In all of the above sections, the
cascading.operation.BaseOperation class was
subclassed. This class is an implementation of the
cascading.operation.Operation interface, and
provides a few default method implementations. It is not strictly
required to extend
implementing this interface, but it is very convenient to do so.
When developing custom operations, the developer may need to
initialize and destroy a resource. For example, when doing pattern
matching, you might need to initialize a
java.util.regex.Matcher and use it in a
thread-safe way. Or you might need to open, and eventually close, a
remote connection. But for performance reasons, the operation should not
create or destroy the connection for each Tuple or every Tuple group
that passes through.
For this reason, the interface
Operation declares two methods:
cleanup(). In the case of Hadoop and MapReduce,
cleanup() methods are called once per Map or
Reduce task. The
prepare() method is called
before any argument Tuple is passed in, and the
cleanup() method is called after all Tuple
arguments have been operated on. Within each of these methods, the
developer can initialize a "context" object that can hold an open socket
Matcher instance. This context is
user defined, and is the same mechanism used by the
Aggregator operation - except that the
Aggregator is also given the opportunity to
initialize and destroy its context, via the
Note that if a "context" object is used, its type should be declared in the subclass class declaration using the Java Generics notation.
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