Cloudera Manager Agents
The Cloudera Manager Agent is a Cloudera Manager component that works with the Cloudera Manager Server to manage the processes that map to role instances.
In a Cloudera Manager managed cluster, you can only start or stop role
instance processes using Cloudera Manager. Cloudera Manager uses an open source process
management tool called supervisord, that starts processes, takes care of
redirecting log files, notifying of process failure, setting the effective user ID of the
calling process to the right user, and so on. Cloudera Manager supports automatically
restarting a crashed process. It will also flag a role instance with a bad health flag if its
process crashes repeatedly right after start up.
The Agent is started by init.d at
start-up. It, in turn, contacts the Cloudera Manager Server and determines
which processes should be running. The Agent is monitored as part of
Cloudera Manager's host monitoring. If the Agent stops heartbeating, the
host is marked as having bad health.
One of the Agent's main responsibilities is to start
and stop processes. When the Agent detects a new process from the Server
heartbeat, the Agent creates a directory for it in
/var/run/cloudera-scm-agent and unpacks the
configuration. It then contacts supervisord, which starts
the process.
cm_processes
To enable Cloudera Manager to run scripts in subdirectories of
/var/run/cloudera-scm-agent, (because /var/run is
mounted noexec in many Linux distributions), Cloudera Manager mounts a
tmpfs, named cm_processes, for process subdirectories.
A tmpfs defaults to a max size of 50% of physical RAM
but this space is not allocated until its used, and tmpfs is paged out
to swap if there is memory pressure.
The lifecycle actions of cmprocesses can be described
by the following statements:
- Created when the Agent starts up for the first time with a new supervisord process.
- If it already exists without
noexec, reused when the Agent is started usingstartand not recreated. - Remounted if Agent is started using
clean_restart. - Unmounting and remounting cleans out the contents (since it is mounted as a tmpfs).
- Unmounted when the host is rebooted.
- Not unmounted when the Agent is stopped.
