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> On 5 May 2015, at 1:19 pm, Zhou Zheng Sheng / 周征晟 <zhengsheng at awcloud.com> wrote:
> 
> Thank you Andrew.
> 
> on 2015/05/05 08:03, Andrew Beekhof wrote:
>>> On 28 Apr 2015, at 11:15 pm, Bogdan Dobrelya <bdobrelia at mirantis.com> wrote:
>>> 
>>>> Hello,
>>> Hello, Zhou
>>> 
>>>> I using Fuel 6.0.1 and find that RabbitMQ recover time is long after
>>>> power failure. I have a running HA environment, then I reset power of
>>>> all the machines at the same time. I observe that after reboot it
>>>> usually takes 10 minutes for RabittMQ cluster to appear running
>>>> master-slave mode in pacemaker. If I power off all the 3 controllers and
>>>> only start 2 of them, the downtime sometimes can be as long as 20 minutes.
>>> Yes, this is a known issue [0]. Note, there were many bugfixes, like
>>> [1],[2],[3], merged for MQ OCF script, so you may want to try to
>>> backport them as well by the following guide [4]
>>> 
>>> [0] https://bugs.launchpad.net/fuel/+bug/1432603
>>> [1] https://review.openstack.org/#/c/175460/
>>> [2] https://review.openstack.org/#/c/175457/
>>> [3] https://review.openstack.org/#/c/175371/
>>> [4] https://review.openstack.org/#/c/170476/
>> Is there a reason you’re using a custom OCF script instead of the upstream[a] one?
>> Please have a chat with David (the maintainer, in CC) if there is something you believe is wrong with it.
>> 
>> [a] https://github.com/ClusterLabs/resource-agents/blob/master/heartbeat/rabbitmq-cluster
> 
> I'm using the OCF script from the Fuel project, specifically from the
> "6.0" stable branch [alpha].

Ah, I’m still learning who is who... i thought you were part of that project :-) 

> 
> Comparing with upstream OCF code, the main difference is that Fuel
> RabbitMQ OCF is a master-slave resource. Fuel RabbitMQ OCF does more
> bookkeeping, for example, blocking client access when RabbitMQ cluster
> is not ready. I beleive the upstream OCF should be OK to use as well
> after I read the code, but it might not fit into the Fuel project. As
> far as I test, the Fuel OCF script is good except sometimes the full
> reassemble time is long, and as I find out, it is mostly because the
> Fuel MySQL Galera OCF script keeps pacemaker from promoting RabbitMQ
> resource, as I mentioned in the previous emails.
> 
> Maybe Vladimir and Sergey can give us more insight on why Fuel needs a
> master-slave RabbitMQ.

That would be good to know.
Browsing the agent, promote seems to be a no-op if rabbit is already running.

> I see Vladimir and Sergey works on the original
> Fuel blueprint "RabbitMQ cluster" [beta].
> 
> [alpha]
> https://github.com/stackforge/fuel-library/blob/stable/6.0/deployment/puppet/nova/files/ocf/rabbitmq
> [beta]
> https://blueprints.launchpad.net/fuel/+spec/rabbitmq-cluster-controlled-by-pacemaker
> 
>>>> I have a little investigation and find out there are some possible causes.
>>>> 
>>>> 1. MySQL Recovery Takes Too Long [1] and Blocking RabbitMQ Clustering in
>>>> Pacemaker
>>>> 
>>>> The pacemaker resource p_mysql start timeout is set to 475s. Sometimes
>>>> MySQL-wss fails to start after power failure, and pacemaker would wait
>>>> 475s before retry starting it. The problem is that pacemaker divides
>>>> resource state transitions into batches. Since RabbitMQ is master-slave
>>>> resource, I assume that starting all the slaves and promoting master are
>>>> put into two different batches. If unfortunately starting all RabbitMQ
>>>> slaves are put in the same batch as MySQL starting, even if RabbitMQ
>>>> slaves and all other resources are ready, pacemaker will not continue
>>>> but just wait for MySQL timeout.
>>> Could you please elaborate the what is the same/different batches for MQ
>>> and DB? Note, there is a MQ clustering logic flow charts available here
>>> [5] and we're planning to release a dedicated technical bulletin for this.
>>> 
>>> [5] http://goo.gl/PPNrw7
>>> 
>>>> I can re-produce this by hard powering off all the controllers and start
>>>> them again. It's more likely to trigger MySQL failure in this way. Then
>>>> I observe that if there is one cloned mysql instance not starting, the
>>>> whole pacemaker cluster gets stuck and does not emit any log. On the
>>>> host of the failed instance, I can see a mysql resource agent process
>>>> calling the sleep command. If I kill that process, the pacemaker comes
>>>> back alive and RabbitMQ master gets promoted. In fact this long timeout
>>>> is blocking every resource from state transition in pacemaker.
>>>> 
>>>> This maybe a known problem of pacemaker and there are some discussions
>>>> in Linux-HA mailing list [2]. It might not be fixed in the near future.
>>>> It seems in generally it's bad to have long timeout in state transition
>>>> actions (start/stop/promote/demote). There maybe another way to
>>>> implement MySQL-wss resource agent to use a short start timeout and
>>>> monitor the wss cluster state using monitor action.
>>> This is very interesting, thank you! I believe all commands for MySQL RA
>>> OCF script should be as well wrapped with timeout -SIGTERM or -SIGKILL
>>> as we did for MQ RA OCF. And there should no be any sleep calls. I
>>> created a bug for this [6].
>>> 
>>> [6] https://bugs.launchpad.net/fuel/+bug/1449542
>>> 
>>>> I also find a fix to improve MySQL start timeout [3]. It shortens the
>>>> timeout to 300s. At the time I sending this email, I can not find it in
>>>> stable/6.0 branch. Maybe the maintainer needs to cherry-pick it to
>>>> stable/6.0 ?
>>>> 
>>>> [1] https://bugs.launchpad.net/fuel/+bug/1441885
>>>> [2] http://lists.linux-ha.org/pipermail/linux-ha/2014-March/047989.html
>>>> [3] https://review.openstack.org/#/c/171333/
>>>> 
>>>> 
>>>> 2. RabbitMQ Resource Agent Breaks Existing Cluster
>>>> 
>>>> Read the code of the RabbitMQ resource agent, I find it does the
>>>> following to start RabbitMQ master-slave cluster.
>>>> On all the controllers:
>>>> (1) Start Erlang beam process
>>>> (2) Start RabbitMQ App (If failed, reset mnesia DB and cluster state)
>>>> (3) Stop RabbitMQ App but do not stop the beam process
>>>> 
>>>> Then in pacemaker, all the RabbitMQ instances are in slave state. After
>>>> pacemaker determines the master, it does the following.
>>>> On the to-be-master host:
>>>> (4) Start RabbitMQ App (If failed, reset mnesia DB and cluster state)
>>>> On the slaves hosts:
>>>> (5) Start RabbitMQ App (If failed, reset mnesia DB and cluster state)
>>>> (6) Join RabbitMQ cluster of the master host
>>>> 
>>> Yes, something like that. As I mentioned, there were several bug fixes
>>> in the 6.1 dev, and you can also check the MQ clustering flow charts.
>>> 
>>>> As far as I can understand, this process is to make sure the master
>>>> determined by pacemaker is the same as the master determined in RabbitMQ
>>>> cluster. If there is no existing cluster, it's fine. If it is run
>>> after
>>> 
>>> Not exactly. There is no master in mirrored MQ cluster. We define the
>>> rabbit_hosts configuration option from Oslo.messaging. What ensures all
>>> queue masters will be spread around all of MQ nodes in a long run. And
>>> we use a master abstraction only for the Pacemaker RA clustering layer.
>>> Here, a "master" is the MQ node what joins the rest of the MQ nodes.
>>> 
>>>> power failure and recovery, it introduces the a new problem.
>>> We do erase the node master attribute in CIB for such cases. This should
>>> not bring problems into the master election logic.
>>> 
>>>> After power recovery, if some of the RabbitMQ instances reach step (2)
>>>> roughly at the same time (within 30s which is hard coded in RabbitMQ) as
>>>> the original RabbitMQ master instance, they form the original cluster
>>>> again and then shutdown. The other instances would have to wait for 30s
>>>> before it reports failure waiting for tables, and be  reset to a
>>>> standalone cluster.
>>>> 
>>>> In RabbitMQ documentation [4], it is also mentioned that if we shutdown
>>>> RabbitMQ master, a new master is elected from the rest of slaves. If we
>>> (Note, the RabbitMQ documentation mentions *queue* masters and slaves,
>>> which are not the case for the Pacemaker RA clustering abstraction layer.)
>>> 
>>>> continue to shutdown nodes in step (3), we reach a point that the last
>>>> node is the RabbitMQ master, and pacemaker is not aware of it. I can see
>>>> there is code to bookkeeping a "rabbit-start-time" attribute in
>>>> pacemaker to record the most long lived instance to help pacemaker
>>>> determine the master, but it does not cover the case mentioned above.
>>> We made an assumption what the node with the highest MQ uptime should
>>> know the most about recent cluster state, so other nodes must join it.
>>> RA OCF does not work with queue masters directly.
>>> 
>>>> A
>>>> recent patch [5] checks existing "rabbit-master" attribute but it
>>>> neither cover the above case.
>>>> 
>>>> So in step (4), pacemaker determines a different master which was a
>>>> RabbitMQ slave last time. It would wait for its original RabbitMQ master
>>>> for 30s and fail, then it gets reset to a standalone cluster. Here we
>>>> get some different clusters, so in step (5) and (6), it is likely to
>>>> report error in log saying timeout waiting for tables or fail to merge
>>>> mnesia database schema, then the those instances get reset. You can
>>>> easily re-produce the case by hard resetting power of all the controllers.
>>>> 
>>>> As you can see, if you are unlucky, there would be several "30s timeout
>>>> and reset" before you finally get a healthy RabbitMQ cluster.
>>> The full MQ cluster reassemble logic is far from the perfect state,
>>> indeed. This might erase all mnesia files, hence any custom entities,
>>> like users or vhosts, would be removed as well. Note, we do not
>>> configure durable queues for Openstack so there is nothing to care about
>>> here - the full cluster downtime assumes there will be no AMQP messages
>>> stored at all.
>>> 
>>>> I find three possible solutions.
>>>> A. Using rabbitmqctl force_boot option [6]
>>>> It will skips waiting for 30s and resetting cluster, but just assume the
>>>> current node is the master and continue to operate. This is feasible
>>>> because the original RabbitMQ master would discards the local state and
>>>> sync with the new master after it joins a new cluster [7]. So we can be
>>>> sure that after step (4) and (6), the pacemaker determined master
>>>> instance is started unconditionally, and it will be the same as RabbitMQ
>>>> master, and all operations run without 30s timeout. I find this option
>>>> is only available in newer RabbitMQ release, and updating RabbitMQ might
>>>> introduce other compatibility problems.
>>> Yes, this option is only supported for newest RabbitMQ versions. But we
>>> definitely should look how this could help.
>>> 
>>>> B. Turn RabbitMQ into cloned instance and use pause_minority instead of
>>>> autoheal [8]
>>> Indeed, there are cases when MQ's autoheal can do nothing with existing
>>> partitions and remains partitioned for ever, for example:
>>> 
>>> Masters: [ node-1 ]
>>> Slaves: [ node-2 node-3 ]
>>> root at node-1:~# rabbitmqctl cluster_status
>>> Cluster status of node 'rabbit at node-1' ...
>>> [{nodes,[{disc,['rabbit at node-1','rabbit at node-2']}]},
>>> {running_nodes,['rabbit at node-1']},
>>> {cluster_name,<<"rabbit at node-2">>},
>>> {partitions,[]}]
>>> ...done.
>>> root at node-2:~# rabbitmqctl cluster_status
>>> Cluster status of node 'rabbit at node-2' ...
>>> [{nodes,[{disc,['rabbit at node-2']}]}]
>>> ...done.
>>> root at node-3:~# rabbitmqctl cluster_status
>>> Cluster status of node 'rabbit at node-3' ...
>>> [{nodes,[{disc,['rabbit at node-1','rabbit at node-2','rabbit at node-3']}]},
>>> {running_nodes,['rabbit at node-3']},
>>> {cluster_name,<<"rabbit at node-2">>},
>>> {partitions,[]}]
>>> 
>>> So we should test the pause-minority value as well.
>>> But I strongly believe we should make MQ multi-state clone to support
>>> many masters, related bp [7]
>>> 
>>> [7]
>>> https://blueprints.launchpad.net/fuel/+spec/rabbitmq-pacemaker-multimaster-clone
>>> 
>>>> This works like MySQL-wss. It let RabbitMQ cluster itself deal with
>>>> partition in a manner similar to pacemaker quorum mechanism. When there
>>>> is network partition, instances in the minority partition pauses
>>>> themselves automatically. Pacemaker does not have to track who is the
>>>> RabbitMQ master, who lives longest, who to promote... It just starts all
>>>> the clones, done. This leads to huge change in RabbitMQ resource agent,
>>>> and the stability and other impact is to be tested.
>>> Well, we should not mess the queue masters and multi-clone master for MQ
>>> resource in the pacemaker.
>>> As I said, pacemaker RA has nothing to do with queue masters. And we
>>> introduced this "master" mostly in order to support the full cluster
>>> reassemble case - there must be a node promoted and other nodes should join.
>>> 
>>>> C. Creating a "force_load" file
>>>> After reading RabbitMQ source code, I find that the actual thing it does
>>>> in solution A is just creating an empty file named "force_load" in
>>>> mnesia database dir, then mnesia thinks it is the last node shut down in
>>>> the last time and boot itself as the master. This implementation keeps
>>>> the same from v3.1.4 to the latest RabbitMQ master branch. I think we
>>>> can make use of this little trick. The change is adding just one line in
>>>> "try_to_start_rmq_app()" function.
>>>> 
>>>> touch "${MNESIA_FILES}/force_load" && \
>>>> chown rabbitmq:rabbitmq "${MNESIA_FILES}/force_load"
>>> This is a very good point, thank you.
>>> 
>>>> [4] http://www.rabbitmq.com/ha.html
>>>> [5] https://review.openstack.org/#/c/169291/
>>>> [6] https://www.rabbitmq.com/clustering.html
>>>> [7] http://www.rabbitmq.com/partitions.html#recovering
>>>> [8] http://www.rabbitmq.com/partitions.html#automatic-handling
>>>> 
>>>> Maybe you have better ideas on this. Please share your thoughts.
>>> Thank you for a thorough feedback! This was a really great job.
>>> 
>>>> ----
>>>> Best wishes!
>>>> Zhou Zheng Sheng / ???  Software Engineer
>>>> Beijing AWcloud Software Co., Ltd.
>>>> 
>>> 
>>> -- 
>>> Best regards,
>>> Bogdan Dobrelya,
>>> Skype #bogdando_at_yahoo.com
>>> Irc #bogdando
>>> 
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