YARN-2052, we changed containerId format: upper 10 bits are for epoch, lower 22 bits are for sequence number of Ids. This is for preserving semantics of ContainerId#getId(), ContainerId#toString(), ContainerId#compareTo(), ContainerId#equals, and ConverterUtils#toContainerId. One concern is epoch can overflow after RM restarts 1024 times.
To avoid the problem, its better to make containerId long. We need to define the new format of container Id with preserving backward compatibility on this JIRA.