Github user srdo commented on a diff in the pull request:
https://github.com/apache/storm/pull/1808#discussion_r91347833
--- Diff: docs/storm-kafka-client.md ---
@@ -1,90 +1,222 @@
-#Storm Kafka Spout with New Kafka Consumer API
+#Storm Kafka integration using the kafka-client jar
-Apache Storm Spout implementation to consume data from Apache Kafka
versions 0.10 onwards (please see [Apache Kafka Version Compatibility]
(#compatibility)).
+##Compatability
-The Apache Storm Spout allows clients to consume data from Kafka starting
at offsets as defined by the offset strategy specified in
`FirstPollOffsetStrategy`.
-In case of failure, the Kafka Spout will re-start consuming messages from
the offset that matches the chosen `FirstPollOffsetStrategy`.
+Apache Kafka versions 0.10 onwards (please see [Apache Kafka Version
Compatibility] (#compatibility)).
-The Kafka Spout implementation allows you to specify the stream
(`KafkaSpoutStream`) associated with each topic or topic wildcard.
`KafkaSpoutStream` represents the stream and output fields. For named topics
use `KafkaSpoutStreamsNamedTopics`, and for topic wildcards use
`KafkaSpoutStreamsWildcardTopics`.
+##Writing to Kafka as part of your topology
+You can create an instance of org.apache.storm.kafka.bolt.KafkaBolt and
attach it as a component to your topology or if you
+are using trident you can use org.apache.storm.kafka.trident.TridentState,
org.apache.storm.kafka.trident.TridentStateFactory and
+org.apache.storm.kafka.trident.TridentKafkaUpdater.
-The `KafkaSpoutTuplesBuilder` wraps all the logic that builds `Tuple`s
from `ConsumerRecord`s. The logic is provided by the user through implementing
the appropriate number of `KafkaSpoutTupleBuilder` instances. For named topics
use `KafkaSpoutTuplesBuilderNamedTopics`, and for topic wildcard use
`KafkaSpoutTuplesBuilderWildcardTopics`.
+You need to provide implementations for the following 2 interfaces
-Multiple topics and topic wildcards can use the same
`KafkaSpoutTupleBuilder` implementation, as long as the logic to build `Tuple`s
from `ConsumerRecord`s is identical.
+###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 a Kafka
key and a Kafka message. If you just want one field
+as key and one field as value, then you can use the provided
FieldNameBasedTupleToKafkaMapper.java
+implementation. In the KafkaBolt, the implementation always looks for a
field with field name "key" and "message" if you
+use the default constructor to construct FieldNameBasedTupleToKafkaMapper
for backward compatibility
+reasons. Alternatively you could also specify a different key and message
field by using the non default constructor.
+In the TridentKafkaState you must specify what is the field name for key
and message as there is no default constructor.
+These should be specified while constructing and instance of
FieldNameBasedTupleToKafkaMapper.
+
+###KafkaTopicSelector and trident KafkaTopicSelector
+This interface has only one method
+```java
+public interface KafkaTopicSelector {
+ String getTopics(Tuple/TridentTuple tuple);
+}
+```
+The implementation of this interface should return the topic to which the
tuple's key/message mapping needs to be published
+You can return a null and the message will be ignored. If you have one
static topic name then you can use
+DefaultTopicSelector.java and set the name of the topic in the constructor.
+`FieldNameTopicSelector` and `FieldIndexTopicSelector` use to support
decided which topic should to push message from tuple.
--- End diff --
Phrasing is a little awkward here. Maybe something like "... can be used to
decide which topic a message should go to based on a field in the tuple"
instead?
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