[pgpool-hackers: 2496] Re: New Feature with patch: Quorum and Consensus for backend failover

[Muhammad Usama]

On Thu, Aug 24, 2017 at 4:34 AM, Tatsuo Ishii <ishii at sraoss.co.jp> wrote:

> After applying the patch, many of regression tests fail. It seems
> pgpool.conf.sample has bogus comment which causes the pgpool.conf
> parser to complain parse error.
>
> 2017-08-24 08:22:36: pid 6017: FATAL:  syntex error in configuration file
> "/home/t-ishii/work/pgpool-II/current/pgpool2/src/test/
> regression/tests/004.watchdog/standby/etc/pgpool.conf"
> 2017-08-24 08:22:36: pid 6017: DETAIL:  parse error at line 568 '*' token
> = 8
>

Really sorry, Somehow I overlooked the sample config file changes I made at
the last minute.
Will send you the updated version.

Thanks
Best Regards
Muhammad Usama

>
> Best regards,
> --
> Tatsuo Ishii
> SRA OSS, Inc. Japan
> English: http://www.sraoss.co.jp/index_en.php
> Japanese:http://www.sraoss.co.jp
>
> > Usama,
> >
> > Thanks for the patch. I am going to review it.
> >
> > In the mean time when I apply your patch, I got some trailing
> > whitespace errors. Can you please fix them?
> >
> > /home/t-ishii/quorum_aware_failover.diff:470: trailing whitespace.
> >
> > /home/t-ishii/quorum_aware_failover.diff:485: trailing whitespace.
> >
> > /home/t-ishii/quorum_aware_failover.diff:564: trailing whitespace.
> >
> > /home/t-ishii/quorum_aware_failover.diff:1428: trailing whitespace.
> >
> > /home/t-ishii/quorum_aware_failover.diff:1450: trailing whitespace.
> >
> > warning: squelched 3 whitespace errors
> > warning: 8 lines add whitespace errors.
> >
> > Best regards,
> > --
> > Tatsuo Ishii
> > SRA OSS, Inc. Japan
> > English: http://www.sraoss.co.jp/index_en.php
> > Japanese:http://www.sraoss.co.jp
> >
> >> Hi
> >>
> >> I was working on the new feature to make the backend node failover
> quorum
> >> aware and on the half way through the implementation I also added the
> >> majority consensus feature for the same.
> >>
> >> So please find the first version of the patch for review that makes the
> >> backend node failover consider the watchdog cluster quorum status and
> seek
> >> the majority consensus before performing failover.
> >>
> >> *Changes in the Failover mechanism with watchdog.*
> >> For this new feature I have modified the Pgpool-II's existing failover
> >> mechanism with watchdog.
> >> Previously as you know when the Pgpool-II require to perform a node
> >> operation (failover, failback, promote-node) with the watchdog. The
> >> watchdog used to propagated the failover request to all the Pgpool-II
> nodes
> >> in the watchdog cluster and as soon as the request was received by the
> >> node, it used to initiate the local failover and that failover was
> >> synchronised on all nodes using the distributed locks.
> >>
> >> *Now Only the Master node performs the failover.*
> >> The attached patch changes the mechanism of synchronised failover, and
> now
> >> only the Pgpool-II of master watchdog node performs the failover, and
> all
> >> other standby nodes sync the backend statuses after the master
> Pgpool-II is
> >> finished with the failover.
> >>
> >> *Overview of new failover mechanism.*
> >> -- If the failover request is received to the standby watchdog node(from
> >> local Pgpool-II), That request is forwarded to the master watchdog and
> the
> >> Pgpool-II main process is returned with the FAILOVER_RES_WILL_BE_DONE
> >> return code. And upon receiving the FAILOVER_RES_WILL_BE_DONE from the
> >> watchdog for the failover request the requesting Pgpool-II moves forward
> >> without doing anything further for the particular failover command.
> >>
> >> -- Now when the failover request from standby node is received by the
> >> master watchdog, after performing the validation, applying the consensus
> >> rules the failover request is triggered on the local Pgpool-II .
> >>
> >> -- When the failover request is received to the master watchdog node
> from
> >> the local Pgpool-II (On the IPC channel) the watchdog process inform the
> >> Pgpool-II requesting process to proceed with failover (provided all
> >> failover rules are satisfied).
> >>
> >> -- After the failover is finished on the master Pgpool-II, the failover
> >> function calls the *wd_failover_end*() which sends the backend sync
> >> required message to all standby watchdogs.
> >>
> >> -- Upon receiving the sync required message from master watchdog node
> all
> >> Pgpool-II sync the new statuses of each backend node from the master
> >> watchdog.
> >>
> >> *No More Failover locks*
> >> Since with this new failover mechanism we do not require any
> >> synchronisation and guards against the execution of failover_commands by
> >> multiple Pgpool-II nodes, So the patch removes all the distributed locks
> >> from failover function, This makes the failover simpler and faster.
> >>
> >> *New kind of Failover operation NODE_QUARANTINE_REQUEST*
> >> The patch adds the new kind of backend node operation NODE_QUARANTINE
> which
> >> is effectively same as the NODE_DOWN, but with node_quarantine the
> >> failover_command is not triggered.
> >> The NODE_DOWN_REQUEST is automatically converted to the
> >> NODE_QUARANTINE_REQUEST when the failover is requested on the backend
> node
> >> but watchdog cluster does not holds the quorum.
> >> This means in the absence of quorum the failed backend nodes are
> >> quarantined and when the quorum becomes available again the Pgpool-II
> >> performs the failback operation on all quarantine nodes.
> >> And again when the failback is performed on the quarantine backend node
> the
> >> failover function does not trigger the failback_command.
> >>
> >> *Controlling the Failover behaviour.*
> >> The patch adds three new configuration parameters to configure the
> failover
> >> behaviour from user side.
> >>
> >> *failover_when_quorum_exists*
> >> When enabled the failover command will only be executed when the
> watchdog
> >> cluster holds the quorum. And when the quorum is absent and
> >> failover_when_quorum_exists is enabled the failed backend nodes will get
> >> quarantine until the quorum becomes available again.
> >> disabling it will enable the old behaviour of failover commands.
> >>
> >>
> >> *failover_require_consensus*This new configuration parameter can be
> used to
> >> make sure we get the majority vote before performing the failover on the
> >> node. When *failover_require_consensus* is enabled then the failover is
> >> only performed after receiving the failover request from the majority or
> >> Pgpool-II nodes.
> >> For example in three nodes cluster the failover will not be performed
> until
> >> at least two nodes ask for performing the failover on the particular
> >> backend node.
> >>
> >> It is also worthwhile to mention here that *failover_require_consensus*
> >> only works when failover_when_quorum_exists is enables.
> >>
> >>
> >> *enable_multiple_failover_requests_from_node*
> >> This parameter works in connection with *failover_require_consensus*
> >> config. When enabled a single Pgpool-II node can vote for failover
> multiple
> >> times.
> >> For example in the three nodes cluster if one Pgpool-II node sends the
> >> failover request of particular node twice that would be counted as two
> >> votes in favour of failover and the failover will be performed even if
> we
> >> do not get a vote from other two nodes.
> >>
> >> And when *enable_multiple_failover_requests_from_node* is disabled,
> Only
> >> the first vote from each Pgpool-II will be accepted and all other
> >> subsequent votes will be marked duplicate and rejected.
> >> So in that case we will require a majority votes from distinct nodes to
> >> execute the failover.
> >> Again this *enable_multiple_failover_requests_from_node* only becomes
> >> effective when both *failover_when_quorum_exists* and
> >> *failover_require_consensus* are enabled.
> >>
> >>
> >> *Controlling the failover: The Coding perspective.*
> >> Although the failover functions are made quorum and consensus aware but
> >> there is still a way to bypass the quorum conditions, and requirement of
> >> consensus.
> >>
> >> For this the patch uses the existing request_details flags in
> >> POOL_REQUEST_NODE to control the behaviour of failover.
> >>
> >> Here are the newly added flags values.
> >>
> >> *REQ_DETAIL_WATCHDOG*:
> >> Setting this flag while issuing the failover command will not send the
> >> failover request to the watchdog. But this flag may not be useful in any
> >> other place than where it is already used.
> >> Mostly this flag can be used to avoid the failover command from going to
> >> watchdog that is already originated from watchdog. Otherwise we can end
> up
> >> in infinite loop.
> >>
> >> *REQ_DETAIL_CONFIRMED*:
> >> Setting this flag will bypass the *failover_require_consensus*
> >> configuration and immediately perform the failover if quorum is present.
> >> This flag can be used to issue the failover request originated from PCP
> >> command.
> >>
> >> *REQ_DETAIL_UPDATE*:
> >> This flag is used for the command where we are failing back the
> quarantine
> >> nodes. Setting this flag will not trigger the failback_command.
> >>
> >> *Some conditional flags used:*
> >> I was not sure about the configuration of each type of failover
> operation.
> >> As we have three main failover operations NODE_UP_REQUEST,
> >> NODE_DOWN_REQUEST, and PROMOTE_NODE_REQUEST
> >> So I was thinking do we need to give the configuration option to the
> users,
> >> if they want to enable/disable quorum checking and consensus for
> individual
> >> failover operation type.
> >> For example: is it a practical configuration where a user would want to
> >> ensure quorum while preforming NODE_DOWN operation while does not want
> it
> >> for NODE_UP.
> >> So in this patch I use three compile time defines to enable disable the
> >> individual failover operation, while we can decide on the best solution.
> >>
> >> NODE_UP_REQUIRE_CONSENSUS: defining it will enable quorum checking
> feature
> >> for NODE_UP_REQUESTs
> >>
> >> NODE_DOWN_REQUIRE_CONSENSUS: defining it will enable quorum checking
> >> feature for NODE_DOWN_REQUESTs
> >>
> >> NODE_PROMOTE_REQUIRE_CONSENSUS: defining it will enable quorum checking
> >> feature for PROMOTE_NODE_REQUESTs
> >>
> >> *Some Point for Discussion:*
> >>
> >> *Do we really need to check ReqInfo->switching flag before enqueuing
> >> failover request.*
> >> While working on the patch I was wondering why do we disallow enqueuing
> the
> >> failover command when the failover is already in progress? For example
> in
> >> *pcp_process_command*() function if we see the *Req_info->switching*
> flag
> >> set we bailout with the error instead of enqueuing the command. Is is
> >> really necessary?
> >>
> >> *Do we need more granule control over each failover operation:*
> >> As described in section "Some conditional flags used" I want the
> opinion on
> >> do we need configuration parameters in pgpool.conf to enable disable
> quorum
> >> and consensus checking on individual failover types.
> >>
> >> *Which failover should be mark as Confirmed:*
> >> As defined in the above section of REQ_DETAIL_CONFIRMED, We can mark the
> >> failover request to not need consensus, currently the requests from the
> PCP
> >> commands are fired with this flag. But I was wondering there may be more
> >> places where we many need to use the flag.
> >> For example I currently use the same confirmed flag when failover is
> >> triggered because of *replication_stop_on_mismatch*.
> >>
> >> I think we should think this flag for each place of failover, like when
> the
> >> failover is triggered
> >> because of health_check failure.
> >> because of replication mismatch
> >> because of backend_error
> >> e.t.c
> >>
> >> *Node Quarantine behaviour.*
> >> What do you think about the node quarantine used by this patch. Can you
> >> think of some problem which can be caused by this?
> >>
> >> *What should be the default values for each newly added config
> parameters.*
> >>
> >>
> >>
> >> *TODOs*
> >>
> >> -- Updating the documentation is still todo. Will do that once every
> aspect
> >> of the feature will be finalised.
> >> -- Some code warnings and cleanups are still not done.
> >> -- I am still little short on testing
> >> -- Regression test cases for the feature
> >>
> >>
> >> Thoughts and suggestions are most welcome.
> >>
> >> Thanks
> >> Best regards
> >> Muhammad Usama
> > _______________________________________________
> > pgpool-hackers mailing list
> > pgpool-hackers at pgpool.net
> > http://www.pgpool.net/mailman/listinfo/pgpool-hackers
>
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