Manual Data Model Evolution Capabilities - the user documentation

1. Overview

   The automatic data model schema evolution implemented in ROOT makes it possible
to read back the serialized data object in the situation when the definition of
the classes those objects represent changed slightly (some of the data members were
removed or some new ones added). It is also possible to manually specify the rules
for more sophisticated data transformations done while reading to load the serialized
objects into data structures that changed quite significantly.

   ROOT provides two interface enabling users to specify the conversion rules. The
first way is to define a rule in the dictionary file and the second way is to insert
it to the TClass object using the C++ API.

   There are two types of conversion rules. The first of them, the normal rules, are
the ones that should be used in the most of the cases. They provide a buffered input
data and an address of the in-memory target object and allow user to specify the
conversion function mapping the data being read to the output format. The second type
of the rules, the raw rules, also provide the pointer to the target object but the
input is a raw TBuffer object containing the input data member declared as an input
to the rule. This type of a rule is provided mainly to handle the file format changes
that couldn't have been handled otherwise and in general should not be used unless there
is no other option.

2. The dictionaries

   The most convenient place to specify the conversion rules is a dictionary. One can
do that either in CINT's LinkDef file or in the selection xml file being fed to genreflex.
The syntax of the rules is the following:

   * For CINT dictionaries:

#pragma read                                              \
    sourceClass="ClassA"                                  \
    source="double m_a; double m_b; double m_c"           \
    version="[4-5,7,9,12-]"                               \
    checksum="[12345,123456]"                             \
    targetClass="ClassB"                                  \
    target="m_x"                                          \
    embed="true"                                          \
    include="iostream,cstdlib"                            \
    code="{m_x = onfile.m_a * onfile.m_b * onfile.m_c; }" \


#pragma readraw           \
      sourceClass="TAxis" \
      source="fXbins"     \
      targetClass="TAxis" \
      target="fXbins"     \
      version="[-5]"      \
      include="TAxis.h"   \
      code="\
{\
Float_t * xbins=0; \
Int_t n = buffer.ReadArray( xbins ); \
fXbins.Set( xbins ); \
}"


   * For REFLEX dictionaries:

<ioread sourceClass="ClassA"
        source="double m_a; double m_b; double m_c"
        version="[4-5,7,9,12-]"
        checksum="[12345,123456]"
        targetClass="ClassB"
        target="m_x"
        embed="true"
        include="iostream,cstdlib">
<![CDATA[
   m x = onfile.m a * onfile .m b * onfile .m c;
]] >
</ioread>

<ioreadraw sourceClass="TAxis"
           source="fXbins"
           targetClass="TAxis"
           target="fXbins"
           version="[-5]"
           include="TAxis.h">
<![CDATA[
      Float_t *xbins = 0;
      Int_t n = buffer.ReadArray( xbins ) ;
      fXbins.Set( xbins );
]] >
</ioreadraw>

   The vairables in the rules have the following meaning:

  * sourceClass - The field defines the on-disk class that is the input for the rule.
  * source      - A semicolon-separated list of values defining the source class data members
                  that need to be cached and accessible via object proxy when the rule is
                  executed. The values are either the names of the data members or the type-name
                  pairs (separated by a space). If types are specified then the ondisk structure
                  can be generated and used in the code snippet defined by the user.
  * version     - A list of versions of the source class that can be an input for this rule.
                  The list has to be enclosed in a square bracket and be a comma-separated
                  list of versions or version ranges. The version is an integer number, whereas
                  the version range is one of the following:
     – "a-b" - a and b are integers and the expression means all the numbers between
               and including a and b
     – "-a"  - a is an integer and the expression means all the version numbers smaller
               than or equal to a
     – "a-"  - a is an integer and the expression means all the version numbers greater
               than or equal to a
  * checksum    - A list of checksums of the source class that can be an input for this
                  rule. The list has to be enclosed in a square brackets and is a
                  comma-separated list of integers.
  * targetClass - The field is obligatory and defines the name of the in-memory class that
                  this rule can be applied to.
  * target      - A semicolon-separated list of target class data member names that this rule
                  is capable of calculating.
  * embed       - This property tells the system if the rule should be written in the output
                  file is some objects of this class are serialized.
  * include     - A list of header files that should be included in order to provide the func-
                  tionality used in the code snippet; the list is comma delimited.
  * code        - An user specified code snippet

   The user can assume that in the provided code snippet the following variables
will be defined:

    The user provided code snippets have to consist of valid C++ code. The system can do
some preprocessing before wrapping the code into function calls and declare some variables to
facilitate the rule definitions. The user can expect the following variables being predeclared:

   * newObj     - variable representing the target in-memory object, it’s type is that of the
                  target object
   * oldObj     - in normal conversion rules, an object of TVirtualObject class representing the
                  input data, guaranteed to hold the data members declared in the source property
                  of the rule
   * buffer     - in raw conversion rules, an object of TBuﬀer class holding the data member
                  declared in source property of the rule
   * names of the data members of the target object declared in the target property of the
                  rule declared to be the appropriate type
   * onfile.xxx - in normal conversion rules, names of the variables of basic types declared
                  in the source property of the rule

3. The C++ API

   The schema evolution C++ API consists of two classes: ROOT::TSchemaRuleSet and ROOT::TSchemaRule.
Objects of the TSchemaRule class represent the rules and their fields have exactly the same
meaning as the ones of rules specified in the dictionaries. TSchemaRuleSet objects
manage the sets of rules and ensure their consistency. There can be no conflicting
rules in the rule sets. The rule sets are owned by the TClass objects corresponding to the
target classes defined in the rules and can be accessed using TClass::{Get|Adopt}SchemaRules
