This class defines the semantic assertions that can be added to functions.
The assertions are realized as java annotations, to be added to the class declaration of
the class that implements the functions. For example, to declare the
ConstantFields
annotation for a map-type function that simply copies some fields, use it the following way:
\@ConstantFields({"0->0,1", "1->2"})
public class MyMapper extends FlatMapFunction, Tuple3>
{
public void flatMap(Tuple3 value, Collector> out) {
value.f2 = value.f1
value.f1 = value.f0;
out.collect(value);
}
}
All annotations takes String arrays. The Strings represent the source and destination fields.
The transition is represented by the arrow "->".
Fields are described by their tuple position (and later also the names of the fields in the objects).
The left hand side of the arrow always describes the fields in the input value(s), i.e. the value that
is passed as a parameter to the function call, or the values obtained from the input iterator. The right
hand side of the arrow describes the field in the value returned from the function. If the right hand side
is omitted, the a field is assumed to stay exactly the same, i.e. the field itself is unmodified, rather
than that the value is placed into another field.
It is very important to follow a conservative strategy when specifying constant fields.
Only fields that are always constant (regardless of value, stub call, etc.) to the output may be
declared as such! Otherwise, the correct execution of a program can not be guaranteed. So if in doubt,
do not add a field to this set.
Be aware that some annotations should only be used for functions with as single input
(RichMapFunction, RichReduceFunction) and some only for stubs with two inputs
(RichCrossFunction, RichFlatJoinFunction, RichCoGroupFunction).