Github user Parth-Brahmbhatt commented on a diff in the pull request:
https://github.com/apache/incubator-storm/pull/195#discussion_r15545039
--- Diff: external/storm-kafka/README.md ---
@@ -44,6 +44,105 @@ use Kafka 0.8.1.1 built against Scala 2.10, you would
use the following dependen
Note that the ZooKeeper and log4j dependencies are excluded to prevent
version conflicts with Storm's dependencies.
+##Writing to Kafka as part of your topology
+You can create an instance of storm.kafka.bolt.KafkaBolt and attach it as
a component to your topology or if you
+are using trident you can use storm.kafka.trident.TridentState,
storm.kafka.trident.TridentStateFactory and
+storm.kafka.trident.TridentKafkaUpdater.
+
+You need to provide implementation of following 2 interfaces
+
+###TupleToKafkaMapper and TridentTupleToKafkaMapper
+These interfaces have 2 methods defined:
+
+```java
+ K getKeyFromTuple(Tuple/TridentTuple tuple);
+ V getMessageFromTuple(Tuple/TridentTuple tuple);
+```
+
+as the name suggests these methods are called to map a tuple to kafka key
and kafka message. If you just want one field
+as key and one field as value then you can use the provided
FieldNameBasedTupleToKafkaKeyAndMessageMapper.java
+implementation. In the KafkaBolt the implementation always looks for a
field with field name "key" and "message" for
+backward compatibility reasons.
+In the TridentKafkaState you can specify what is the field name for key
and message. These should be specified while
+constructing and instance of FieldNameBasedTupleToKafkaKeyAndMessageMapper.
+
+###KafkaTopicSelector and trident KafkaTopicSelector
+This interface has only one method
+```java
+public interface KafkaTopicSelector {
+ List<String> getTopics(Tuple/TridentTuple tuple);
+}
+```
+The implementation of this interface should return topics to which the
tuple's key/message mapping needs to be published
+You can return an empty list but null is not acceptable. If you have one
static topic name then you can use
+DefaultTopicSelector.java and set the name of the topic in the constructor.
+
--- End diff --
fixed.
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