App Timeline Server not starting Or Downgrade Ambari

This can also be used as a downgrade guide from Ambari 1.7.0 to 1.6.1.

If using HDP 2.1.2 with Ambari 1.7.0 your App Timeline Server does not start, you come to the right place. 

Symptoms: running the ATS from Ambari throws:

Fail: Execution of ‘ls /var/run/hadoop-yarn/yarn/yarn-yarn-timelineserver.pid >/dev/null 2>&1 && ps cat /var/run/hadoop-yarn/yarn/yarn-yarn-timelineserver.pid` >/dev/null 2>&1′ returned 1.

All services work fine. I have set the recommended configuration for HDP 2.1.2

yarn.timeline-service.store-class = org.apache.hadoop.yarn.server.applicationhistoryservice.timeline.LeveldbTimelineStore

The History Server is running fine. 

Not the ideal solution but it worked for me. Since this is kind of related to Ambari versions I reverted back to 1.6.1, steps:
1. Stopped and removed ambari server and all agents
2. Deleted repo and any directories for ambari
3. Downloaded and installed ambari 1.6.1
4. Re-configured/installed cluster, as HDP version remained the same
5. Formatted namenode and hbase
6. Change the config: 

yarn.timeline-service.store-class = org.apache.hadoop.yarn.server.applicationhistoryservice.timeline.LeveldbTimelineStore

7. Start ATS, failed, checked logs for historyserver, error: 

Permission denied on /hadoop/yarn/timeline/leveldb.timeline-store.ldb/LOCK

8. Deleted the leveldb-timeline-store.ldb
9. Restarted ATS, worked fine!

Usually I never got this issue for other cluster installs using HDP 2.2 and Ambari 1.7.0.