Database Reference
In-Depth Information
and summation operations. Instead, implementations of the
Aggregator
interface handle the task of counting events.
Aggregators come about in Trident because it is a fundamentally
batch-oriented system. An aggregator takes a batch as input and produces
an output according to the aggregation function. Although it is possible to
write custom aggregators most of the time, the built-in
Count
and
Sum
aggregations are sufficient to accomplish most tasks. The following code
implements a trivial event counter in Trident using the topology submitter
interface developed in Chapter 5:
public
StormTopology topology(String[] args) {
TridentTopology topology =
new
TridentTopology();
topology.newStream("input",
SimpleKafkaSpout.
spout
().configure(args))
.aggregate(
new
Count(),
new
Fields("count"))
.each(
new
Fields("count"),
new PostFilter
());
;
return
topology.build();
}
The
PostFilter
class is a simple class designed to interface with the
node.js delivery mechanisms developed in Chapter 7, “Delivering Streaming
Metrics.” It implements the same functionality as the
IRichBolt
counter
in the previous section, pushing its results to a
node.js
application
listening on port 3000 by default.
