<div dir="ltr">Hi Ishii-San<br><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Nov 28, 2017 at 8:09 AM, Tatsuo Ishii <span dir="ltr"><<a href="mailto:ishii@sraoss.co.jp" target="_blank">ishii@sraoss.co.jp</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi Usama,<br>
<br>
I have a question regarding the condition for "quorum exists".<br>
I think "g_cluster.quorum_status >= 0" is the condition for "quorum exists".<br>
Am I correct?<br>
<br>
BTW, I observed from watchdog.c:<br>
<br>
When there are only two watchdog nodes participating and they are<br>
alive, then g_cluster.quorum_status == 1, which means "quorum exists",<br>
because following condition is satisfied<br>
(get_mimimum_nodes_required_<wbr>for_quorum() returns<br>
(g_cluster.remoteNodeCount - 1 ) / 2), which is 0).<br>
<br>
if ( g_cluster.clusterMasterInfo.<wbr>standby_nodes_count > get_mimimum_nodes_required_<wbr>for_quorum())<br>
{<br>
g_cluster.quorum_status = 1;<br>
}<br>
<br>
In this case, if pool_config->failover_when_<wbr>quorum_exists is on, and<br>
pool_config->failover_require_<wbr>consensus is on, then failover will always perform because:<br>
<br>
if (failoverObj->request_count <= get_mimimum_nodes_required_<wbr>for_quorum())<br>
<br>
always fails (get_mimimum_nodes_required_<wbr>for_quorum() returns 0).<br>
<br>
So, it seems for a two-watchdog-cluster, regardless the setting of<br>
failover_when_quorum_exists and failover_require_consensus, Pgpool-II<br>
behaves as before. Is my understanding correct?<br></blockquote><div><br></div><div>Yes that is correct, This is because in watchdog we consider the existence of quorum if 50% of nodes are</div><div>alive, So for two node cluster only one node is enough to complete the quorum.</div><div>Thats why we recommend to have minimum three and odd number of nodes.</div><div><br></div><div>Do you have any reservations on this and want to have a changed behaviour??</div><div><br></div><div>Thanks</div><div>Best Regards</div><div>Muhammad Usama</div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div class="HOEnZb"><div class="h5"><br>
Best regards,<br>
--<br>
Tatsuo Ishii<br>
SRA OSS, Inc. Japan<br>
English: <a href="http://www.sraoss.co.jp/index_en.php" rel="noreferrer" target="_blank">http://www.sraoss.co.jp/index_<wbr>en.php</a><br>
Japanese:<a href="http://www.sraoss.co.jp" rel="noreferrer" target="_blank">http://www.sraoss.co.<wbr>jp</a><br>
<br>
> Hi Usama,<br>
><br>
> While writing a presentation material of Pgpool-II 3.7, I am not sure<br>
> I understand the behavior the quorum consusens behavior.<br>
><br>
>> *enable_multiple_failover_<wbr>requests_from_node*<br>
>> This parameter works in connection with *failover_require_consensus*<br>
>> config. When enabled a single Pgpool-II node can vote for failover multiple<br>
>> times.<br>
><br>
> In what situation a Pgpool-II node could send multiple failover<br>
> requests? My guess is in the following scenario:<br>
><br>
> 1) Pgpool-II watchdog standby health check process detects the failure<br>
> of backend A and send a faiover request to the master Pgpool-II.<br>
><br>
> 2) Since the vote does not satisfy the quorum consensus, failver is<br>
> not occurred. Just backend_info->quarantine is set and<br>
> backend_info->backend_status is set to CON_DOWN.<br>
><br>
> 3) Pgpool-II watchdog standby health check process detects the failure<br>
> of backend A again, then sent a failover request to the master<br>
> Pgpool-II again. If enable_multiple_failover_<wbr>requests_from_node is<br>
> set, failover will happen.<br>
><br>
> But after thinking more, I realized that in step 3, since<br>
> backend_status is already set to CON_DOWN, health check will not be<br>
> performed against backend A. So the watchdog standby will not send<br>
> multiple vote.<br>
><br>
> Apparently I am missing something here.<br>
><br>
> Can you please tell what is the scenario in that a watchdog sends<br>
> multiple votes for failover?<br>
><br>
> Best regards,<br>
> --<br>
> Tatsuo Ishii<br>
> SRA OSS, Inc. Japan<br>
> English: <a href="http://www.sraoss.co.jp/index_en.php" rel="noreferrer" target="_blank">http://www.sraoss.co.jp/index_<wbr>en.php</a><br>
> Japanese:<a href="http://www.sraoss.co.jp" rel="noreferrer" target="_blank">http://www.sraoss.co.<wbr>jp</a><br>
><br>
> From: Muhammad Usama <<a href="mailto:m.usama@gmail.com">m.usama@gmail.com</a>><br>
> Subject: New Feature with patch: Quorum and Consensus for backend failover<br>
> Date: Tue, 22 Aug 2017 00:18:27 +0500<br>
> Message-ID: <CAEJvTzUbz-d8dfsJdLt=<wbr>XNYWdOMxKf06sp+p=uAbxyjvG=<a href="mailto:vS3A@mail.gmail.com">vS3A<wbr>@mail.gmail.com</a>><br>
><br>
>> Hi<br>
>><br>
>> I was working on the new feature to make the backend node failover quorum<br>
>> aware and on the half way through the implementation I also added the<br>
>> majority consensus feature for the same.<br>
>><br>
>> So please find the first version of the patch for review that makes the<br>
>> backend node failover consider the watchdog cluster quorum status and seek<br>
>> the majority consensus before performing failover.<br>
>><br>
>> *Changes in the Failover mechanism with watchdog.*<br>
>> For this new feature I have modified the Pgpool-II's existing failover<br>
>> mechanism with watchdog.<br>
>> Previously as you know when the Pgpool-II require to perform a node<br>
>> operation (failover, failback, promote-node) with the watchdog. The<br>
>> watchdog used to propagated the failover request to all the Pgpool-II nodes<br>
>> in the watchdog cluster and as soon as the request was received by the<br>
>> node, it used to initiate the local failover and that failover was<br>
>> synchronised on all nodes using the distributed locks.<br>
>><br>
>> *Now Only the Master node performs the failover.*<br>
>> The attached patch changes the mechanism of synchronised failover, and now<br>
>> only the Pgpool-II of master watchdog node performs the failover, and all<br>
>> other standby nodes sync the backend statuses after the master Pgpool-II is<br>
>> finished with the failover.<br>
>><br>
>> *Overview of new failover mechanism.*<br>
>> -- If the failover request is received to the standby watchdog node(from<br>
>> local Pgpool-II), That request is forwarded to the master watchdog and the<br>
>> Pgpool-II main process is returned with the FAILOVER_RES_WILL_BE_DONE<br>
>> return code. And upon receiving the FAILOVER_RES_WILL_BE_DONE from the<br>
>> watchdog for the failover request the requesting Pgpool-II moves forward<br>
>> without doing anything further for the particular failover command.<br>
>><br>
>> -- Now when the failover request from standby node is received by the<br>
>> master watchdog, after performing the validation, applying the consensus<br>
>> rules the failover request is triggered on the local Pgpool-II .<br>
>><br>
>> -- When the failover request is received to the master watchdog node from<br>
>> the local Pgpool-II (On the IPC channel) the watchdog process inform the<br>
>> Pgpool-II requesting process to proceed with failover (provided all<br>
>> failover rules are satisfied).<br>
>><br>
>> -- After the failover is finished on the master Pgpool-II, the failover<br>
>> function calls the *wd_failover_end*() which sends the backend sync<br>
>> required message to all standby watchdogs.<br>
>><br>
>> -- Upon receiving the sync required message from master watchdog node all<br>
>> Pgpool-II sync the new statuses of each backend node from the master<br>
>> watchdog.<br>
>><br>
>> *No More Failover locks*<br>
>> Since with this new failover mechanism we do not require any<br>
>> synchronisation and guards against the execution of failover_commands by<br>
>> multiple Pgpool-II nodes, So the patch removes all the distributed locks<br>
>> from failover function, This makes the failover simpler and faster.<br>
>><br>
>> *New kind of Failover operation NODE_QUARANTINE_REQUEST*<br>
>> The patch adds the new kind of backend node operation NODE_QUARANTINE which<br>
>> is effectively same as the NODE_DOWN, but with node_quarantine the<br>
>> failover_command is not triggered.<br>
>> The NODE_DOWN_REQUEST is automatically converted to the<br>
>> NODE_QUARANTINE_REQUEST when the failover is requested on the backend node<br>
>> but watchdog cluster does not holds the quorum.<br>
>> This means in the absence of quorum the failed backend nodes are<br>
>> quarantined and when the quorum becomes available again the Pgpool-II<br>
>> performs the failback operation on all quarantine nodes.<br>
>> And again when the failback is performed on the quarantine backend node the<br>
>> failover function does not trigger the failback_command.<br>
>><br>
>> *Controlling the Failover behaviour.*<br>
>> The patch adds three new configuration parameters to configure the failover<br>
>> behaviour from user side.<br>
>><br>
>> *failover_when_quorum_exists*<br>
>> When enabled the failover command will only be executed when the watchdog<br>
>> cluster holds the quorum. And when the quorum is absent and<br>
>> failover_when_quorum_exists is enabled the failed backend nodes will get<br>
>> quarantine until the quorum becomes available again.<br>
>> disabling it will enable the old behaviour of failover commands.<br>
>><br>
>><br>
>> *failover_require_consensus*<wbr>This new configuration parameter can be used to<br>
>> make sure we get the majority vote before performing the failover on the<br>
>> node. When *failover_require_consensus* is enabled then the failover is<br>
>> only performed after receiving the failover request from the majority or<br>
>> Pgpool-II nodes.<br>
>> For example in three nodes cluster the failover will not be performed until<br>
>> at least two nodes ask for performing the failover on the particular<br>
>> backend node.<br>
>><br>
>> It is also worthwhile to mention here that *failover_require_consensus*<br>
>> only works when failover_when_quorum_exists is enables.<br>
>><br>
>><br>
>> *enable_multiple_failover_<wbr>requests_from_node*<br>
>> This parameter works in connection with *failover_require_consensus*<br>
>> config. When enabled a single Pgpool-II node can vote for failover multiple<br>
>> times.<br>
>> For example in the three nodes cluster if one Pgpool-II node sends the<br>
>> failover request of particular node twice that would be counted as two<br>
>> votes in favour of failover and the failover will be performed even if we<br>
>> do not get a vote from other two nodes.<br>
>><br>
>> And when *enable_multiple_failover_<wbr>requests_from_node* is disabled, Only<br>
>> the first vote from each Pgpool-II will be accepted and all other<br>
>> subsequent votes will be marked duplicate and rejected.<br>
>> So in that case we will require a majority votes from distinct nodes to<br>
>> execute the failover.<br>
>> Again this *enable_multiple_failover_<wbr>requests_from_node* only becomes<br>
>> effective when both *failover_when_quorum_exists* and<br>
>> *failover_require_consensus* are enabled.<br>
>><br>
>><br>
>> *Controlling the failover: The Coding perspective.*<br>
>> Although the failover functions are made quorum and consensus aware but<br>
>> there is still a way to bypass the quorum conditions, and requirement of<br>
>> consensus.<br>
>><br>
>> For this the patch uses the existing request_details flags in<br>
>> POOL_REQUEST_NODE to control the behaviour of failover.<br>
>><br>
>> Here are the newly added flags values.<br>
>><br>
>> *REQ_DETAIL_WATCHDOG*:<br>
>> Setting this flag while issuing the failover command will not send the<br>
>> failover request to the watchdog. But this flag may not be useful in any<br>
>> other place than where it is already used.<br>
>> Mostly this flag can be used to avoid the failover command from going to<br>
>> watchdog that is already originated from watchdog. Otherwise we can end up<br>
>> in infinite loop.<br>
>><br>
>> *REQ_DETAIL_CONFIRMED*:<br>
>> Setting this flag will bypass the *failover_require_consensus*<br>
>> configuration and immediately perform the failover if quorum is present.<br>
>> This flag can be used to issue the failover request originated from PCP<br>
>> command.<br>
>><br>
>> *REQ_DETAIL_UPDATE*:<br>
>> This flag is used for the command where we are failing back the quarantine<br>
>> nodes. Setting this flag will not trigger the failback_command.<br>
>><br>
>> *Some conditional flags used:*<br>
>> I was not sure about the configuration of each type of failover operation.<br>
>> As we have three main failover operations NODE_UP_REQUEST,<br>
>> NODE_DOWN_REQUEST, and PROMOTE_NODE_REQUEST<br>
>> So I was thinking do we need to give the configuration option to the users,<br>
>> if they want to enable/disable quorum checking and consensus for individual<br>
>> failover operation type.<br>
>> For example: is it a practical configuration where a user would want to<br>
>> ensure quorum while preforming NODE_DOWN operation while does not want it<br>
>> for NODE_UP.<br>
>> So in this patch I use three compile time defines to enable disable the<br>
>> individual failover operation, while we can decide on the best solution.<br>
>><br>
>> NODE_UP_REQUIRE_CONSENSUS: defining it will enable quorum checking feature<br>
>> for NODE_UP_REQUESTs<br>
>><br>
>> NODE_DOWN_REQUIRE_CONSENSUS: defining it will enable quorum checking<br>
>> feature for NODE_DOWN_REQUESTs<br>
>><br>
>> NODE_PROMOTE_REQUIRE_<wbr>CONSENSUS: defining it will enable quorum checking<br>
>> feature for PROMOTE_NODE_REQUESTs<br>
>><br>
>> *Some Point for Discussion:*<br>
>><br>
>> *Do we really need to check ReqInfo->switching flag before enqueuing<br>
>> failover request.*<br>
>> While working on the patch I was wondering why do we disallow enqueuing the<br>
>> failover command when the failover is already in progress? For example in<br>
>> *pcp_process_command*() function if we see the *Req_info->switching* flag<br>
>> set we bailout with the error instead of enqueuing the command. Is is<br>
>> really necessary?<br>
>><br>
>> *Do we need more granule control over each failover operation:*<br>
>> As described in section "Some conditional flags used" I want the opinion on<br>
>> do we need configuration parameters in pgpool.conf to enable disable quorum<br>
>> and consensus checking on individual failover types.<br>
>><br>
>> *Which failover should be mark as Confirmed:*<br>
>> As defined in the above section of REQ_DETAIL_CONFIRMED, We can mark the<br>
>> failover request to not need consensus, currently the requests from the PCP<br>
>> commands are fired with this flag. But I was wondering there may be more<br>
>> places where we many need to use the flag.<br>
>> For example I currently use the same confirmed flag when failover is<br>
>> triggered because of *replication_stop_on_mismatch*<wbr>.<br>
>><br>
>> I think we should think this flag for each place of failover, like when the<br>
>> failover is triggered<br>
>> because of health_check failure.<br>
>> because of replication mismatch<br>
>> because of backend_error<br>
>> e.t.c<br>
>><br>
>> *Node Quarantine behaviour.*<br>
>> What do you think about the node quarantine used by this patch. Can you<br>
>> think of some problem which can be caused by this?<br>
>><br>
>> *What should be the default values for each newly added config parameters.*<br>
>><br>
>><br>
>><br>
>> *TODOs*<br>
>><br>
>> -- Updating the documentation is still todo. Will do that once every aspect<br>
>> of the feature will be finalised.<br>
>> -- Some code warnings and cleanups are still not done.<br>
>> -- I am still little short on testing<br>
>> -- Regression test cases for the feature<br>
>><br>
>><br>
>> Thoughts and suggestions are most welcome.<br>
>><br>
>> Thanks<br>
>> Best regards<br>
>> Muhammad Usama<br>
</div></div></blockquote></div><br></div></div>