactory base class, we can make it automatically available via the config() method.

CPP

class ReconstructedElectrons_factory : public JOmniFactory<ReconstructedElectrons_factory, ElectronReconstructionConfig> {
    ...
}

Overriding parameters via a generator

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If you use a Config object for your parameters, you can pass it in directly to the factory generator:

CPP

    app->Add(new JOmniFactoryGeneratorT<BasicTestAlg>(
        "FunTest", {"MyHits"}, {"MyClusters"}, 
        {
          .threshold = 6.1,
          .bucket_count = 22
        },
        app));

Overriding parameters via the command line

---

We can override parameters on the command line like so:

BASH

eicrecon -PFunTest:threshold=12.0 in.root

Accessing services from a factory

---

Services are singletons that provide access to resources such as loggers, geometry, magnetic field maps, etc. Services need to be thread-safe because they are shared by different threads. The most relevant service right now is DD4hep_service. You obtain a service using a registered member like this:

CPP

    Service<DD4hep_service> m_geoSvc {this};

Oftentimes we want to retrieve a resource from a Service and refresh it whenever the run number changes. OmniFactory provides Resource for this purpose.

Challenge

Exercise

• Give your factory a Config struct.
• Give your Config struct some parameters.
• Experiment with overriding parameter values in the generator and on the command line.

Show me the solution

Add a struct with a couple of fields (for example a double threshold) and pass it as the second template argument to JOmniFactory. Register ParameterRef members pointing at those fields. You can then set them in the generator with brace-initialization ({.threshold = 6.1}) or on the command line with -P<prefix>:threshold=12.0; the command-line value takes precedence.

Key Points

• Parameters are registered members; in EICrecon we back them with a Config struct via ParameterRef.
• Pass the Config type as the second JOmniFactory template argument to make it available through config().
• Override parameters in the generator with brace-initialization, or on the command line with -P<prefix>:<name>=<value>.
• Access shared resources (geometry, fields) through thread-safe Service members.