Databases Reference
In-Depth Information
CHAPTER 5
Bolts
As you have seen, bolts are key components in a Storm cluster. In this chapter, you'll
look at a bolt's life cycle, some strategies for bolt design, and some examples of how
to implement them.
Bolt Lifecycle
A bolt is a component that takes tuples as input and produces tuples as output. When
writing a bolt, you will usually implement the
IRichBolt
interface. Bolts are created on
the client machine, serialized into the topology, and submitted to the master machine
of the cluster. The cluster launches workers that deserialize the bolt, call
prepare
on it,
and then start processing tuples.
To customize a bolt, you should set parameters in its constructor and
save them as instance variables so they will be serialized when submit-
ting the bolt to the cluster.
Bolt Structure
Bolts have the following methods:
declareOutputFields(OutputFieldsDeclarer declarer)
Declare the output schema for this bolt
prepare(java.util.Map stormConf, TopologyContext context, OutputCollector col
lector)
Called just before the bolt starts processing tuples
execute(Tuple input)
Process a single tuple of input
cleanup()
Called when a bolt is going to shut down
