cascading.flow.planner.rule

Class RuleExpression

java.lang.Object

cascading.flow.planner.rule.RuleExpression

Direct Known Subclasses:

BufferAfterEveryExpression, EveryAfterBufferExpression, LogicalMergeAnnotatorExpression, LoneGroupExpression, MissingGroupExpression, NonBlockedBlockedJoinAnnotatorExpression, NonBlockedBlockedJoinJoinAnnotatorExpression, NoOpPipeExpression, NoOpPipeMultiGraphExpression, PlannerLevelExpression, SplitBeforeEveryExpression

public class RuleExpression
extends java.lang.Object

A RuleExpression describes where a Rule is applied in an element graph. Where a Rule can be a RuleAssert, RuleTransformer, or ExpressionRulePartitioner.

To pin down how a Rule behaves and where, a RuleExpression relies on the ExpressionGraph class, where an ExpressionGraph is an actual graph of ElementExpression nodes and ScopeExpression edges.

This expression graph is analogous to a text Regular Expression. A regular expression is used to match a sub-set of text in a string. This enables efficient string replacement and parsing.

The expression graph is used to match a smaller graph inside a larger one so that the larger graph can be manipulated. As in a regular expression, elements of the captured graph can be addressed and used.

The simplest application is to use a single ExpressionGraph to match a portion of a larger graph. Once found, the calling rule can fire. Most commonly this is useful for the RuleAssert rule that looks for a given structure, and if founds throws an error pinpointing the element in the graph that violates the assertion rule.

The second application is to have one ExpressionGraph identify distinguished elements in a larger graph, and remove them. This is called graph contraction. The structure of the original graph is retained where possible. Think of this as hiding elements in the larger graph so that a second ExpressionGraph can be applied to look for a sub-graph.

If the second sub-graph is found, the calling Rule can execute. This most commonly used to partition a graph into smaller graphs. For example, any sub-graph with source Taps and a single sink Group (Pipe) is a process Node (Mapper).

The third application is similar to the second. An ExpressionGraph is used to create a contracted graph of only distinguished elements. The second ExpressionGraph finds a sub-graph in the contracted graph. This contracted sub-graph is then isolated and all hidden elements are restored within the bounds of the sub-graph.

Finally a third ExpressionGraph is used to identify a location within the new sub-graph so the Rule can execute. This is most commonly used to perform transformations within a graph. For example, to insert a temporary Tap into the full assembly element graph to force boundaries between MapReduce jobs.

Field Summary

Fields

Modifier and Type	Field and Description
protected ExpressionGraph	contractedMatchExpression
protected ExpressionGraph	contractionExpression
protected ExpressionGraph	matchExpression

Constructor Summary

Constructors

Constructor and Description
RuleExpression(ExpressionGraph matchExpression)
RuleExpression(ExpressionGraph contractionExpression, ExpressionGraph matchExpression)
RuleExpression(ExpressionGraph contractionExpression, ExpressionGraph contractedMatchExpression, ExpressionGraph matchExpression)

Method Summary

Modifier and Type	Method and Description
ExpressionGraph	getContractedMatchExpression()
ExpressionGraph	getContractionExpression()
java.lang.String	getExpressionName()
ExpressionGraph	getMatchExpression()

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

contractionExpression

protected final ExpressionGraph contractionExpression

contractedMatchExpression

protected final ExpressionGraph contractedMatchExpression

matchExpression

protected final ExpressionGraph matchExpression

Constructor Detail

RuleExpression

public RuleExpression(ExpressionGraph matchExpression)

RuleExpression

public RuleExpression(ExpressionGraph contractionExpression,
                      ExpressionGraph matchExpression)

RuleExpression

public RuleExpression(ExpressionGraph contractionExpression,
                      ExpressionGraph contractedMatchExpression,
                      ExpressionGraph matchExpression)

Method Detail

getExpressionName

public java.lang.String getExpressionName()

getContractionExpression

public ExpressionGraph getContractionExpression()

getContractedMatchExpression

public ExpressionGraph getContractedMatchExpression()

getMatchExpression

public ExpressionGraph getMatchExpression()