Kafka Streams and Logstash are two completely different things
Kafka Streams is a client library that you can use to write an application to stream and process data stored in Kafka Brokers, you need to write your own application in Java.
Logstash is an ETL tool that you can use to extract/receive data from multiple sources, process this data using a wide range of filters and send it to different outputs, like elasticsearch, file, s3, kafka and many others.
It is very common to use Logstash and Kafka together, which Kafka working as a message queue for the messages that logstash will consume and process, you have shippers like Filebeat sending data to Kafka Brokers and then you use Logstash to consume this data.
You can build your own applications in Java using the Kafka Streams library to collect, process and ship the data to Elasticsearch, but this will be very complex in comparison with using the tools of the stack, Filebeat to collect logs, Logstash to receive/process, Elasticsearch to store.
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