In my current research, a set of log files need to be processed by Logstash before sent to ElasticSearch. After a the first run, I realized that the log format was changed once, which made my Logstash configuration fail in processing half of the log lines. I updated the filter and ran Logstash again, hoping that it would reparse the log file with the new configuration. Unfortunately, no line was parsed! This blog post tell about my experience in making Logstash to parse old files without changing files names.