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The second property “serializer.class” defines what Serializer to use when preparing the message for transmission to the Broker. In our example we use a simple String encoder provided as part of Kafka. Note that the encoder must accept the same type as defined in the KeyedMessage object in the next step.

It is possible to change the Serializer for the Key (see below) of the message by defining "key.serializer.class" appropriately. By default it is set to the same value as "serializer.class".

The third property  "partitioner.class" defines what class to use to determine which Partition in the Topic the message is to be sent to. This is optional, but for any non-trivial implementation you are going to want to implement a partitioning scheme. More about the implementation of this class later. If you include a value for the key but haven't defined a partitioner.class Kafka will use the default partitioner. If the key is null, then the Producer will assign the message to a random Partition.

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