Check out example codes for "Error: com.mysql.cj.jdbc.Driver not loaded. Are you sure you've included the correct jdbc driver in :jdbc_driver_library?". It will help you in understanding the concepts better.
Code Example 1
for logstash 6.2.x and above, prefer adding the drivers under: /usr/share/logstash/logstash-core/lib/jars/ and rather keep the jdbc_driver_library empty. This has been quoted in many of the elastic.co forums too.
Code Example 2
You might be using mysql-connector-java_5.1.45, and specifying "com.mysql.cj.jdbc.Driver" as the driver class. You should use either mysql-connector-java_8.0.21, or use the old driver class "com.mysql.jdbc.Driver"
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