define viral propagation

Semantics

The define viral propagation construct registers the algorithm that must be applied whenever an operator propagates the value of a viral Attribute. The propagation can be attached either to a Represented Variable (i.e., to the Attribute itself) or to the Value Domain used by the Attribute and is automatically reused by every operator that states that viral Attributes must be propagated. When a propagation rule is declared on a Value Domain it acts as the default for all Attributes using that Value Domain, unless a more specific rule is defined directly on the Attribute’s Represented Variable. It is a semantic error to define more than one propagation rule for the same Value Domain or for the same Variable.

The input of a propagation rule is a pair of values from the viral Attribute. The propagation algorithm is a binary operation that is associative, commutative, and reflexive. When an operator involves more than two operands (e.g., an aggregate function or an N-ary arithmetic operation), the propagation rule is applied recursively to the values pair by pair. The final result is independent of the order in which the pairs are evaluated.

Two modelling families are supported:

• Enumerated rules. These rules are intended for Attributes that are based on enumerated Value Domains. They are expressed through ordered when … then clauses. Each clause describes a combination of input values and the resulting value that shall be produced. The clauses are evaluated in order, but binary clauses (specifying a pair of values) always take precedence over unary clauses (specifying a single value).

  • A binary clause (e.g., when "A" and "B" then "C") matches if the input pair consists of exactly those two values.

  • A unary clause (e.g., when "A" then "A") matches if the specified value is present in the input pair (regardless of what the other value is).

  The first matching clause determines the result. To guarantee determinism and associativity, the rule must be defined such that the result is consistent regardless of grouping.

  Optionally, an else clause can provide the default result for any combination that is not explicitly listed; when it is absent, missing combinations yield NULL. Every combination of values can be described only once. The use of the or operator is not allowed; every combination must be listed explicitly.

• Aggregate rules. These rules are meant for Attributes that use non-enumerated Value Domains (for example dates or numbers). The rule simply names the aggregate function that must collapse the set of operand values to a single result. Only a dedicated list of aggregate keywords (min, max, sum, avg) can be used so that the propagation stays deterministic and implementation-independent.

Operators do not accept propagation instructions directly: once the propagation is defined (either on the Value Domain or on the Variable), every operator that propagates viral Attributes must apply that definition (e.g., the typical behaviour rules for binary, aggregate, analytic, or join operators).

Syntax

define viral propagation propagationName ( vpSignature ) is

vpBody

end viral propagation

vpSignature ::= valuedomain valueDomain | variable variable

vpBody ::= enumeratedClause { ; enumeratedClause }* [ ; defaultClause ]

aggregationClause

vpClause ::= enumeratedClause | aggregationClause | defaultClause

enumeratedClause ::= { ruleName : } when enumerationCondition then vpResult

aggregationClause ::= aggregate aggregateFunction

defaultClause ::= else vpResult

enumerationCondition ::= enumerationTerm [ and enumerationTerm ]

enumerationTerm ::= scalarLiteral

aggregateFunction ::= min | max | sum | avg

Syntax description

propagationName	the name assigned to the propagation definition. It can be used to reference the rule in documentation or metadata repositories.
vpSignature	the signature of the propagation definition. It can target either a Value Domain or a Represented Variable. When a Value Domain is specified, every Attribute using that Value Domain inherits the propagation unless it defines its own Variable-level rule.
valueDomain	the Value Domain for which the propagation is defined. Only one propagation definition can exist for the same Value Domain.
variable	the Represented Variable owning the viral Attribute. If more than one propagation definition is declared for the same Variable, a semantic error is raised. Variable-level definitions override any Value Domain rule that might exist for the Attribute.
vpClause	a clause that participates in the definition. Clauses are evaluated in the order in which they are written. Aggregation clauses cannot be mixed with enumerated clauses: a propagation uses either a single aggregation clause or a sequence of enumerated clauses (plus an optional default clause).
enumeratedClause	describes a combination of input values (a pair or a single value). The clause is triggered when the input pair matches the condition. Binary clauses (two values) are checked before unary clauses (one value).
enumerationCondition	specifies the values to match. It can be a single value (unary) or two values separated by and (binary).
vpResult	the literal value that must be propagated when the clause is satisfied. It must belong to the same Value Domain as the viral Attribute unless NULL is explicitly desired.
defaultClause	an optional fallback clause triggered after all other enumerated clauses when none of them matches. If omitted and no enumerated clause matches, NULL is propagated.
aggregationClause	specifies that the viral Attribute uses a non-enumerated Value Domain and that the input set must be reduced by applying the indicated aggregate function. Only one aggregation clause is allowed inside a propagation definition.
aggregateFunction	the keyword naming the allowed aggregate to be applied on the input set: min, max, sum, or avg. The semantics of these aggregates follow the standard VTL definitions.

Examples

Enumerated Attribute with strict priority (the CL_CONF list with C > N > F):

define viral propagation CONF_priority (variable CONF) is
   when "C" then "C";
   when "N" then "N";
   else "F"
end viral propagation

Enumerated Attribute defined once for the Value Domain so that every Attribute using CL_OBS_STATUS inherits it. Note that all combinations are listed explicitly:

define viral propagation OBS_STATUS_default (valuedomain CL_OBS_STATUS) is
   when "M" then "M";
   when "L" then "L";
   when "F" then "F";
   when "E" then "F";
   when "Q" and "P" then "Q";
   else "A"
end viral propagation

Enumerated Attribute where the combination of input values produces a new code (computed missing status). The binary rule "C" and "M" takes precedence over the unary rule "M":

define viral propagation COMPUTED_missing (variable COMPUTED) is
   when "C" and "M" then "N";
   when "M" then "M";
   else " "
end viral propagation

Non-enumerated Attribute where the most restrictive embargo time has to prevail across operations:

define viral propagation EMBARGO_time (variable EMBARGO_TIME) is
   aggregate max
end viral propagation