Using SKIP LOCK in MySQL For Queue Processing

A small thing that brings huge help.

The other day I was writing some code to process a very large amount of items coming from a social media API. My items were ending in a queue in MySQL and then needed to be processed and eventually moved.

The task was not so strange, but what I have to do is to develop a queue processor. Now when you need to process a queue you have two types of queue: static and dynamic.

The static comes in a batch of N number of items in a given time interval and is normally easier to process given you have a defined number of items that you can split in chunks and process in parallel.

The dynamic is… well... more challenging. One option is to wait to have a predefined number of items, and then process them as if they were a static queue.

But this approach is not very good, given it is possible that it will delay a lot the processing of an item for all the time it has to wait to reach the desired queue’s size.

The other possibility is to have the processing jobs work on a single item and not in chunk/batch. But, this is not optimal when given the chance to process a queue in batch to speed up the processing time.

My incoming queue is a bit unpredictable, a mix of fixed sizes and a few thousand coming sparse, without a clear interval. So I was there thinking on how to process this and already starting to design a quite complex mechanism to dynamically calculate the size of the possible chunks and the number of jobs, when…

An aside: some colleagues know my habit to read the whole MySQL manual, from A to Z, at least once a year. It's a way for me to review what is going on and sometimes to dig in more in some aspects. This normally also gives me a good level of confidence about new features and other changes on top of reading the release notes.

...When … looking at the documentation for something else, my attention was captured by:

“To avoid waiting for other transactions to release row locks, NOWAIT and SKIP LOCKED options may be used with SELECT ... FOR UPDATE or SELECT ... FOR SHARE locking read statements.”

Wait - what???

Let me dig in a bit:

“SKIP LOCKED. A locking read that uses SKIP LOCKED never waits to acquire a row lock. The query executes immediately, removing locked rows from the result set.”

Wow, how could I have missed that?

It was also not new but in MySQL 8.0.1, the milestone release. Having experience with Oracle, I knew what SKIP LOCKED does and how to use it. But I was really not aware that it was also available in MySQL.

In short, SKIP LOCKED allows you to lock a row (or set of them), bypassing the rows already locked by other transactions.

The classic example is:

# Session 1:
mysql> CREATE TABLE t (i INT, PRIMARY KEY (i)) ENGINE = InnoDB;
mysql> INSERT INTO t (i) VALUES(1),(2),(3);
mysql> START TRANSACTION;
mysql> SELECT * FROM t WHERE i = 2 FOR UPDATE;
+---+
| i |
+---+
| 2 |
+---+

# Session 2:
mysql> START TRANSACTION;
mysql> SELECT * FROM t FOR UPDATE SKIP LOCKED;
+---+
| i |
+---+
| 1 |
| 3 |
+---+

But what is important for me is that given an N number of jobs running, I can bypass all the effort of calculating the dynamic chunks, given that using SKIP LOCKED that will happen as well, if in a different way.

All good, but what performance will I have using SKIP LOCK in comparison to the other two solutions?

I have run the following tests on my laptop, so not a real server, and used a fake queue processor I wrote on the fly to test the things you can find on GitHub here.

What I do is to create a table like this:

CREATE TABLE `jobs` (
  `jobid` int unsigned NOT NULL AUTO_INCREMENT,
  `time_in` bigint NOT NULL,
  `time_out` bigint DEFAULT '0',
  `worked_time` bigint DEFAULT '0',
  `processer` int unsigned DEFAULT '0',
  `info` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`jobid`),
  KEY `idx_time_in` (`time_in`,`time_out`),
  KEY `idx_time_out` (`time_out`,`time_in`),
  KEY `processer` (`processer`)
) ENGINE=InnoDB AUTO_INCREMENT=0 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

Then I will do three different methods of processing:

Simple process, reading and writing a row a time
Use chunks to split my queue
Use SKIP LOCKED

To clarify the difference existing between the 3 different way of processing the queue let us use 3 simple images:

proc1

In simple processing each row represent a possible lock that the other processes must wait for.

proc2

In chunk processing given each process knows what records to lock they can go in parallel.

proc3

In SKIP LOCKED also if each process have no idea of what rows they need to lock, is enough to say the size of the chunk, and MySQL will return the records available.

I will repeat the tests for a static queue, and after for a dynamic queue for 20 seconds. Let's see what happens.

Test one - simple processing and static queue:

queue Processor report:
=========================
Start Time                = Tue Jul 28 13:28:25 CEST 2020
End Time                  = Tue Jul 28 13:28:31 CEST 2020
Total Time taken(ms)      =     6325.0
Number of jobs            =          5
Number of Items processed =      15000
Avg records proc time     =     220308

Test 2 use the chunks and static queue

Chunk no loop
--------------
queue Processor report:
=========================
Start Time                = Tue Jul 28 13:30:18 CEST 2020
End Time                  = Tue Jul 28 13:30:22 CEST 2020
Total Time taken(ms)      =     4311.0
Number of jobs            =          5
Number of Items processed =      15000
Avg records proc time     =     391927

Test three - use SKIP LOCKED and static queue:

SKIP LOCK - no loop
------------------------
queue Processor report:
=========================
Start Time                = Tue Jul 28 13:32:07 CEST 2020
End Time                  = Tue Jul 28 13:32:11 CEST 2020
Total Time taken(ms)      =     4311.0
Number of jobs            =          5
Number of Items processed =      15000
Avg records proc time     =     366812

So far, so good.

Time is almost the same (actually in this test, it's exactly the same), normally fluctuating a bit up and down by a few ms.

Average execution by commit fluctuates a bit:

Here, the base execution is faster for the simple reason that the application is processing one record against a batch of records of the other two.

Now it is time to see what happens if instead of a static batch, I have a process that fills the queue constantly. If you want picture what will happen on each test, just imagine this:

Some pipes will put water in a huge tank
The base solution will try to empty the tank using a small glass of water but acting very fast at each run
With Chunk it will wait for the water to reach a specific level, then will use a fixed-size bucket
Using SKIP LOCK, it will constantly look at the tank and will choose the size of the bucket based on the quantity of the water present.

To simulate that, I will use five threads to write new items, five to process the queue, and will run the test for 20 seconds.

Here we will have some leftovers; that is how much water remains in the tank because the application was not emptied with the given bucket. We can say it is a way to measure the efficiency of the processing, where the optimal sees the tank empty.

Test one - simple processing and static queue:

Basic no loop
--------------
queue Processor report:
=========================
Start Time                = Tue Jul 28 13:42:37 CEST 2020
End Time                  = Tue Jul 28 13:43:25 CEST 2020
Total Time taken(ms)      =    48586.0
Number of jobs            =          5
Number of Items processed =      15000
Total Loops executed (sum)=         85
Avg records proc time     =     243400

Leftover
+----------+------------+
| count(*) | max(jobId) |
+----------+------------+
|   143863 |     225000 |
+----------+------------+

Test 2 use the chunks and static queue:

Chunk no loop
--------------
queue Processor report:
=========================
Start Time                = Tue Jul 28 13:44:56 CEST 2020
End Time                  = Tue Jul 28 13:45:44 CEST 2020
Total Time taken(ms)      =    47946.0
Number of jobs            =          5
Number of Items processed =      15000
Total Loops executed (sum)=         70
Avg records proc time     =     363559

Leftover
+----------+------------+
| count(*) | max(jobId) |
+----------+------------+
|       53 |     195000 |
+----------+------------+

Test 3 use SKIP LOCKED and static queue:

queue Processor report:
=========================
Start Time                = Tue Jul 28 14:46:45 CEST 2020
End Time                  = Tue Jul 28 14:47:32 CEST 2020
Total Time taken(ms)      =    46324.0
Number of jobs            =          5
Number of Items processed =      15000
Total Loops executed (sum)=       1528
Avg records proc time     =     282658

Leftover
+----------+------------+
| count(*) | max(jobId) |
+----------+------------+
|        0 |       NULL |
+----------+------------+

Here, the scenario is a bit different than the one we had with the static queue.

Record processing when comparing by chunk and SKIP LOCK is again more efficient in the second one. This is because it optimizes the size of the “bucket” and given that it can sometimes process fewer records per loop.

As we can see when using SKIP LOCK, the application was able to execute 1528 loops to process the queue against the 70 of the chunk and 85 of the basic approach.

In the end, the only one that was able to empty the tank was the solution with SKIP LOCK.

Conclusion

Processing queues can be simple when we need to process a fixed number of items, but if you need an adaptive approach, then the situation changes. You can find yourself writing quite complex algorithms to optimize the processing.

Using SKIP LOCK helps you in keeping the code/solution simple and move the burden of identifying the record to process onto the RDBMS.

SKIP LOCK is something that other technologies like Oracle-DB and Postgres already implemented, and their developer communities use.

MySQL implementation comes a bit later, and the option is not widely known or used in the developer’s community using MySQL, but it should.

Give it a try and let us know!

NOTE !!

SKIP LOCK is declared unsafe for statement replication, you MUST use ROW based replication if you use it.

References

MySQL 8.0 Hot Rows with NOWAIT and SKIP LOCKED

MySQL 8.0 Reference Manual: Locking Reads

{jcomments on}

Deadlocks are our Friends

Why another article on this Marco?

Deadlocks is a topic covered many times and with a lot of articles on the web, also from Percona.
I suggest you review the reference section for articles on how to identify Deadlocks and from where they are generated.
So why another article?
The answer is that messages like the following are still very common:

User (John): “Marco our MySQL is having problems”
Marco: “Ok John what problems. Can you be a bit more specific?”
John: “Our log scraper is collecting that MySQL has a lot of errors”
Marco: “Ok can you share the MySQL log so I can review it?”
John: “Errors are in the application log, will share one application log”

Marco reviews the log and in it he founds:

“ERROR 1213 (40001): Deadlock found when trying to get lock;
try restarting transaction”

Marco reaction is: "Oh my ..." headdesk

Why? Because deadlocks are not what is express in the message, and of course we have a problem of mindset and last but not least terminology.

In this very short article I will try to change your point of view around Deadlocks.

What is a deadlock?

A deadlock is a situation wherein two or more competing actions are waiting for the other to finish. As a consequence, neither ever does.
In computer science, deadlock refers to a specific condition when two or more processes are each waiting for each other to release a resource.
In order for a deadlock to happen 4 conditions (Coffman conditions) should exists:
Mutual exclusion: At least one resource must be held in a non-shareable mode. Otherwise, the processes would not be prevented from using the resource when necessary. Only one process can use the resource at any given instant of time.
Hold and wait or resource holding: a process is currently holding at least one resource and requesting additional resources which are being held by other processes.
No preemption: a resource can be released only voluntarily by the process holding it.
Circular wait: each process must be waiting for a resource which is being held by another process, which in turn is waiting for the first process to release the resource.

All the above illustrates conditions that are not bound to RDBMS only but to any system dealing with data transaction processing. In any case it is a fact that today in most cases deadlocks are not avoidable unless to prevent one of the above conditions to happen without compromising the system execution integrity. Breaking or ignoring one of the above rules, especially for RDBMS, could affect data integrity, which will go against the reason to exist of a RDBMS.

Just to help us to better contextualize, let us review a simple case of Deadlock.
Say I have MySQL with the World schema loaded, and I have the TWO transactions running, both looking for the same 2 cities in Tuscany (Firenze and Prato) but in different order.

mysql> select * from City where CountryCode = 'ITA' and District='Toscana';
+------+---------+-------------+----------+------------+
| ID   | Name    | CountryCode | District | Population |
+------+---------+-------------+----------+------------+
| 1471 | Firenze | ITA    | Toscana       | 376662     | <---
| 1483 | Prato   | ITA    | Toscana       |  172473    | <--- ...
+------+---------+-------------+----------+------------+

And both transactions are updating the population: 

Connection 1 will have: connection1 > start transaction;
Query OK, 0 rows affected (0.01 sec)

connection1 > select * from City where ID=1471;
+------+---------+-------------+----------+------------+
| ID   | Name    | CountryCode | District | Population |
+------+---------+-------------+----------+------------+
| 1471 | Firenze | ITA         | Toscana  | 376662     |
+------+---------+-------------+----------+------------+
1 row in set (0.00 sec)

connection1 > update City set Population=Population + 1 where ID = 1471;
Query OK, 1 row affected (0.05 sec)
Rows matched: 1 Changed: 1 Warnings: 0

connection1 > update City set Population=Population + 1 where ID = 1483;
Query OK, 1 row affected (2.09 sec)
Rows matched: 1 Changed: 1 Warnings: 0

Connection 2 will have:
connection 2 >start transaction;
Query OK, 0 rows affected (0.01 sec)

connection 2 >select * from City where ID=1483;
+------+-------+-------------+----------+------------+
| ID   | Name  | CountryCode | District | Population |
+------+-------+-------------+----------+------------+
| 1483 | Prato | ITA         | Toscana  | 172473     |
+------+-------+-------------+----------+------------+
1 row in set (0.01 sec)

connection 2 >update City set Population=Population + 1 where ID = 1483;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0

connection 2 >update City set Population=Population + 1 where ID = 1471;
ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction

This is a very simple example of deadlock detection
An image may help:

dl ff 1

If we stop a second and ignore the word “ERROR” in the message, what is really happening is that MySQL is preventing us from modifying the data in the wrong way. If the locks would not be in place one of the two transactions would modify the population incrementing a number that is not valid anymore.
The right thing to do is to abort one of the two transactions and NOTIFY the application that, if you really need to perform the action, in this case increase the population, better to redo the execution and be sure it is still the case.
Just think, it could happen that the application re-run transactions 2 and identify there is no need to increase the value because it is already what it is supposed to be.

Think if you are calculating the financial situation of your company and you and your colleague are processing the same data but for different tasks.
Without locks & deadlocks you may end up in corrupting each other's interpretation of the data, and perform wrong operations. As a result you may end up paying the wrong salaries or worse.

Given that, and more, deadlocks (and locks) needs to be seen as friends helping us in keeping our data consistent.
The problem raise, when we have applications poorly designed and developed, and unfortunately by the wrong terminology (in my opinion) in MySQL.

Let us start with MySQL, Deadlock detection is detecting an intrinsic inevitable condition in the RDBMS/ACID world. As such defining it an ERROR is totally misleading. A deadlock is a CONDITION, and its natural conclusion is the abortion of one of the transactions reason of the deadlock.
The message should be a NOTIFICATION not an ERROR.

The problem in the apps instead, is that normally the isolation and validation of the data is demanded to RDBMS, which is fine. But then only seldom can we see applications able to deal with messages like lock-timeout or deadlock. This is of course a huge pitfall, because while it is natural to have the RDBMS dealing with the data consistency, it is not, and should not, be responsible for the retry that is bound to the application logic.
Nowadays we have a lot of applications that require very fast execution, and locks and deadlocks are seen as enemies because they have a cost in time.

But this is a mistake, a design mistake. Because if you are more willing to have speed instead of data consistency, then you should not use a RDBMS that must respect specific rules, at any (time) cost.
Other systems to store data (eventually consistent) will be more appropriate in your case.

While if you care about your data, then you need to listen to your RDBMS and write the code in a way, you will get all the benefit out of it, also when it comes to deadlocks.

Conclusion

Deadlocks (and locks), should be seen as friends. They are mechanisms that exist to keep our data consistent. We should not bypass them unless willing to compromise our data.

As previously indicated, if you want to understand in the details how to diagnose a deadlock review the links in the reference.

References
https://www.percona.com/blog/2012/09/19/logging-deadlocks-errors/
https://www.percona.com/blog/2014/10/28/how-to-deal-with-mysql-deadlocks/
https://www.percona.com/community-blog/2018/09/24/minimize-mysql-deadlocks-3-steps/

Achieving Consistent Read and High Availability with Percona XtraDB Cluster 8.0 (Part 2)

Back to part 1

Now that we had seen how to setup our environment with PXC8 is time to see how our setup will behave and what can be done in case of issues.

We will now analyse the following common situations:
- Put a node in maintenance or remove a node and insert it back
- node crash
- network failure
- proxysql node crash

The following tests are done using a java application connecting using straight JDBC connection and no connection pooling. I choose to use that more than a benchmarking tool such as sysbench because I was looking for something as close to a real application more than a benchmark. Especially I was interested in having dependencies cross requests and real data retrieve and error management.

As soon as we start the schedule script, the script checks the nodes and identify that we have writer_is_also_reader=0 so it removes the entry for the current writer

2020/07/03 06:43:50.130:[INFO] Writer is also reader disabled removing node from reader Hostgroup 192.168.1.5;3306;201;5000 Retry #1
2020/07/03 06:43:50.138:[WARN] DELETE node:192.168.1.5;3306;201;5000 SQL: DELETE from mysql_servers WHERE hostgroup_id=201 AND hostname='192.168.1.5' AND port='3306'
2020/07/03 06:43:50.143:[INFO] END EXECUTION Total Time(ms):277.705907821655

Test 1 Put a node in maintenance

PXC has a very useful feature pxc_maint_mode to deal with maintenance and to notify applications and midlevel architectural blocks (such as ProxySQL) that a node is going to be under maintenance.

With pxc_maint_mode you can specifies the maintenance mode for taking a node down without adjusting settings in ProxySQL.

The following values are available:

DISABLED: This is the default state that tells ProxySQL to route traffic to the node as usual.
SHUTDOWN: This state is set automatically when you initiate node shutdown.
MAINTENANCE: You can manually change to this state if you need to perform maintenance on a node without shutting it down.

The First test is to put a reader in maintenance and put it back.
current scenario in ProxySQL:

+--------+--------------+-------------+--------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+--------+----------+----------+--------+---------+-------------+
| 10000  | 200          | 192.168.1.5 | ONLINE | 2        | 3        | 5      | 0	| 5           |
| 10000  | 201          | 192.168.1.7 | ONLINE | 2        | 15       | 17     | 0	| 17          |
| 10000  | 201          | 192.168.1.6 | ONLINE | 0        | 12       | 12     | 0	| 12          |
| 998    | 8200         | 192.168.1.7 | ONLINE | 0        | 0        | 0      | 0	| 0           |
| 999    | 8200         | 192.168.1.6 | ONLINE | 0        | 0        | 0      | 0.      | 0           |
| 1000   | 8200         | 192.168.1.5 | ONLINE | 0        | 0        | 0      | 0	| 0           |
| 1000   | 8201         | 192.168.1.7 | ONLINE | 0        | 0        | 0      | 0	| 0           |
| 1000   | 8201         | 192.168.1.6 | ONLINE | 0        | 0        | 0      | 0	| 0           |
| 1000   | 8201         | 192.168.1.5 | ONLINE | 0        | 0        | 0      | 0	| 0           |
+--------+--------------+-------------+--------+----------+----------+--------+---------+-------------+

Putting down the 192.168.1.6 node:

(root localhost) [(none)]>set global pxc_maint_mode=maintenance;
Query OK, 0 rows affected (10.00 sec)

+--------+--------------+-------------+--------------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status       | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+--------------+----------+----------+--------+---------+-------------+
| 10000  | 200          | 192.168.1.5 | ONLINE       | 0        | 5        | 5      | 0       | 5           |
| 10000  | 201          | 192.168.1.7 | ONLINE       | 1        | 15       | 17     | 0       | 17          |
| 10000  | 201          | 192.168.1.6 | OFFLINE_SOFT | 0        | 0        | 12     | 0       | 12          | 
| 998    | 8200         | 192.168.1.7 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 999    | 8200         | 192.168.1.6 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8200         | 192.168.1.5 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.7 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.6 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.5 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
+--------+--------------+-------------+--------------+----------+----------+--------+---------+-------------+

Node is in OFFLINE_SOFT as such it will allow any existing connection to finish while not accepting any new.
We can wait for running connection to end, then do whatever kind of maintenance we need without affecting our production.

Once done we can move the node back:

[(none)]>set global pxc_maint_mode=disabled;
Query OK, 0 rows affected (0.00 sec)

In the script log we will see that the node is identify as ready to be put back:

2020/07/03 06:58:23.871:[INFO] Evaluate nodes state 192.168.1.6;3306;201;1000 Retry #1
2020/07/03 06:58:23.882:[WARN] Move node:192.168.1.6;3306;201;1000 SQL: UPDATE mysql_servers SET status='ONLINE' WHERE hostgroup_id=201 AND hostname='192.168.1.6' AND port='3306'
2020/07/03 06:58:23.887:[INFO] END EXECUTION Total Time(ms):265.045881271362

In all this we never had a moment of service interruption.

The above test was the easy one.

Let us now see what happens if we put the writer in maintenance.

This is a much more impacting action, given the node is accepting write transactions and is in single mode.
Ler us put the writer 192.168.1.5 in maintenance:

set global pxc_maint_mode=maintenance;
Query OK, 0 rows affected (10.00 sec)

And in few seconds:

+--------+--------------+-------------+--------------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status       | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+--------------+----------+----------+--------+---------+-------------+
| 999    | 200          | 192.168.1.6 | ONLINE       | 3        | 2        | 5      | 0       | 5           |
| 10000  | 200          | 192.168.1.5 | OFFLINE_SOFT | 0        | 0        | 5      | 0       | 5           |
| 10000  | 201          | 192.168.1.7 | ONLINE       | 4        | 12       | 17     | 0       | 17          |
| 998    | 8200         | 192.168.1.7 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 999    | 8200         | 192.168.1.6 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8200         | 192.168.1.5 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.7 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.6 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.5 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
+--------+--------------+-------------+--------------+----------+----------+--------+---------+-------------+

What happened?

In the script log:

...
2020/07/03 08:11:22.110:[INFO] END EXECUTION Total Time(ms):231.402158737183
2020/07/03 08:11:24.299:[WARN] PXC maintenance on single writer, is asking for failover. Fail-over in action Using Method = 1
2020/07/03 08:11:24.307:[WARN] Move node:192.168.1.5;3306;200;3020 SQL: UPDATE mysql_servers SET status='OFFLINE_SOFT' WHERE hostgroup_id=200 AND hostname='192.168.1.5' AND port='3306'
2020/07/03 08:11:24.313:[INFO] Special Backup - Group found! I am electing a node to writer following the indications
 This Node Try to become the new WRITER for HG 200 Server details: 192.168.1.6:3306:HG8200
2020/07/03 08:11:24.313:[INFO] This Node Try to become a WRITER promoting to HG 200 192.168.1.6:3306:HG 8200
2020/07/03 08:11:24.313:[WARN] DELETE from writer group as: SQL:DELETE from mysql_servers where hostgroup_id in (200,9200) AND STATUS = 'ONLINE'
2020/07/03 08:11:24.720:[WARN] Move node:192.168.1.6:33069992000 SQL:INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.6',200,3306,999,2000);
2020/07/03 08:11:24.720:[WARN] !!!! FAILOVER !!!!!
 Cluster was without WRITER I have try to restore service promoting a node
2020/07/03 08:11:24.720:[INFO] END EXECUTION Total Time(ms):685.911178588867
...

The script identify the need to shift production to another node.
It set the current writer as offline in the staging environment and identify which node from the special group 8000 is the appropriate replacement.

Then push all the changes to runtime
All is done in ~700 ms, so the whole process takes less then a second and the production was not impacted.

Test 2 writer node crash

Note: In the text below MySQL is set to work on CEST while system is EDT.

This is of course a much more impacting scenario, we need to keep in to account not only the the situation of the node in ProxySQL, but the need for PXC to rebuild the Primary view getting quorum etc..

Initial picture:

+--------+--------------+-------------+--------------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status       | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+--------------+----------+----------+--------+---------+-------------+
| 999    | 200          | 192.168.1.6 | ONLINE       | 3        | 2        | 5      | 0       | 5           |
| 10000  | 201          | 192.168.1.7 | ONLINE       | 4        | 12       | 17     | 0       | 17          |
| 998    | 8200         | 192.168.1.7 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 999    | 8200         | 192.168.1.6 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8200         | 192.168.1.5 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.7 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.6 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.5 | ONLINE       | 0        | 0        | 0      | 0       | 0           |
+--------+--------------+-------------+--------------+----------+----------+--------+---------+-------------+/pre>

Abruptly killing the writer with kill -9 signal:

[root@galera1h1n6 gal8_2]# ps aux|grep gal8_2
root 5265 0.0 0.0 113452 876 ? S Jun26 0:00 /bin/sh /opt/mysql_templates/PXC8/bin/mysqld_safe --defaults-file=/opt/mysql_instances/gal8_2/my.cnf
mysql 8745 7.7 72.9 9355104 5771476 ? Sl Jun26 882:54 /opt/mysql_templates/PXC8/bin/mysqld

PXC cluster start to identify the issue:

2020-07-04T10:05:41.011531Z 0 [Note] [MY-000000] [Galera] (27260297, 'tcp://192.168.1.5:4567') turning message relay requesting on, nonlive peers: tcp://192.168.1.6:4567

And script as well:

2020/07/04 06:05:41.000:[ERROR] Cannot connect to DBI:mysql:host=192.168.1.6;port=3306 as monitor
2020/07/04 06:05:41.001:[ERROR]  Node is not responding setting it as SHUNNED (ProxySQL bug - #2658)192.168.1.6:3306:HG200

2020/07/04 06:05:41.052:[WARN] PXC maintenance on single writer, is asking for failover. Fail-over in action Using Method = 1
2020/07/04 06:05:41.061:[INFO] Special Backup - Group found! I am electing a node to writer following the indications
 This Node Try to become the new WRITER for HG 200 Server details: 192.168.1.5:3306:HG8200
2020/07/04 06:05:41.062:[INFO] This Node Try to become a WRITER promoting to HG 200 192.168.1.5:3306:HG 8200
2020/07/04 06:05:41.062:[WARN]  DELETE from writer group as:  SQL:DELETE from mysql_servers where hostgroup_id in (200,9200) AND STATUS = 'ONLINE'

As said there is also the need from the cluster to rebuild the cluster view and get a quorum (see below):

2020-07-04T10:05:45.685154Z 0 [Note] [MY-000000] [Galera] Current view of cluster as seen by this node
view (view_id(PRIM,27260297,16)
memb {
	27260297,1
	7e4d3144,1
	}
joined {
	}
left {
	}
partitioned {
	3eb94984,1
	}
)

As soon as the view is available the script can perform the failover:

2020/07/04 06:05:46.318:[WARN] Move node:192.168.1.5:330610002000 SQL:INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.5',200,3306,1000,2000);
2020/07/04 06:05:46.318:[WARN] !!!! FAILOVER !!!!!
 Cluster was without WRITER I have try to restore service promoting a node
2020/07/04 06:05:46.318:[INFO] END EXECUTION Total Time(ms):5551.42211914062

the whole exercise takes 5 seconds.

Which for a server crash is not bad at all.

In my case given the Java application was design to deal with minimal service interruption with retry loop, I did not had any error, but this depends on how you had wrote the application layer.

+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status  | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+
| 1000   | 200          | 192.168.1.5 | ONLINE  | 0        | 4        | 5      | 0       | 5           |
| 10000  | 201          | 192.168.1.7 | ONLINE  | 0        | 26       | 30     | 0       | 28          |
| 998    | 8200         | 192.168.1.7 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 999    | 8200         | 192.168.1.6 | SHUNNED | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8200         | 192.168.1.5 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.7 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.6 | SHUNNED | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.5 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+

Node also if SHUNNED in the special groups will be normally managed and in the need put ONLINE.

Crash of a reader

Let us now do the same with a reader

[root galera1h1n7 gal8_3]# date; kill -9 5259 8739
Sat Jul 4 06:43:46 EDT 2020

The script will fail to connect :

2020/07/04 06:43:46.923:[ERROR] Cannot connect to DBI:mysql:host=192.168.1.7;port=3306 as monitor
2020/07/04 06:43:46.923:[ERROR]  Node is not responding setting it as SHUNNED (ProxySQL bug - #2658)192.168.1.7:3306:HG8201

2020-07-04T10:43:47.998377Z 0 [Note] [MY-000000] [Galera] (27260297, 'tcp://192.168.1.5:4567') reconnecting to 7e4d3144 (tcp://192.168.1.7:4567), attempt 0

The node is internally SHUNNED by the script given not accessible, while waiting for ProxySQL to take action and shun the node. All reads request are managed by ProxySQL.
The final picture will be:

+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status  | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+
| 1000   | 200          | 192.168.1.5 | ONLINE  | 0        | 5        | 6      | 0       | 5           |
| 10000  | 201          | 192.168.1.7 | SHUNNED | 0        | 0        | 30     | 98      | 28          |
| 1000   | 201          | 192.168.1.6 | ONLINE  | 1	   | 24       | 25     | 0	 | 25          |
| 998    | 8200         | 192.168.1.7 | SHUNNED | 0        | 0        | 0      | 0       | 0           |
| 999    | 8200         | 192.168.1.6 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8200         | 192.168.1.5 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.7 | SHUNNED | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.6 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.5 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+

No service interruption from the read side thanks to ProxySQL.

Test 3 network failure

Network failures are always tricky, they can happen as a simple single instance, or for a large but unpredictable window of time.
Network can have a slow down first such that the node affect the whole cluster, then fully resolve themselves, leaving you with very limited data to understand what and why that happens.

Given that is important to try to take action to limit negative effects as much as possible.
That level of actions are above the scope of a scheduler Script, who is in charge only of the layout of the nodes.
Given that what it should do is not to solve the possible impact at PXC level but reduce the possible confusion in distributing the traffic with ProxySQL.

The initial picture is:

+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status  | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+
| 1000   | 200          | 192.168.1.5 | ONLINE  | 0        | 5        | 6      | 0       | 5           |
| 10000  | 201          | 192.168.1.7 | SHUNNED | 0        | 0        | 30     | 98      | 28          |
| 1000   | 201          | 192.168.1.6 | ONLINE  | 1	   | 24       | 25     | 0	 | 25          |
| 998    | 8200         | 192.168.1.7 | SHUNNED | 0        | 0        | 0      | 0       | 0           |
| 999    | 8200         | 192.168.1.6 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8200         | 192.168.1.5 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.7 | SHUNNED | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.6 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.5 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+

On node 192.168.1.5 I will stop the network interface.

enp0s8: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
 inet 192.168.1.5 netmask 255.255.255.0 broadcast 192.168.1.255
[vagrant galera1h1n5 ~]$ date;sudo ifdown enp0s8
Sat Jul 4 13:20:32 EDT 2020
Device 'enp0s8' successfully disconnected.

As soon as I stop it, the PXC cluster identify the node is not reachable:

2020-07-04T17:20:35.980786Z 0 [Note] [MY-000000] [Galera] (7e4d3144, 'tcp://192.168.1.7:4567') connection to peer 27260297 with addr tcp://192.168.1.5:4567 timed out, no messages seen in PT3S (gmcast.peer_timeout)

The script is running and must timeout the attempt to connect (6 seconds), and fencing other process to start:

2020/07/04 13:20:36.729:[ERROR] Another process is running using the same HostGroup and settings,
 Or orphan pid file. check in /tmp/proxysql_galera_check_200_W_201_R.pid
2020/07/04 13:20:38.806:[ERROR] Another process is running using the same HostGroup and settings,
 Or orphan pid file. check in /tmp/proxysql_galera_check_200_W_201_R.pid

Finally script node connection time out and failover starts

2020/07/04 13:20:40.699:[ERROR] Cannot connect to DBI:mysql:host=192.168.1.5;port=3306;mysql_connect_timeout=6 as monitor
2020/07/04 13:20:40.699:[ERROR] Node is not responding setting it as SHUNNED (internally) (ProxySQL bug - #2658)192.168.1.5:3306:HG200
2020/07/04 13:20:40.804:[WARN] Fail-over in action Using Method = 1
2020/07/04 13:20:40.805:[INFO] Special Backup - Group found! I am electing a node to writer following the indications
 This Node Try to become the new WRITER for HG 200 Server details: 192.168.1.6:3306:HG8200
2020/07/04 13:20:40.805:[INFO] This Node Try to become a WRITER promoting to HG 200 192.168.1.6:3306:HG 8200
2020/07/04 13:20:40.805:[WARN] DELETE from writer group as: SQL:DELETE from mysql_servers where hostgroup_id in (200,9200) AND STATUS = 'ONLINE'
2020/07/04 13:20:42.622:[WARN] Move node:192.168.1.6:33069992000 SQL:INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.1.6',200,3306,999,2000);
2020/07/04 13:20:42.623:[WARN] !!!! FAILOVER !!!!!
 Cluster was without WRITER I have try to restore service promoting a node
2020/07/04 13:20:42.623:[INFO] END EXECUTION Total Time(ms):7983.0858707428

Script does failover due to network in 7 seconds, 10 seconds from network issue

final picture:

+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status  | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+
| 999    | 200          | 192.168.1.6 | ONLINE  | 0        | 5        | 5      | 0       | 5           |
| 10000  | 201          | 192.168.1.7 | ONLINE  | 3        | 16       | 60     | 721     | 28          |
| 998    | 8200         | 192.168.1.7 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 999    | 8200         | 192.168.1.6 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8200         | 192.168.1.5 | SHUNNED | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.7 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.6 | ONLINE  | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.5 | SHUNNED | 0        | 0        | 0      | 0       | 0           |
+--------+--------------+-------------+---------+----------+----------+--------+---------+-------------+

Given the level of complexity a network failure brings, I think this is a pretty good results, also if it will be inevitable to have some application errors given the connection was on the fly, and for that there is no way to prevent, except re-run the whole transaction.

Test 4 ProxySQL node crash

Last test is what happen if a proxysql node crash? We have set a VIP that we use to connect to ProxySQL (192.168.1.194), and set Keepalived to deal with the move of the VIP cross nodes will that be efficient enough?

Let us try killing one of the ProxySQL node.

Picture before:

+--------+--------------+-------------+--------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+--------+----------+----------+--------+---------+-------------+
| 999    | 200          | 192.168.1.6 | ONLINE | 3        | 1        | 4      | 0	    | 4           |
| 10000  | 201          | 192.168.1.7 | ONLINE | 3        | 7        | 10     | 0	    | 10          |
| 1000   | 201          | 192.168.1.5 | ONLINE | 0        | 2        | 2      | 0	    | 2           |
| 998    | 8200         | 192.168.1.7 | ONLINE | 0        | 0        | 0      | 0	    | 0           |
| 999    | 8200         | 192.168.1.6 | ONLINE | 0        | 0        | 0      | 0	    | 0           |
| 1000   | 8200         | 192.168.1.5 | ONLINE | 0        | 0        | 0      | 0	    | 0           |
| 1000   | 8201         | 192.168.1.7 | ONLINE | 0        | 0        | 0      | 0	    | 0           |
| 1000   | 8201         | 192.168.1.6 | ONLINE | 0        | 0        | 0      | 0	    | 0           |
| 1000   | 8201         | 192.168.1.5 | ONLINE | 0        | 0        | 0      | 0	    | 0           |
+--------+--------------+-------------+--------+----------+----------+--------+---------+-------------+

I connect on the current Proxy node that is in charge of the `vip and:

[root proxy1 proxysql]# date;kill -9 24890 24891
Sat Jul 4 14:02:29 EDT 2020

In the system log of the next in the chain Proxy server (different machine), Keepalived identify the node is down and swap the VIP:

Jul 4 14:02:31 proxy2 Keepalived_vrrp[6691]: VRRP_Instance(VI_01) forcing a new MASTER election
Jul 4 14:02:32 proxy2 Keepalived_vrrp[6691]: VRRP_Instance(VI_01) Transition to MASTER STATE
Jul 4 14:02:33 proxy2 Keepalived_vrrp[6691]: VRRP_Instance(VI_01) Entering MASTER STATE
Jul 4 14:02:33 proxy2 Keepalived_vrrp[6691]: VRRP_Instance(VI_01) setting protocol VIPs.
Jul 4 14:02:33 proxy2 Keepalived_vrrp[6691]: Sending gratuitous ARP on enp0s8 for 192.168.1.194
Jul 4 14:02:33 proxy2 Keepalived_vrrp[6691]: VRRP_Instance(VI_01) Sending/queueing gratuitous ARPs on enp0s8 for 192.168.1.194
Jul 4 14:02:33 proxy2 Keepalived_vrrp[6691]: Sending gratuitous ARP on enp0s8 for 192.168.1.194
Jul 4 14:02:33 proxy2 Keepalived_vrrp[6691]: Sending gratuitous ARP on enp0s8 for 192.168.1.194

Application identify few connection that were close unexpectedly and try to restore them:

20/07/04 14:02:33 ERROR [ACTIONS2]: ##### Connection was closed at server side unexpectedly. I will try to recover it
20/07/04 14:02:33 ERROR [ACTIONS2]: ##### Connection was closed at server side unexpectedly. I will try to recover it
20/07/04 14:02:33 ERROR [ACTIONS2]: ##### Connection was closed at server side unexpectedly. I will try to recover it
20/07/04 14:02:33 ERROR [ACTIONS2]: ##### Connection was closed at server side unexpectedly. I will try to recover it

So in few seconds ProxySQL on the Proxy2 node is up and running able to manage the traffic:

+--------+--------------+-------------+--------+----------+----------+--------+---------+-------------+
| weight | hostgroup_id | srv_host    | status | ConnUsed | ConnFree | ConnOK | ConnERR | MaxConnUsed |
+--------+--------------+-------------+--------+----------+----------+--------+---------+-------------+
| 999    | 200          | 192.168.1.6 | ONLINE | 0        | 6        | 6      | 0	| 6           |
| 10000  | 201          | 192.168.1.7 | ONLINE | 3        | 29       | 32     | 0	| 32          |
| 1000   | 201          | 192.168.1.5 | ONLINE | 0        | 2        | 2      | 0	| 2           |
| 998    | 8200         | 192.168.1.7 | ONLINE | 0        | 0        | 0      | 0	| 0           |
| 999    | 8200         | 192.168.1.6 | ONLINE | 0        | 0        | 0      | 0	| 0           |
| 1000   | 8200         | 192.168.1.5 | ONLINE | 0        | 0        | 0      | 0	| 0           |
| 1000   | 8201         | 192.168.1.7 | ONLINE | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.6 | ONLINE | 0        | 0        | 0      | 0       | 0           |
| 1000   | 8201         | 192.168.1.5 | ONLINE | 0        | 0        | 0      | 0	| 0           |
+--------+--------------+-------------+--------+----------+----------+--------+---------+-------------+

Full failover done in 4 Seconds !!!!

Also this to me sounds a quite good achievement.

Conclusions

We can achieve very highly customisable setup with the use of the scheduler+script in ProxySQL.

The flexibility we get is such that we can create a very high available solution without the need to accept compromises impose by native galera support.

All this without reducing the efficiency of the solution.
In terms of time we had (all times may have a max additional +2 seconds due the scheduler interval we had define):

Maintenance on writer - All is done in ~700 ms no service interruption.
Writer crash - whole failover takes 5 seconds. It may have the need to retry transactions.
Reader crash - no service interruption
Network issue (with PXC cluster quorum) between 7 and 10 seconds
ProxySQL node crash - 4 seconds to fail over another ProxySQL node

Of course applications must be design to be fault tolerant and retry the transactions if any problem raise, I would say this goes without saying and is in any programming Best Practices.
If you do not do that and fully rely on the data layer, better for you to review your developers team.

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Using SKIP LOCK in MySQL For Queue Processing

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References

Deadlocks are our Friends

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Achieving Consistent Read and High Availability with Percona XtraDB Cluster 8.0 (Part 2)

Test 1 Put a node in maintenance

Test 2 writer node crash

Crash of a reader

Test 3 network failure

Test 4 ProxySQL node crash

Conclusions

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