- All Superinterfaces:
- DeclaresResults, Operation<Context>
- All Known Implementing Classes:
public interface Buffer<Context>
- extends Operation<Context>
A Buffer is similar to an
For the case where a Buffer follows a CoGroup, the method
Aggregator by the fact that it operates on unique groups of values. It differs
by the fact that an
Iterator is provided and it is the responsibility
operate(cascading.flow.FlowProcess, BufferCall) method to iterate overall all the input
arguments returned by this Iterator, if any.
will be called for every unique group whether or not there are values available to iterate over. This may be
counter-intuitive for the case of an 'inner join' where the left or right stream may have a null grouping key value.
Regardless, the current grouping value can be retrieved through
Buffer is very useful when header or footer values need to be inserted into a grouping, or if values need to be
inserted into the middle of the group values. For example, consider a stream of timestamps. A Buffer could
be used to add missing entries, or to calculate running or moving averages over a smaller "window" within the grouping.
By default, if a result is emitted from the Buffer before the argumentsIterator is started or after it is
argumentsIterator.hasNext() == false), non-grouping values are forced to null (to allow for header
and footer tuple results).
true in the
Operation.prepare(cascading.flow.FlowProcess, OperationCall) method, the last seen Tuple values will not be
nulled after completion and will be treated as the current incoming Tuple when merged with the Buffer result Tuple
via the Every outgoing selector.
There may be only one Buffer after a
CoGroup. And there
may not be any additional
Every pipes before or after the buffers Every pipe instance. A
PlannerException will be thrown if these rules are violated.
Buffer implementations should be re-entrant. There is no guarantee a Buffer instance will be executed in a
unique vm, or by a single thread. Also, note the Iterator will return the same
instance, but with new values in its child
As of Cascading 2.5, if the previous CoGroup uses a
BufferJoin as the
Joiner, a Buffer may be used to implement differing Joiner strategies.
Instead of calling
BufferCall.getArgumentsIterator() (which will return null),
BufferCall.getJoinerClosure() will return an
instance with direct access to each CoGrouped Iterator.
|Fields inherited from interface cascading.operation.Operation
void operate(FlowProcess flowProcess,
- Method operate is called once for each grouping.
BufferCall passes in an
that returns an argument
TupleEntry for each value in the grouping defined by the
argument selector on the parent Every pipe instance.
TupleEntry entry, or entry.getTuple() should not be stored directly in a collection or modified.
A copy of the tuple should be made via the
new Tuple( entry.getTuple() ) copy constructor.
This method is called for every unique group, whether or not there are values in the arguments Iterator.
flowProcess - of type FlowProcess
bufferCall - of type BufferCall
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