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From Ivan Pavlukhin <vololo...@gmail.com>
Subject Re: [DISCUSSION] Maintenance Mode feature
Date Sun, 06 Sep 2020 05:08:24 GMT
Sergey,

Thank you for your answer!

Might be I am looking at the subject from a different angle.

> I think of a node in MM as an almost normal one
I cannot think of such a mode as a normal one, because it apparently
does not perform usual cluster node functions. It is not a part of a
cluster, caches data is not available, Discovery and Communication are
not needed.

I fear that with "node started in a special mode" approach we will get
an additional flag in the code making the code more complex and
fragile. Should not I worry about it?

2020-09-02 10:45 GMT+03:00, Sergey Chugunov <sergey.chugunov@gmail.com>:
> Vladislav, Ivan,
>
> Thank you for your questions and suggestions. Let me answer them.
>
> Vladislav,
>
> If I understood you correctly, you're talking about a node performing some
> automatic actions to fix the problem and then join the cluster as usual.
>
> However the original ticket [1] where we faced the need for Maintenance
> Mode is about exactly the opposite: avoid doing automatic actions and give
> a user the ability to decide what to do.
>
> Also the idea of Maintenance Mode is that the node is able to accept
> commands, expose metrics and so on, thus we need all components to be
> initialized (some of them may be partially initialized due to their own
> maintenance).
> To achieve that we need to go through a full cycle of node initialization
> including discovery initialization. When discovery is initialized (in
> special isolated mode) I don't think it is easy to switch back to normal
> operations without a restart.
>
> Ivan,
>
> I think of a node in MM as an almost normal one (maybe with some components
> skipped some steps of their initialization). Commands are accepted,
> appropriate metrics are exposed e.g. through JMX API and so on.
>
> So as I see it we'll have special commands for control.{sh|bat} CLI
> allowing user to see reasons why node switched to maintenance mode and/or
> trigger actions to fix the problem (I'm still thinking about proper design
> of these actions though).
>
> Of course the user should also be able to fix the problem manually e.g. by
> manually deleting corrupted PDS files when node is down. Ideally
> Maintenance Mode should be smart enough to figure that out and switch to
> normal operations without a restart but I'm not sure if it is possible
> without invasive changes of our components' lifecycle.
> So I believe this model (node truly started in Maintenance Mode and new
> commands in control.{sh|bat}) is a good fit for our current APIs and ways
> to interact with the node.
>
> Does it sound reasonable to you?
>
> Thank you!
>
> [1] https://issues.apache.org/jira/browse/IGNITE-13366
>
> On Tue, Sep 1, 2020 at 2:07 PM Ivan Pavlukhin <vololo100@gmail.com> wrote:
>
>> Sergey,
>>
>> Actually, I missed the point that the discussed mode affects a single
>> node but not a whole cluster. Perhaps I mixed terms "mode" and
>> "state".
>>
>> My next thoughts about maintenance routines are about special
>> utilities. As far as I remember MySQL provides a bunch of scripts for
>> various maintenance purposes. What user interface for maintenance
>> tasks execution is assumed? And what do we mean by "starting" a node
>> in a maintenance mode? Can we do some routines without "starting"
>> (e.g. try to recover PDS or cleanup)?
>>
>> 2020-08-31 23:41 GMT+03:00, Vladislav Pyatkov <vldpyatkov@gmail.com>:
>> > Hi Sergey.
>> >
>> > As I understand any switching from/to MM possible only through manual
>> > restart a node.
>> > But in your example that look like a technical actions, that only
>> possible
>> > in the case.
>> > Do you plan to provide a possibility for client where he can make a
>> > decision without a manual intervention?
>> >
>> > For example: Start node and manually agree with an option and after
>> > automatically resolve conflict and back to topology as a stable node.
>> >
>> > On Mon, Aug 31, 2020 at 5:41 PM Sergey Chugunov <
>> sergey.chugunov@gmail.com>
>> > wrote:
>> >
>> >> Hello Ivan,
>> >>
>> >> Thank you for raising the good question, I didn't think of Maintenance
>> >> Mode
>> >> from that perspective.
>> >>
>> >> In short, Maintenance Mode isn't related to Cluster States concept.
>> >> According to javadoc documentation of ClusterState enum [1] it is
>> >> solely
>> >> about cache operations and to some extent doesn't affect other
>> components
>> >> of Ignite node.
>> >> From APIs perspective putting the methods to manage Cluster State to
>> >> IgniteCluster interface doesn't look ideal to me but it is as it is.
>> >>
>> >> On the other hand Maintenance Mode as I see it will be managed through
>> >> different APIs than a ClusterState and this difference definitely will
>> be
>> >> reflected in the documentation of the feature.
>> >>
>> >> Ignite node is a complex piece of many components interacting with
>> >> each
>> >> other, they may have different lifecycles and states; states of
>> different
>> >> components cannot be reduced to the lowest common denominator.
>> >>
>> >> However if you have an idea of how to call the feature better to let
>> >> the
>> >> user easier distinguish it from other similar features please share it
>> >> with
>> >> us. Personally I'm very welcome to any suggestions that make design
>> >> more
>> >> intuitive and easy-to-use.
>> >>
>> >> Thanks!
>> >>
>> >> [1]
>> >>
>> >>
>> https://github.com/apache/ignite/blob/master/modules/core/src/main/java/org/apache/ignite/cluster/ClusterState.java
>> >>
>> >> On Mon, Aug 31, 2020 at 12:32 PM Ivan Pavlukhin <vololo100@gmail.com>
>> >> wrote:
>> >>
>> >> > Hi Sergey,
>> >> >
>> >> > Thank you for bringing attention to that important subject!
>> >> >
>> >> > My note here is about one more cluster mode. As far as I know
>> >> > currently we already have 3 modes (inactive, read-only, read-write)
>> >> > and the subject is about one more. From the first glance it could be
>> >> > hard for a user to understand and use all modes properly. Do we
>> >> > really
>> >> > need all spectrum? Could we simplify things somehow?
>> >> >
>> >> > 2020-08-27 15:59 GMT+03:00, Sergey Chugunov
>> >> > <sergey.chugunov@gmail.com>:
>> >> > > Hello Nikolay,
>> >> > >
>> >> > > Created one, available by link [1]
>> >> > >
>> >> > > Initially there was an intention to develop it under IEP-47 [2]
>> >> > > and
>> >> there
>> >> > > is even a separate section for Maintenance Mode there.
>> >> > > But it looks like this feature is useful in more cases and
>> >> > > deserves
>> >> > > its
>> >> > own
>> >> > > IEP.
>> >> > >
>> >> > > [1]
>> >> > >
>> >> >
>> >>
>> https://cwiki.apache.org/confluence/display/IGNITE/IEP-53%3A+Maintenance+Mode
>> >> > > [2]
>> >> > >
>> >> >
>> >>
>> https://cwiki.apache.org/confluence/display/IGNITE/IEP-47:+Native+persistence+defragmentation
>> >> > >
>> >> > > On Thu, Aug 27, 2020 at 11:01 AM Nikolay Izhikov
>> >> > > <nizhikov@apache.org>
>> >> > > wrote:
>> >> > >
>> >> > >> Hello, Sergey!
>> >> > >>
>> >> > >> Thanks for the proposal.
>> >> > >> Let’s have IEP for this feature.
>> >> > >>
>> >> > >> > 27 авг. 2020 г., в 10:25, Sergey Chugunov <
>> >> sergey.chugunov@gmail.com>
>> >> > >> написал(а):
>> >> > >> >
>> >> > >> > Hello Igniters,
>> >> > >> >
>> >> > >> > I want to start a discussion about new supporting feature
that
>> >> > >> > could
>> >> > be
>> >> > >> > very useful in many scenarios where persistent storage
is
>> >> > >> > involved:
>> >> > >> > Maintenance Mode.
>> >> > >> >
>> >> > >> > *Summary*
>> >> > >> > Maintenance Mode (MM for short) is a special state of
Ignite
>> >> > >> > node
>> >> when
>> >> > >> node
>> >> > >> > doesn't serve user requests nor joins the cluster but
waits for
>> >> > >> > user
>> >> > >> > commands or performs automatic actions for maintenance
>> >> > >> > purposes.
>> >> > >> >
>> >> > >> > *Motivation*
>> >> > >> > There are situations when node cannot participate in
regular
>> >> > operations
>> >> > >> but
>> >> > >> > at the same time should not be shut down.
>> >> > >> >
>> >> > >> > One example is a ticket [1] where I developed the first
draft
>> >> > >> > of
>> >> > >> > Maintenance Mode.
>> >> > >> > Here we get into a situation when node has potentially
>> >> > >> > corrupted
>> >> > >> > PDS
>> >> > >> > thus
>> >> > >> > cannot proceed with restore routine and join the cluster
as
>> usual.
>> >> > >> > At the same time node should not fail nor be stopped
for manual
>> >> > >> > cleanup.
>> >> > >> > Manual cleanup is not always an option (e.g. restricted
access
>> >> > >> > to
>> >> file
>> >> > >> > system); in managed environments failed node will be
restarted
>> >> > >> > automatically so user won't have time for performing
necessary
>> >> > >> operations.
>> >> > >> > Thus node needs to function in a special mode allowing
user to
>> >> connect
>> >> > >> > to
>> >> > >> > it and perform necessary actions.
>> >> > >> >
>> >> > >> > Another example is described in IEP-47 [2] where
>> >> > >> > defragmentation
>> >> > >> > is
>> >> > >> > being
>> >> > >> > developed. Node defragmenting its PDS should not join
the
>> >> > >> > cluster
>> >> > until
>> >> > >> the
>> >> > >> > process is finished so it needs to enter Maintenance
Mode as
>> well.
>> >> > >> >
>> >> > >> > *Suggested design*
>> >> > >> > I suggest MM to work as follows:
>> >> > >> > 1. Node enters MM if special markers are found on disk.
These
>> >> markers
>> >> > >> > called Maintenance Records could be created automatically
(e.g.
>> >> > >> > when
>> >> > >> > storage component detects corrupted storage) or by user
request
>> >> (when
>> >> > >> user
>> >> > >> > requests defragmentation of some caches). So entering
MM
>> >> > >> > requires
>> >> node
>> >> > >> > restart.
>> >> > >> > 2. Started in MM node doesn't join the cluster but finishes
>> >> > >> > startup
>> >> > >> routine
>> >> > >> > so it is able to receive commands and provide metrics
to the
>> user.
>> >> > >> > 3. When all necessary maintenance operations are finished,
>> >> Maintenance
>> >> > >> > Records for these operations are deleted from disk and
node
>> >> restarted
>> >> > >> again
>> >> > >> > to enter normal service.
>> >> > >> >
>> >> > >> > *Example*
>> >> > >> > To put it into a context let's consider an example of
how I see
>> >> > >> > the
>> >> MM
>> >> > >> > workflow in case of PDS corruption.
>> >> > >> >
>> >> > >> >   1. Node has failed in the middle of checkpoint when
WAL is
>> >> disabled
>> >> > >> > for
>> >> > >> >   a particular cache -> data files of the cache are
potentially
>> >> > >> corrupted.
>> >> > >> >   2. On next startup node detects this situation, creates
>> >> Maintenance
>> >> > >> >   Record on disk and shuts down.
>> >> > >> >   3. On next startup node sees Maintenance Record, enters
>> >> Maintenance
>> >> > >> Mode
>> >> > >> >   and waits for user to do specific actions: clean potentially
>> >> > >> > corrupted
>> >> > >> PDS.
>> >> > >> >   4. When user has done necessary actions he/she removes
>> >> > >> > Maintenance
>> >> > >> >   Record using Maintenance Mode API exposed via
>> >> > >> > control.{sh|bat}
>> >> > script
>> >> > >> or
>> >> > >> >   JMX.
>> >> > >> >   5. On next startup node goes to normal operations as
>> maintenance
>> >> > >> > reason
>> >> > >> >   is fixed.
>> >> > >> >
>> >> > >> >
>> >> > >> > I prepared a PR [3] for ticket [1] with draft implementation.
>> >> > >> > It
>> >> > >> > is
>> >> > not
>> >> > >> > ready to be merged to master branch but is already fully
>> >> > >> > functional
>> >> > and
>> >> > >> can
>> >> > >> > be reviewed.
>> >> > >> >
>> >> > >> > Hope you'll share your feedback on the feature and/or
any
>> thoughts
>> >> on
>> >> > >> > implementation.
>> >> > >> >
>> >> > >> > Thank you!
>> >> > >> >
>> >> > >> > [1] https://issues.apache.org/jira/browse/IGNITE-13366
>> >> > >> > [2]
>> >> > >> >
>> >> > >>
>> >> >
>> >>
>> https://cwiki.apache.org/confluence/display/IGNITE/IEP-47:+Native+persistence+defragmentation
>> >> > >> > [3] https://github.com/apache/ignite/pull/8189
>> >> > >>
>> >> > >>
>> >> > >
>> >> >
>> >> >
>> >> > --
>> >> >
>> >> > Best regards,
>> >> > Ivan Pavlukhin
>> >> >
>> >>
>> >
>> >
>> > --
>> > Vladislav Pyatkov
>> >
>>
>>
>> --
>>
>> Best regards,
>> Ivan Pavlukhin
>>
>


-- 

Best regards,
Ivan Pavlukhin

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