datamodel_syntax.md

Data Models and Data Model Definitions

To describe a data model PODIO provides a data model definition syntax. This is to ease the creation of optimised data formats, which may take advantage of a so-called struct-of-array data layout. From the user-provided data model description PODIO creates all the files necessary to use this data model.

Basic Concepts and Supported Features

PODIO encourages the usage of composition, rather than inheritance. One of the reasons for doing so is the focus on efficiency friendly plain-old-data. For this reason, PODIO does not support inheritance within the defined data model. Instead, users can combine multiple components to build a to be used datatype. Additionally, if several datatypes should be callable through the same interface, the interface category can be used.

To allow the datatypes to be real PODs, the data stored within the data model are constrained to be POD-compatible data. Those are

1. basic types like int, double, etc as well as a limited set of fixed width integers from <cstdint> (the _leastN or _fastN types as well as all 8 bit types are not allowed).
2. components built up from basic types or other components
3. Fixed sized arrays of those types

In addition, PODIO supports the storage of one-to-one and one-to-many relations between objects. In the following the syntax for defining a given data model is explained. A later section contains more details about the code being created from this.

Mandatory fields

A datamodel definition has only one mandatory field: schema_version. This defines the current version of the datamodel schema and allows to do schema evolution. All other fields described below are in principle optional.

Definition of custom components

A component is just a flat struct containing data. it can be defined via:

    components:
      # My example component
      MyComponent:
        Members:
          - float x
          - float y
          - float z
          - AnotherComponent a

Definition of custom data classes

Here an excerpt from "datamodel.yaml" for a simple class, just containing one member of the type int.

    datatypes :
      EventInfo :
        Description : "My first data type"
        Author : "It's me"
        Members :
        - int Number // event number

Using this definition, three classes will be created: EventInfo, EventInfoData and EventInfoCollection. These have the following signature:

    class EventInfoData {
      public:
        int Number;
    }

    class EventInfo {
      public:
      ...
        int Number() const;
        void Number(int);
      ...
    }

Defining members

The definition of a member is done in the Members section in the form:

    Members:
      <type> <name> // <comment>

where type can be any builtin-type or a component.

It is also possible to specify default values for members via

    Members:
      <type> <name>{<default-value>} // <comment>

Note that in this case it is extremely expensive to check whether the provided default-value results in valid c++. Hence, there is only a very basic syntax check, but no actual type check, and wrong default values will be caught only when trying to compile the datamodel.

For describing physics quantities it is important to know their units. Thus it is possible to add the units to the member definition:

    Members:
      <type> <name>{<default-value>} [<unit>] // <comment>

Definition of references between objects:

There can be one-to-one-relations and one-to-many relations being stored in a particular class. This happens either in the OneToOneRelations or OneToManyRelations section of the data definition. The definition has again the form:

    OneToOneRelations:
      <type> <name> // <comment>
    OneToManyRelations:
      <type> <name> // <comment>

Explicit definition of methods

In a few cases, it makes sense to add some more functionality to the created classes. Thus this library provides two ways of defining additional methods and code. Either by defining them inline or in external files. Extra code has to be provided separately for immutable and mutable additions. Note that the immutable (ExtraCode) will also be placed into the mutable classes, so that there is no need for duplicating the code. Only if some additions should only be available for the mutable classes it is necessary to fill the MutableExtraCode section. The includes will be add to the header files of the generated classes.

    ExtraCode:
      includes: <newline separated list of strings (optional)>
      declaration: <string>
      implementation : <string>
      declarationFile: <string>
      implementationFile: <string>
    MutableExtraCode:
      includes: <newline separated list of strings (optional)>
      declaration: <string>
      implementation : <string>
      declarationFile: <string>
      implementationFile: <string>

The code being provided has to use the macro {name} in place of the concrete name of the class. The paths to the files given in declarationFile and implementationFile should be either absolute or relative to the datamodel yaml file. The cmake users are encouraged to specify these file via the DEPENDS option of PODIO_GENERATE_DATAMODEL to add the files as configuration dependency of the datamodel and cause the datamodel re-generation when they change.

Definition of custom interfaces

An interface type can be defined as follows (again using an example from the example datamodel)

    interfaces:
      TypeWithEnergy:
        Description: "A generic interface for types with an energy member"
        Author: "Someone"
        Types:
          - ExampleHit
          - ExampleMC
          - ExampleCluster
        Members:
          - double energy // the energy

This definition will yield a class called TypeWithEnergy that has one public method to get the energy. Here the syntax for defining more accessors is exactly the same as it is for the datatypes. Additionally, it is necessary to define which Types can be used with this interface class, in this case the ExampleHit, ExampleMC or an ExampleCluster. NOTE: interface types do not allow for mutable access to their data. They can be used in relations between objects, just like normal datatypes.

Assigning to interface types

Interface types support the same functionality as normal (immutable) datatypes. The main additional functionality they offer is the possibility to directly assign to them (if they type is supported) and to query them for the type that is currently being held internally.

auto energyType = TypeWithEnergy{}; // An empty interface is possible but useless
bool valid = energyType.isValid();  // <-- false

auto hit = ExampleHit{};
energyType = hit;     // assigning a hit to the interface type
energyType.energy();  // <-- get the energy from the underlying hit

auto cluster = ExampleCluster{};
energyType = cluster; // ra-assigning another object is possible
bool same = (energyType == cluster); // <-- true (comparisons work as expected)

bool isCluster = energyType.isA<ExampleCluster>(); // <-- true
bool isHit     = energyType.isA<ExampleHit>();     // <-- false

auto newCluster = energyType.as<ExampleCluster>(); // <-- "cast back" to original type

// "Casting" only works if the types match. Otherwise there will be an exception
auto newHit = energyType.as<ExampleHit>(); // <-- exception

Global options

Some customization of the generated code is possible through flags. These flags are listed in the section options:

    options:
      getSyntax: False
      exposePODMembers: True
    components:
      # My simple component
      ExampleComponent:
        Members:
          - int x
    datatypes:
      ExampleType:
        Description: "My datatype with a component member"
        Author: "Mr me"
        Members:
         - ExampleComponent comp // component from above

• getSyntax: steers the naming of get and set methods. If set to true, methods are prefixed with get and set following the capitalized member name, otherwise the member name is used for both.
• exposePODMembers: whether get and set methods are also generated for members of a member-component. In the example corresponding methods would be generated to directly set / get x through ExampleType.

Extending a datamodel / using types from an upstream datamodel

It is possible to extend another datamodel with your own types, resp. use some datatypes or components from an upstream datamodel in your own datamodel. This can be useful for prototyping new datatypes or for accommodating special requirements without having to reimplement / copy a complete datamodel.

To pass an upstream datamodel to the class generator use the --upstream-edm option that takes the package name as well as the yaml definition file of the upstream datamodel separated by a colon (':'). This will effectively make all components and datatypes of the upstream datamodel available to the current definition for validation and generation of the necessary includes. Nevertheless, only the code for the datatypes and components defined in the current yaml file will be generated. The podio PODIO_GENERATE_DATAMODEL cmake macro has gained an additional parameter UPSTREAM_EDM to pass the arguments to the generator via the cmake macros.

Potential pitfalls

• The cmake macros do not handle linking against an upstream datamodel automatically. It is the users responsibility to explicitly link against the upstream datamodel.
• When generating two datamodels side-by-side and into the same output directory and using the ROOT backend, the generated selection.xml file might be overwritten if both datamodels are generated into the same output directory.

Limiting this functionality to one upstream datamodel is a conscious choice to limit the potential for abuse of this feature.