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From "Dmitry Lysnichenko (JIRA)" <>
Subject [jira] [Commented] (AMBARI-4323) Add ability to an agent to clear the ActionQueue
Date Mon, 24 Feb 2014 15:51:19 GMT


Dmitry Lysnichenko commented on AMBARI-4323:

[], yes, we perform the same action as for commands that timed out:
we kill the process and all child subprocesses.

I assume when a CANCEL is issued for a task the state of the task in the server side is IN_PROGRESS.
Only, when the agent returns the command status that the server will update the state to FAILED.
CANCEL command may be issued for any task id (both IN_PROGRESS or just scheduled)

Are we also implementing canceling of IN_PROGRESS commands? Just wanted to confirm.
yes, we may cancel even in_progress commands. If command has been already successfully finished
by the moment when agent receives relevant CANCEL command, we display command as COMPLETED
(cancellation has not effect)

clears entire action queue on every registration: When expected and received heartbeat ids
are not the same the server asks agents to re-register. I do not know if in this case server
assumes that in progress commands are aborted. If server does not assumes that commands are
aborted then agent should not clear the action queue. Can you verify the behavior here?
 According to code /org/apache/ambari/server/agent/ , we clear queue
server-side queue on heartbeat lost
        //Purge action queue
        //notify action manager
But I'll recheck that when implementing the jira

we pass a stage timeout instead of task timeout as a parameter: That part of the ActionScheduler
always seems more complex than needed. Anyway, I have a question. Today stage timeout is the
largest "sum of task timeouts on a single host". One a one node, during install the last_attempt_time
is basically the same for all install tasks. So if we pass "task timeout + some small time"
while checking timeOutActionNeeded() for the later tasks - ones in the end of the queue in
Agent - will we not have a problem?
Good point. Actually, we currently are passing stage timeout value, but it is named taskTimeout.
So the fix is just to rename a variable.

> Add ability to an agent to clear the ActionQueue
> ------------------------------------------------
>                 Key: AMBARI-4323
>                 URL:
>             Project: Ambari
>          Issue Type: Improvement
>          Components: agent, controller
>    Affects Versions: 1.5.0
>            Reporter: Dmitry Lysnichenko
>            Assignee: Dmitry Lysnichenko
>             Fix For: 1.5.0
> h2. Implementation proposal:
> 1. Add a new command type CANCEL_COMMAND to agent-server protocol. CANCEL_COMMAND contains
identifier (task_id + stage_id) of an exact command for cancellation and an arbitrary text
string (reasoning for command cancelation).  So CANCEL_COMMAND looks like 
> {code}
> {
>   target_task_id: "4-3"
>   reason: "Aborted by user via API"
> }
> {code}
> 2. At the server side, commands of this type are issued automagically when tasks are
considered timed out. I'm going to do that here: org.apache.ambari.server.actionmanager.ActionScheduler#processInProgressStage.
Also we will implement (in a separate jira) an ability to cancel arbitrary order via server
API. A new method addCancelCommandAction() at org/apache/ambari/server/controller/
will become the endpoint that forms up a new CANCEL_COMMAND.
> 3. At the agent side, CANCEL_COMMANDs are executed inside right after arrival
(they are not put into ActionQueue). If command mentioned by the CANCEL_COMMAND is not present
in the ActionQueue (it is already in progress or completed) and command is not IN_PROGRESS,
CANCEL_COMMAND is silently ignored. After executing  CANCEL_COMMAND, agent starts executing
next EXECUTION_COMMAND from the ActionQueue.
> 4. Also, agent clears entire action queue on every registration (disconnected from the
server or the re-registration is requested). I'm going to add an appropriate logic to src.main.python.ambari_agent.Controller.Controller#registerAndHeartbeat.
The motivation for doing that is to make a recovery from the network/server fail more reliable
and fast (agent will have an empty ActionQueue and will be able to execute new EXECUTION_COMMANDS
and STATUS_COMMANDS immediately after registration). Currently, after re-registration agent
is locked up and continues to execute stale EXECUTION_COMMANDS.
> 5. -In both cases described above (executing a single CANCEL_COMMAND or clearing entire
ActionQueue) EXECUTION_COMMANDS are considered transactional-like. I mean that EXECUTION_COMMANDs
that are already IN_PROGRESS are never interrupted. Thus we decrease chanses of leaving system
in misconfigured/unpredictable state.- When appropriate CANCEL_COMMAND is received, EXECUTION_COMMAND
is cancelled even if it is IN_PROGRESS. 
> 6. Agent forms up command reports for cancelled commands just like it is done for COMPLETE
and FAILED commands. Command statuses for cancelled commands are set to FAILED. I did not
find enough reasoning for adding a new command report state CANCELED, feedback is welcome.
Reasoning text (why command has been cancelled) is appended to command stderr and to command
> So, cancelled command report looks like:
> {code}
> {
>   taskId: "4-3"
>   status : FAILED
>   stderr : ".... some text ... \n Command was aborted because of: Aborted by user via
>   stdout : ".... some text ... \n Command was aborted because of: Aborted by user via
>   exitcode: 999
> }
> {code}
> Also, I'm going to fix a bug at org.apache.ambari.server.actionmanager.ActionScheduler#processInProgressStage
. Here, we pass a stage timeout instead of task timeout as a parameter for org.apache.ambari.server.actionmanager.ActionScheduler#timeOutActionNeeded
. After bugfix, task timeout + some small time will be passed as a parameter value. Additional
smal time (10-30 seconds) is needed to avoid sending CANCEL_COMMAND when it is not absolutely
necessary (task timeouts at agent automatically without server actions in most cases).
> This implementation should also solve another related jira AMBARI-4324

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