[pgpool-hackers: 2507] Re: New Feature with patch: Quorum and Consensus for backend failover

Fri Aug 25 20:17:21 JST 2017

On Fri, Aug 25, 2017 at 12:53 PM, Tatsuo Ishii <ishii at sraoss.co.jp> wrote:

> Usama,
>
> With the new patch, the regression tests all passed.
>

Glad to hear that :-)
Did you had a chance to look at the node quarantine state I added. What are
your thoughts on that ?

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
>
> > Hi Ishii-San
> >
> > Please fine the updated patch, It fixes the regression issue you were
> > facing and also another bug which I encountered during my testing.
> >
> > -- Adding Yugo to the thread,
> > Hi Yugo,
> >
> > Since you are an expert of watchdog feature, So I thought you might have
> > something to say especially regarding the discussion points mentioned in
> > the initial mail.
> >
> >
> > Thanks
> > Best Regards
> > Muhammad Usama
> >
> >
> > On Thu, Aug 24, 2017 at 11:25 AM, Muhammad Usama <m.usama at gmail.com>
> wrote:
> >
> >>
> >>
> >> 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/regre
> >>> ssion/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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