MariaDB 10.4 has being declared GA, and few presentations on Galera4 in the last conferences were hold.

So, I thought, it is time to give it a try and see what is going on.

It is not a secret that I consider the Codership guys my heroes, and that I have push for Galera as solid and very flexible HA solution, for many years.

Given that my first comment is that it was a shame to have Galera4 available only in MariaDB, I would have preferred to test the MySQL vanilla version for many reasons, but mainly because the MySQL/Oracle is and remain the official and main line of the MySQL software, you like it or not, and as such I want to see how the Galera4 behave with that. Anyhow Codership state that the other versions will be out AFTER the summer, and I hope this will be true.

To test the new version given I do not have the vanilla MySQL, I decide to use the other distribution coming from Percona. At the end the test where done comparing MariaDB 10.4.x with PXC 5.7.x. In short Galera4 Vs Galera3.

I setup on the same machines the two different software, and I configure as close as possible. Said that I did 2 main set of tests: Data ingest and OLTP, both running for 90 minutes, not pushing like hell, but gently simulate some traffic. Configuration files can be found here.

Galera4 stream replication was disable, following the Codership instruction (wsrep_trx_fragment_size=0).

Test1 Ingest

For the ingest test I had use my stresstool application (here) with only 10 threads and 50 batch inserts each thread, the schema definition is in the windmills.json file.

As always, an image says more than many words:

ingest execution by thread

In general terms, PXC was able to execute same load in less than MariaDB.

ingest events thread

And PXC was able to deal with a higher number of events per thread as well.

The average galera latency was around 9ms in the writer and around 5ms for the receivers in PXC. With same load, same machines, same configuration:

Screen Shot 2019 07 24 at 21658 PM

The latency in MariaDB was significantly higher around 19ms for the writer, and between 9 and 5 ms for the receivers.

Screen Shot 2019 07 24 at 13350 PM

In short overall PXC 5.7 with galera3 was performing better than MariaDB 10.4 with galera4.

The amount of data on transmitted and received on PXC was higher (good) than Mariadb:

PXC:

Screen Shot 2019 07 24 at 22127 PM

MariaDB:

Screen Shot 2019 07 24 at 14453 PM

OLTP

For oltp test I have sysbenc with oltp r/w tests, 180 threads (90 from two different application nodes), 200K rows for table, 40 tables and 90 minutes run.

Let see what happened:

oltp evens write

PXC was performing better than MariaDB, executing more writes/s and and more events_thread/sec.

Checking the latency, we can see:

oltp latency

Also in this case PXC was having less average latency than MariaDB in the execution.

What about galera?

For PXC/Galera3, the average galera latency was around 3.5ms in the writer and less in the receivers:

Screen Shot 2019 07 26 at 124919 PM

In this case the latency in Galera4 was same or less of the one in Galera3:

Screen Shot 2019 07 26 at 51341 PM

Also analyzing the MAX latency:

Galera3

Screen Shot 2019 07 26 at 124936 PM

Galera4

Screen Shot 2019 07 26 at 51354 PM

We can see that Galera4 was dealing with it much better than the version3.

 

I have done many other checks and it seems to me that in the OLTP, but I do not exclude this is valid for ingest as well, Galera4 is in some way penalize by the MariaDB platform.

I am just at the start of my investigation and I may be wrong, but I cannot confirm or deny until Codership will release the code for MySQL.

Conclusions

Galera4 seems to come with some very good new feature, please review Seppo presentation, and one thing I noticed it comes with optimized node communication, reducing the latency fluctuation.

Just this for me is great, plus we will have stream replication that can be very useful but I have not tested it yet.

Nevertheless, I would not move to it just yet. I would wait to have the other MySQL distribution supported, do some tests, and see where the performance problem is.

Because at the moment also with not heavy load, current version of PXC 5.7/Galera3 runs better than MariaDB/Galera4, so why I should migrate to a platform that locks me in like MariaDB, and do not give me benefit (yet)? Also considering that once Galera4 will be available for the standard MySQL versions, we can have all the good coming from Galera4, without being lock in by MariaDB.

A small note about MariaDB, while I was playing with it, I noticed that by default MariaDB comes with the plugin level BETA, which means potentially run in production code that is still in beta stage, no comment!surprise emo

References

https://github.com/Tusamarco/blogs/tree/master/Galera4

https://galeracluster.com/

Seppo presentation https://www.slideshare.net/SakariKeskitalo/galera-cluster-4-presentation-in-percona-live-austin-2019

https://mariadb.org/mariadb-10-4-4-now-available/

mysql community Mariadb oracle Percona galera codership performance

During the last conferences, I had the chance to discuss a lot with many colleagues. Some of them, like me, feel we have lost ourselves, not totally but a bit. morepowerful together

I had start to think why, and this is my reasoning, not saying is right, but I think is not far from the truth.

Let us start just mentioning few events in the 2019 schedule, February Fosdem, end of May Percona live US, mid-June DataOps Barcelona, September Oracle open World, end of September-October Percona Live Europe, plus an undefined number of secondary events or MariaDB roadshows.

Where is the MySQL (and related) conference?

Do not get me wrong, we talk about MySQL a lot during each one of the above (or MariaDB, tometo/tomato). But it seems to me, that in a subtle way, the community had lost his conference.

There is nothing wrong if each company is trying to get the most out of their investment, so implicitly driving the show to get the best return for their own business.

But … as community are we getting what we need?

If we take a look to the past, we see that the MySQL conference in Santa Clara, was a real milestone. It is true that MySQL was a less mature product, and as such a lot of innovation and discussion were presented every year. But it was not only because that.

In my opinion the singularity of the event was making it a more precious moment, with a lot of focus and will to share.

Not only, the fact that we did not have dozens of events was grouping more people, as speakers/experts and attendee. This was generating more discussions and exchanges, resulting in ideas that we as community had the chance to develop in the forthcoming months.

Here another very important factor, we as part of the community, as engineers, respect each other and we have no limit in sharing knowledge and help each other. No matter if one of us make a mistake, next time he will do it right, and we had always help to go in that direction.

Having the chance to discuss face to face during the event, was great, having the chance to continue the discussion after the event even better, in theory having the chance to do that multiple time in the year should be the best thing. But… there is a but, all these smalls (or smaller) events are companies’ events, not community, sorry to say that but is the reality. What it means is that the interactions are, often, limited, restricted, penalized by the competition between the companies.

Mark Callaghan, some times ago, mention in a short post, that was sad to see so many bright engineers sparse cross companies, and often not able to collaborate to make things better in a common effort, following a shared path (Mark that was my interpretation if that was not your intention, my apology, but also if, I think this is true).

 

This is sad indeed, and a waste of energy. It is also very sad, because we know each other, directly or indirectly. We bring no ressentiment to each other, and we like to discuss, share, help and get help.

Damn we love to work together! No matter the company we are working with, I am sure Oracle developers would love to work with MariaDB guys, as well as Percona’s, and vice versa.

I am not naïve, I understand the need to have sane competition and the need to have “some” differences. But maybe we are not considering the long term, we are a small community, we are a very small fish ball, if we do not help each other we will lose in long term. This is already happening with Postgres growing day by day evolving under our blind eyes.

Looking our small ball, what I see is fragmentation, I see companies trying to find new magic word, or magic trends, to package the same old shit, to survive in the market.

That is not innovation, that is not evolution, that is giving up to what is the most important concept of an opensource community.

Going back to the number of events and their relevance. This increasing number of events, is the natural consequence of the lack of coordination and proper interaction. I do not care how many we will have, if they are bringing us, food for brain.

If they are going to be useful to make MySQL (and related) better.

But if the scope is only promoting the company business, that is ok… just it is not a community event, and in that case, we need to recognize and accept, that we do not have an open conference any longer. And we need one, well two. One for Americas, and another one for EurAsia (get use to the geo definition please, there is where the market is evolving).

We need a strong, unified, focused community. We need to have all the actors collaborate on a common strategy, not doing chicken fights for crumbs. We need a steering committee and the company’s commitment to adhere to the committee indications.

We need that, or we will be forgotten soon because obsolete.

That is what I think… please let me know your ideas, we are a community... let us discuss how to do things better!

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mysql community conferences Mariadb oracle Percona

ProxySQL in its versions up to 1.x did not natively support Percona XtraDB Cluster (PXC). Instead, it relied on the flexibility offered by the scheduler. This approach allowed users to implement their own preferred way to manage the ProxySQL behaviour in relation to the Galera events. From version 2.0 we can use native ProxySQL support for PXC.. The mechanism to activate native support is very similar to the one already in place for group replication. In brief it is based on the table [runtime_]mysql_galera_hostgroups and the information needed is mostly the same:

 

  • writer_hostgroup: the hostgroup ID that refers to the WRITER
  • backup_writer_hostgroup: the hostgoup ID referring to the Hostgorup that will contain the candidate servers
  • reader_hostgroup: The reader Hostgroup ID, containing the list of servers that need to be taken in consideration
  • offline_hostgroup: The Hostgroup ID that will eventually contain the writer that will be put OFFLINE
  • active: True[1]/False[0] if this configuration needs to be used or not
  • max_writers: This will contain the MAX number of writers you want to have at the same time. In a sane setup this should be always 1, but if you want to have multiple writers, you can define it up to the number of nodes.
  • writer_is_also_reader: If true [1] the Writer will NOT be removed from the reader HG
  • max_transactions_behind: The number of wsrep_local_recv_queue after which the node will be set OFFLINE. This must be carefully set, observing the node behaviour.
  • comment: I suggest to put some meaningful notes to identify what is what.

Given the above let us see what we need to do in order to have a working galera native solution. I will have three Servers:

  192.168.1.205 (Node1)
  192.168.1.21  (Node2)
  192.168.1.231 (node3)

As set of Hostgroup, I will have:

Writer  HG-> 100
Reader  HG-> 101
BackupW HG-> 102
offHG   HG-> 9101

To set it up

Servers first:
 
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight) VALUES ('192.168.1.205',101,3306,1000);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight) VALUES ('192.168.1.21',101,3306,1000);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight) VALUES ('192.168.1.231',101,3306,1000);

Then the galera settings:
 
insert into mysql_galera_hostgroups (writer_hostgroup,backup_writer_hostgroup,reader_hostgroup, offline_hostgroup,active,max_writers,writer_is_also_reader,max_transactions_behind) values (100,102,101,9101,0,1,1,16);

As usual if we want to have R/W split we need to define the rules for it:
 
insert into mysql_query_rules (rule_id,proxy_port,schemaname,username,destination_hostgroup,active,retries,match_digest,apply) values(1040,6033,'windmills','app_test',100,1,3,'^SELECT.*FOR UPDATE',1);
insert into mysql_query_rules (rule_id,proxy_port,schemaname,username,destination_hostgroup,active,retries,match_digest,apply) values(1041,6033,'windmills','app_test',101,1,3,'^SELECT.*@@',1);

save mysql query rules to disk;
load mysql query rules to run;

Then another important variable... the server version, please do yourself a good service ad NEVER use the default.
 
update global_variables set variable_value='5.7.0' where variable_name='mysql-server_version';
LOAD MYSQL VARIABLES TO RUNTIME;SAVE MYSQL VARIABLES TO DISK;

Finally activate the whole thing:
 
save mysql servers to disk;
load mysql servers to runtime;

 

 

 

One thing to note before we go ahead. In the list of servers I had:

  1. Filled only the READER HG
  2. Used the same weight

This because of the election mechanism ProxySQL will use to identify the writer, and the (many) problems that may be attached to it.

For now let us go ahead and see what happens when I load this information to runtime. Before running the above commands:

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+--------+-----------+---------------+----------+---------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+
| weight | hostgroup | srv_host      | srv_port | STATUS  | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed | Queries | Queries_GTID_sync | Bytes_data_sent | Bytes_data_recv | Latency_us |
+--------+-----------+---------------+----------+---------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+
+--------+-----------+---------------+----------+---------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+
After:
 
+--------+-----------+---------------+----------+---------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+
| weight | hostgroup | srv_host      | srv_port | STATUS  | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed | Queries | Queries_GTID_sync | Bytes_data_sent | Bytes_data_recv | Latency_us |
+--------+-----------+---------------+----------+---------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+
| 1000   | 100       | 192.168.1.231 | 3306     | ONLINE  | 0        | 0        | 0	 | 0	   | 0           | 0	   | 0                 | 0               | 0               | 501        |
| 1000   | 101       | 192.168.1.231 | 3306     | ONLINE  | 0        | 0        | 0	 | 0	   | 0           | 0	   | 0                 | 0               | 0               | 501        |
| 1000   | 101       | 192.168.1.21  | 3306     | ONLINE  | 0        | 0        | 0	 | 0	   | 0           | 0	   | 0                 | 0               | 0               | 546        |
| 1000   | 101       | 192.168.1.205 | 3306     | ONLINE  | 0        | 0        | 0	 | 0	   | 0           | 0	   | 0                 | 0               | 0               | 467        |
| 1000   | 102       | 192.168.1.21  | 3306     | ONLINE  | 0        | 0        | 0	 | 0	   | 0           | 0	   | 0                 | 0               | 0               | 546        |
| 1000   | 102       | 192.168.1.205 | 3306     | ONLINE  | 0        | 0        | 0	 | 0	   | 0           | 0	   | 0                 | 0               | 0               | 467        |
+--------+-----------+---------------+----------+---------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+

 

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mysql> SELECT * FROM runtime_mysql_galera_hostgroups \G
*************************** 1. row ***************************
       writer_hostgroup: 100
backup_writer_hostgroup: 102
       reader_hostgroup: 101
      offline_hostgroup: 9101
                active: 0  <----------- note this 
            max_writers: 1
  writer_is_also_reader: 1
max_transactions_behind: 16
                comment: NULL
1 row IN SET (0.01 sec)

 

As we can see, ProxySQL had taken the nodes from my READER group and distribute them adding node 1 in the writer and node 2 as backup_writer.
But – there is a but – wasn't my rule set with Active=0? Indeed it was, and I assume this is a bug (#Issue  1902).
The other thing we should note is that ProxySQL had elected as writer node 3 (192.168.1.231).
As I said before what should we do IF we want to have a specific node as preferred writer? We need to modify its weight.
So say we want to have node 1 (192.168.1.205) as writer we will need something like this:

 
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INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight) VALUES ('192.168.1.205',101,3306,10000);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight) VALUES ('192.168.1.21',101,3306,100);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight) VALUES ('192.168.1.231',101,3306,100);
 

Doing that will give us :

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+--------+-----------+---------------+----------+--------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+
| weight | hostgroup | srv_host      | srv_port | STATUS | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed | Queries | Queries_GTID_sync | Bytes_data_sent | Bytes_data_recv | Latency_us |
+--------+-----------+---------------+----------+--------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+
| 10000  | 100       | 192.168.1.205 | 3306     | ONLINE | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 2209       |
| 100    | 101       | 192.168.1.231 | 3306     | ONLINE | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 546        |
| 100    | 101       | 192.168.1.21  | 3306     | ONLINE | 0        | 0        | 0      | 0	  | 0           | 0	  | 0                 | 0               | 0               | 508        |
| 10000  | 101       | 192.168.1.205 | 3306     | ONLINE | 0        | 0        | 0      | 0	  | 0           | 0	  | 0                 | 0               | 0               | 2209       |
| 100    | 102       | 192.168.1.231 | 3306     | ONLINE | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 546        |
| 100    | 102       | 192.168.1.21  | 3306     | ONLINE | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 508        |
+--------+-----------+---------------+----------+--------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+

If you noticed, given we had set the WEIGHT in node 1 higher, this node will become also the most utilized for reads.

We probably do not want that, so let us modify the reader weight.

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UPDATE mysql_servers SET weight=10 WHERE hostgroup_id=101 AND hostname='192.168.1.205';

At this point if we trigger the failover, with set global wsrep_reject_queries=all; on node 1. ProxySQL will take action and will elect another node as writer:

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+--------+-----------+---------------+----------+---------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+
| weight | hostgroup | srv_host      | srv_port | STATUS  | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed | Queries | Queries_GTID_sync | Bytes_data_sent | Bytes_data_recv | Latency_us |
+--------+-----------+---------------+----------+---------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+
| 100    | 100       | 192.168.1.231 | 3306     | ONLINE  | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 562        |
| 100    | 101       | 192.168.1.231 | 3306     | ONLINE  | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 562        |
| 100    | 101       | 192.168.1.21  | 3306     | ONLINE  | 0        | 0        | 0      | 0	      | 0           | 0	      | 0                 | 0               | 0               | 588        |
| 100    | 102       | 192.168.1.21  | 3306     | ONLINE  | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 588        |
| 10000  | 9101      | 192.168.1.205 | 3306     | ONLINE  | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 468        |
+--------+-----------+---------------+----------+---------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+

 

Node 3 (192.168.1.231) is the new writer and node 1 is in the special group for OFFLINE. Let see now what will happen IF we put back node 1.

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+--------+-----------+---------------+----------+---------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+
| weight | hostgroup | srv_host      | srv_port | STATUS  | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed | Queries | Queries_GTID_sync | Bytes_data_sent | Bytes_data_recv | Latency_us |
+--------+-----------+---------------+----------+---------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+
| 10000  | 100       | 192.168.1.205 | 3306     | ONLINE  | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 449        |
| 100    | 101       | 192.168.1.231 | 3306     | ONLINE  | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 532        |
| 100    | 101       | 192.168.1.21  | 3306     | ONLINE  | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 569        |
| 10000  | 101       | 192.168.1.205 | 3306     | ONLINE  | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 449        |
| 100    | 102       | 192.168.1.231 | 3306     | ONLINE  | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 532        |
| 100    | 102       | 192.168.1.21  | 3306     | ONLINE  | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 569        |
+--------+-----------+---------------+----------+---------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+

Ooops the READER has come back with the HIGHEST value and as such it will be the most used node, once more. To fix it, we need to re-run the update as before.

But there is a way to avoid this? In short the answer is NO! This, in my opinion, is BAD and is worth a feature request, because this can really put a node on the knees.

Now this is not the only problem. There is another point that is probably worth discussion, which is the fact ProxySQL is currently doing FAILOVER/FAILBACK.

Failover, is obviously something we want to have, but failback is another discussion.

The point is, once the failover is complete and the cluster has redistributed the incoming requests, doing a failback is an impacting operation that can be a disruptive one too.

If all nodes are treated as equal, there is no real way to prevent it, while if YOU set a node to be the main writer, something can be done, let us see what and how. Say we have:

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INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight) VALUES ('192.168.1.205',101,3306,1000);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight) VALUES ('192.168.1.21',101,3306,100);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight) VALUES ('192.168.1.231',101,3306,100);
 
+--------+-----------+---------------+----------+--------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+
| weight | hostgroup | srv_host      | srv_port | STATUS | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed | Queries | Queries_GTID_sync | Bytes_data_sent | Bytes_data_recv | Latency_us |
+--------+-----------+---------------+----------+--------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+
| 1000   | 100       | 192.168.1.205 | 3306     | ONLINE | 0        | 0        | 0      | 0	  | 0           | 0	  | 0                 | 0               | 0               | 470        |
| 100    | 101       | 192.168.1.231 | 3306     | ONLINE | 0        | 0        | 0      | 0	  | 0           | 0	  | 0                 | 0               | 0               | 558        |
| 100    | 101       | 192.168.1.21  | 3306     | ONLINE | 0        | 0        | 0      | 0	  | 0           | 0	  | 0                 | 0               | 0               | 613        |
| 10     | 101       | 192.168.1.205 | 3306     | ONLINE | 0        | 0        | 0      | 0	  | 0           | 0	  | 0                 | 0               | 0               | 470        |
| 100    | 102       | 192.168.1.231 | 3306     | ONLINE | 0        | 0        | 0      | 0	  | 0           | 0	  | 0                 | 0               | 0               | 558        |
| 100    | 102       | 192.168.1.21  | 3306     | ONLINE | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 613        |
+--------+-----------+---------------+----------+--------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+

Let us put the node down set global wsrep_reject_queries=all; And check:

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+--------+-----------+---------------+----------+---------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+
| weight | hostgroup | srv_host      | srv_port | STATUS  | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed | Queries | Queries_GTID_sync | Bytes_data_sent | Bytes_data_recv | Latency_us |
+--------+-----------+---------------+----------+---------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+
| 100    | 100       | 192.168.1.231 | 3306     | ONLINE  | 0        | 0        | 0      | 0	      | 0           | 0	      | 0                 | 0               | 0               | 519        |
| 100    | 101       | 192.168.1.231 | 3306     | ONLINE  | 0        | 0        | 0      | 0	      | 0           | 0	      | 0                 | 0               | 0               | 519        |
| 100    | 101       | 192.168.1.21  | 3306     | ONLINE  | 0        | 0        | 0      | 0	      | 0           | 0	      | 0                 | 0               | 0               | 506        |
| 100    | 102       | 192.168.1.21  | 3306     | ONLINE  | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 506        |
| 1000   | 9101      | 192.168.1.205 | 3306     | ONLINE  | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 527        |
+--------+-----------+---------------+----------+---------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+

We can now manipulate the weight in the special OFFLINE group and see what happen:

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UPDATE mysql_servers SET weight=10 WHERE hostgroup_id=9101 AND hostname='192.168.1.205'

Then I put the node up again: set global wsrep_reject_queries=none;

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+--------+-----------+---------------+----------+---------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+
| weight | hostgroup | srv_host      | srv_port | STATUS  | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed | Queries | Queries_GTID_sync | Bytes_data_sent | Bytes_data_recv | Latency_us |
+--------+-----------+---------------+----------+---------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+
| 100    | 100       | 192.168.1.231 | 3306     | ONLINE  | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 537        |
| 100    | 101       | 192.168.1.231 | 3306     | ONLINE  | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 537        |
| 100    | 101       | 192.168.1.21  | 3306     | ONLINE  | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 573        |
| 10     | 101       | 192.168.1.205 | 3306     | ONLINE  | 0        | 0        | 0      | 0	   | 0           | 0	   | 0                 | 0               | 0               | 458	|
| 100    | 102       | 192.168.1.21  | 3306     | ONLINE  | 0        | 0        | 0      | 0	   | 0           | 0	   | 0                 | 0               | 0               | 573	|
| 10     | 102       | 192.168.1.205 | 3306     | ONLINE  | 0        | 0        | 0      | 0       | 0           | 0       | 0                 | 0               | 0               | 458        |
+--------+-----------+---------------+----------+---------+----------+----------+--------+---------+-------------+---------+-------------------+-----------------+-----------------+------------+

 

That's it, the node is back but with no service interruption.

At this point we can decide if make this node reader like the others, or wait and plan a proper time of the day when we can put it back as writer, while, in the meanwhile it has a bit of load to warm its bufferpool.

The other point – and important information – is what is ProxySQL is currently checking on Galera? From reading the code Proxy will trap the following:

  • read_only
  • wsrep_local_recv_queue
  • wsrep_desync
  • wsrep_reject_queries
  • wsrep_sst_donor_rejects_queries
  • primary_partition

Plus the standard sanity checks on the node. Finally to monitor the whole situation we can use this:

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mysql> SELECT * FROM mysql_server_galera_log ORDER BY time_start_us DESC LIMIT 10;
+---------------+------+------------------+-----------------+-------------------+-----------+------------------------+-------------------+--------------+----------------------+---------------------------------+-------+
| hostname      | port | time_start_us    | success_time_us | primary_partition | read_only | wsrep_local_recv_queue | wsrep_local_state | wsrep_desync | wsrep_reject_queries | wsrep_sst_donor_rejects_queries | error |
+---------------+------+------------------+-----------------+-------------------+-----------+------------------------+-------------------+--------------+----------------------+---------------------------------+-------+
| 192.168.1.231 | 3306 | 1549982591661779 | 2884            | YES               | NO        | 0                      | 4                 | NO           | NO                   | NO                              | NULL  |
| 192.168.1.21  | 3306 | 1549982591659644 | 2778            | YES               | NO        | 0                      | 4                 | NO           | NO                   | NO                              | NULL  |
| 192.168.1.205 | 3306 | 1549982591658728 | 2794            | YES               | NO        | 0                      | 4                 | NO           | YES                  | NO                              | NULL  |
| 192.168.1.231 | 3306 | 1549982586669233 | 2827            | YES               | NO        | 0                      | 4                 | NO           | NO                   | NO                              | NULL  |
| 192.168.1.21  | 3306 | 1549982586663458 | 5100            | YES               | NO        | 0                      | 4                 | NO           | NO                   | NO                              | NULL  |
| 192.168.1.205 | 3306 | 1549982586658973 | 4132            | YES               | NO        | 0                      | 4                 | NO           | YES                  | NO                              | NULL  |
| 192.168.1.231 | 3306 | 1549982581665317 | 3084            | YES               | NO        | 0                      | 4                 | NO           | NO                   | NO                              | NULL  |
| 192.168.1.21  | 3306 | 1549982581661261 | 3129            | YES               | NO        | 0                      | 4                 | NO           | NO                   | NO                              | NULL  |
| 192.168.1.205 | 3306 | 1549982581658242 | 2786            | YES               | NO        | 0                      | 4                 | NO           | NO                   | NO                              | NULL  |
| 192.168.1.231 | 3306 | 1549982576661349 | 2982            | YES               | NO        | 0                      | 4                 | NO           | NO                   | NO                              | NULL  |
+---------------+------+------------------+-----------------+-------------------+-----------+------------------------+-------------------+--------------+----------------------+---------------------------------+-------+

As you can see above the log table keeps track of what is changed. In this case, it reports that node 1 has wsrep_reject_queries activated, and it will continue to log this until we set wsrep_reject_queries=none.

Conclusions

ProxySQL galera native integration is a useful feature to manage any Galera implementation, no matter whether it's Percona PXC, MariaDB cluster or MySQL/Galera.

The generic approach is obviously a good thing, still it may miss some specific extension like we have in PXC with the performance_schema pxc_cluster_view table.

I've already objected about the failover/failback, and I am here again to remind you: whenever you do a controlled failover REMEMBER to change the weight to prevent an immediate failback.

This is obviously not possible in the case of a real failover, and, for instance, a simple temporary eviction will cause two downtimes instead only one.

Some environments are fine with that others not so. Personally I think there should be a FLAG in the configuration, such that we can decide if failback should be executed or not.  

More Articles ...

  1. How Not to do MySQL High Availability: Geographic Node Distribution with Galera-Based Replication Misuse
  2. MySQL High Availability On-Premises: A Geographically Distributed Scenario
  3. MySQL 8: Load Fine Tuning With Resource Groups
  4. PXC loves firewalls (and System Admins loves iptables)
  5. No orange pants this year
  6. Leveraging ProxySQL with AWS Aurora to Improve Performance
  7. How to Implement ProxySQL with AWS Aurora
  8. ProxySQL server version impersonation
  9. ProxySQL Firewalling
  10. ProxySQL PXC Single Writer Mode and auto failover, re-bootstrap
  11. How ProxySQL deal with schema (and schemaname)
  12. How ProxySQL deal with schema (and schemaname) Long story
  13. Sweet and sour can become bitter
  14. Group-Replication, sweet & sour
  15. ProxySQL and Mirroring what about it?
  16. InnoDB Page Merging and Page Splitting
  17. Setup ProxySQL as High Available (and not a SPOF)
  18. ProxySQL – Percona Cluster (Galera) integration
  19. ProxySQL and MHA integration

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