Advantages of using redis

Look at some documentation when building the ELK.
Sometimes install Redis in front of Logstash.

Even though Logstash provides a buffer, is there any advantage to installing Logstash in front?

If Redis is present but not, but Elasticsearch does not receive logs, the logs are left in a queue or buffer.

You can manually enable persistent queues in logstash if you want logstash to hang on to messages before they are processed or if you think that there could be a communication problem with the output of logstash.
Read this article for reference: