Package org.apache.tinkerpop.gremlin.process.traversal

Interface TraversalStrategy<S extends TraversalStrategy>

All Superinterfaces:
Comparable<Class<? extends TraversalStrategy>>, Serializable
All Known Subinterfaces:
TraversalStrategy.DecorationStrategy, TraversalStrategy.FinalizationStrategy, TraversalStrategy.OptimizationStrategy, TraversalStrategy.ProviderOptimizationStrategy, TraversalStrategy.VerificationStrategy
public interface TraversalStrategy<S extends TraversalStrategy> extends Serializable, Comparable<Class<? extends TraversalStrategy>>
A TraversalStrategy defines a particular atomic operation for mutating a Traversal prior to its evaluation. There are 5 pre-defined "traversal categories": TraversalStrategy.DecorationStrategy, TraversalStrategy.OptimizationStrategy, TraversalStrategy.ProviderOptimizationStrategy, TraversalStrategy.FinalizationStrategy, and TraversalStrategy.VerificationStrategy. Strategies within a category are sorted amongst themselves, and then category sorts are applied in the ordered specified previously. That is, decorations are applied, then optimizations, then provider optimizations, then finalizations, and finally, verifications. If a strategy does not fit within the specified categories, then it can simply implement TraversalStrategy and can have priors/posts that span categories.

A traversal strategy should be a final class as various internal operations on a strategy are based on its ability to be assigned to more general classes. A traversal strategy should typically be stateless with a public static instance() method. However, at limit, a traversal strategy can have a state-defining constructor (typically via a "builder"), but that state cannot mutate once instantiated.

Given that a traversal strategy can completely rewrite a traversal, it must take into account the GValueManager state while doing so. The GValueManager maintains a record of "pinned" and "free" variables. When a new traversal is created, all variables are considered free, as any value may be substituted for a variable without restriction. Strategies may rewrite a traversal in a manner which is valid for the current value of a GValue, but would lead to incorrect behaviour if that variable were later updated arbitrarily. An example of this is EarlyLimitStrategy, which will replace a traversal such as `limit(GValue.of("x", 5)).valueMap().range(5, 10)` with simply `discard()`, as the limit and range prevent the traversal from being productive. This optimization is not generalizable to any values of `x` however, as any value of `x` greater than 5 is expected to produce results. In cases such as these, the strategy must ensure that the variable `x` is "pinned" in the GValueManager, to indicate it is no longer free to be updated arbitrarily.

Author:
Marko A. Rodriguez (http://markorodriguez.com), Matthias Broecheler (me@matthiasb.com)

  • Method Details

    • apply

      void apply(Traversal.Admin<?,?> traversal)
      The transformation the strategy applies to the traversal.

    • applyPrior

      default Set<Class<? extends S>> applyPrior()
      The set of strategies that must be executed before this strategy is executed. If there are no ordering requirements, the default implementation returns an empty set.
      Returns:
      the set of strategies that must be executed prior to this one.

    • applyPost

      default Set<Class<? extends S>> applyPost()
      The set of strategies that must be executed after this strategy is executed. If there are no ordering requirements, the default implementation returns an empty set.
      Returns:
      the set of strategies that must be executed post this one

    • getTraversalCategory

      default Class<S> getTraversalCategory()
      The type of traversal strategy -- i.e. TraversalStrategy.DecorationStrategy, TraversalStrategy.OptimizationStrategy, TraversalStrategy.FinalizationStrategy, or TraversalStrategy.VerificationStrategy.
      Returns:
      the traversal strategy category class

    • getConfiguration

      default org.apache.commons.configuration2.Configuration getConfiguration()
      Get the configuration representation of this strategy. This is useful for converting a strategy into a serialized form.
      Returns:
      the configuration used to create this strategy

    • compareTo

      default int compareTo(Class<? extends TraversalStrategy> otherTraversalCategory)
      Specified by:
      compareTo in interface Comparable<S extends TraversalStrategy>