I have used logstash in between to implement grok filter… does the same achievable without logstash?
Yes you can use extractors with grok or the processing pipelines to extract all the information out.
If i am sending large amount of data through filebeat to graylog(via logstash), it’s about 3 days but still log of a particular machine are not visible in graylog server. as per my understanding the logs are accumulated in logstash until filebeat does not find EOF or file inactive. If i want to see the logs sent till now to logstash, how can i see them?
Logstash is still one moving part more in the field that needs to be debugged. Filebeat is maintaining a registry with the information what is send over and what not. The location depends how you have install filebeat ( https://www.elastic.co/guide/en/beats/filebeat/current/migration-registry-file.html ).
if you miss data in Graylog you should follow the way of logs from the source to Graylog. Check all components if the messages pass-by and then proceed to the next. Until you hit Graylog.
You should check the Graylog logfile and if you find messages of this source when searching for this source and in all-messages. It might be that the system is in the past or the future - so checking time on all sending systems might be a good idea too.