[pgpool-hackers: 2617] Re: New Feature with patch: Quorum and Consensus for backend failover
Tatsuo Ishii
ishii at sraoss.co.jp
Tue Nov 28 12:09:41 JST 2017
Hi Usama,
I have a question regarding the condition for "quorum exists".
I think "g_cluster.quorum_status >= 0" is the condition for "quorum exists".
Am I correct?
BTW, I observed from watchdog.c:
When there are only two watchdog nodes participating and they are
alive, then g_cluster.quorum_status == 1, which means "quorum exists",
because following condition is satisfied
(get_mimimum_nodes_required_for_quorum() returns
(g_cluster.remoteNodeCount - 1 ) / 2), which is 0).
if ( g_cluster.clusterMasterInfo.standby_nodes_count > get_mimimum_nodes_required_for_quorum())
{
g_cluster.quorum_status = 1;
}
In this case, if pool_config->failover_when_quorum_exists is on, and
pool_config->failover_require_consensus is on, then failover will always perform because:
if (failoverObj->request_count <= get_mimimum_nodes_required_for_quorum())
always fails (get_mimimum_nodes_required_for_quorum() returns 0).
So, it seems for a two-watchdog-cluster, regardless the setting of
failover_when_quorum_exists and failover_require_consensus, Pgpool-II
behaves as before. Is my understanding correct?
Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp
> Hi Usama,
>
> While writing a presentation material of Pgpool-II 3.7, I am not sure
> I understand the behavior the quorum consusens behavior.
>
>> *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.
>
> In what situation a Pgpool-II node could send multiple failover
> requests? My guess is in the following scenario:
>
> 1) Pgpool-II watchdog standby health check process detects the failure
> of backend A and send a faiover request to the master Pgpool-II.
>
> 2) Since the vote does not satisfy the quorum consensus, failver is
> not occurred. Just backend_info->quarantine is set and
> backend_info->backend_status is set to CON_DOWN.
>
> 3) Pgpool-II watchdog standby health check process detects the failure
> of backend A again, then sent a failover request to the master
> Pgpool-II again. If enable_multiple_failover_requests_from_node is
> set, failover will happen.
>
> But after thinking more, I realized that in step 3, since
> backend_status is already set to CON_DOWN, health check will not be
> performed against backend A. So the watchdog standby will not send
> multiple vote.
>
> Apparently I am missing something here.
>
> Can you please tell what is the scenario in that a watchdog sends
> multiple votes for failover?
>
> Best regards,
> --
> Tatsuo Ishii
> SRA OSS, Inc. Japan
> English: http://www.sraoss.co.jp/index_en.php
> Japanese:http://www.sraoss.co.jp
>
> From: Muhammad Usama <m.usama at gmail.com>
> Subject: New Feature with patch: Quorum and Consensus for backend failover
> Date: Tue, 22 Aug 2017 00:18:27 +0500
> Message-ID: <CAEJvTzUbz-d8dfsJdLt=XNYWdOMxKf06sp+p=uAbxyjvG=vS3A at mail.gmail.com>
>
>> 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
More information about the pgpool-hackers
mailing list