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https://issues.apache.org/jira/browse/HDFS-7204?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14169617#comment-14169617
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Allen Wittenauer edited comment on HDFS-7204 at 10/13/14 5:47 PM:
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[~benoyantony], I promise there is a method to the madness.
TL;DR: No, yes, no.
Longer:
In branch-2 and previous, "daemons" were handled via wrapping "standard"
command lines. If we concentrate on the functionality (vs. the code rot...)
this has some interesting (and inconsistent) results, especially around logging
and pid files. If you run the *-daemon version, you got a pid file and
hadoop.root.logger is set to be INFO,(something). When a daemon is run in
non-daemon mode (e.g., straight up: 'hdfs namenode'), no pid file is generated
and hadoop.root.logger is kept as INFO,console. With no pid file generated, it
is possible to run, e.g. hdfs namenode, both in *-daemon.sh mode and in
straight up mode again. It also means that one needs to pull apart the process
list to safely determine the status of the daemon since pid files aren't always
created. This made building custom init scripts fraught with danger. This
inconsistency has been a point of frustration for many operations teams.
In branch-3/post-HADOOP-9902, there is a slight change in the above
functionality and one of the key reasons why this is an incompatible change.
Sub-commands that were intended to run as daemons (either fully, e.g., namenode
or partially, e.g. balancer) have all of this handling consolidated, helping to
eliminate code rot as well as providing a consistent user experience across
projects. daemon=true, which is a per-script local, but is consistent across
the hadoop sub-projects, tells the latter parts of the shell code that this
sub-command needs to have some extra-handling enabled beyond the normal
commands. In particular, daemon=true's will always get pid and out files. They
will prevent two being run on the same machine by the same user simultaneously
(see footnote 1, however). They get some extra options on the java command
line. Etc, etc.
So where does \-\-daemon come in? The value of that is stored in a global
called HADOOP_DAEMON_MODE. If the user doesn't set it specifically, it defaults
to 'default'. This was done to allow the code to mostly replicate the behavior
of branch-2 and previous when the *-daemon.sh code was NOT used. In other
words, \-\-daemon default (or no value provided), let's commands like hdfs
namenode still run in the foreground, just now with pid and out files.
\-\-daemon start does the disown (previously a nohup), change the logging
output from HADOOP_ROOT_LOGGER to HADOOP_DAEMON_ROOT_LOGGER, add some extra
command line options, etc, etc similar to the *-daemon.sh commands.
What happens if daemon mode is set for all commands? The big thing is the pid
and out file creation and the checks around it. A user would only ever be able
to execute one 'hadoop fs' command at a time because of the pid file! Less
than ideal. :)
To summarize, daemon=true tells the code that --daemon actually means something
to the sub-command. Otherwise, --daemon is ignored.
1-... unless HADOOP_IDENT_STRING is modified appropriately. This means that in
branch-3, it is now possible to run two secure datanodes on the same machine as
the same user, since all of the logs, pids, and outs, take that into
consideration! QA folks should be very happy. :)
was (Author: aw):
[~benoyantony], I promise there is a method to the madness.
TL;DR: No, yes, no.
Longer:
In branch-2 and previous, "daemons" were handled via wrapping "standard"
command lines. If we concentrate on the functionality (vs. the code rot...)
this has some interesting (and inconsistent) results, especially around logging
and pid files. If you run the *-daemon version, you got a pid file and
hadoop.root.logger is set to be INFO,(something). When a daemon is run in
non-daemon mode (e.g., straight up: 'hdfs namenode'), no pid file is generated
and hadoop.root.logger is kept as INFO,console. With no pid file generated, it
is possible to run, e.g. hdfs namenode, both in *-daemon.sh mode and in
straight up mode again. It also means that one needs to pull apart the process
list to determine safely determine the status of the daemon since pid files
aren't always created. This made building custom init scripts fraught with
danger. This inconsistency has been a point of frustration for many operations
teams.
In branch-3/post-HADOOP-9902, there is a slight change in the above
functionality and one of the key reasons why this is an incompatible change.
Sub-commands that were intended to run as daemons (either fully, e.g., namenode
or partially, e.g. balancer) have all of this handling consolidated, helping to
eliminate code rot as well as providing a consistent user experience across
projects. daemon=true, which is a per-script local, but is consistent across
the hadoop sub-projects, tells the latter parts of the shell code that this
sub-command needs to have some extra-handling enabled beyond the normal
commands. In particular, daemon=true's will always get pid and out files. They
will prevent two being run on the same machine by the same user simultaneously
(see footnote 1, however). They get some extra options on the java command
line. Etc, etc.
So where does \-\-daemon come in? The value of that is stored in a global
called HADOOP_DAEMON_MODE. If the user doesn't set it specifically, it defaults
to 'default'. This was done to allow the code to mostly replicate the behavior
of branch-2 and previous when the *-daemon.sh code was NOT used. In other
words, \-\-daemon default (or no value provided), let's commands like hdfs
namenode still run in the foreground, just now with pid and out files.
\-\-daemon start does the disown (previously a nohup), change the logging
output from HADOOP_ROOT_LOGGER to HADOOP_DAEMON_ROOT_LOGGER, add some extra
command line options, etc, etc similar to the *-daemon.sh commands.
What happens if daemon mode is set for all commands? The big thing is the pid
and out file creation and the checks around it. A user would only ever be able
to execute one 'hadoop fs' command at a time because of the pid file! Less
than ideal. :)
To summarize, daemon=true tells the code that --daemon actually means something
to the sub-command. Otherwise, --daemon is ignored.
1-... unless HADOOP_IDENT_STRING is modified appropriately. This means that in
branch-3, it is now possible to run two secure datanodes on the same machine as
the same user, since all of the logs, pids, and outs, take that into
consideration! QA folks should be very happy. :)
> balancer doesn't run as a daemon
> --------------------------------
>
> Key: HDFS-7204
> URL: https://issues.apache.org/jira/browse/HDFS-7204
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: scripts
> Affects Versions: 3.0.0
> Reporter: Allen Wittenauer
> Assignee: Allen Wittenauer
> Priority: Blocker
> Labels: newbie
> Attachments: HDFS-7204-01.patch, HDFS-7204.patch
>
>
> From HDFS-7184, minor issues with balancer:
> * daemon isn't set to true in hdfs to enable daemonization
> * start-balancer script has usage instead of hadoop_usage
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