A story around replication lag and Flow-Control.

Overview

In the last few months we had 2 main actors in the the MySQL ecosystem, ProxySQL and Group-Replication (with the evolution to InnoDB Cluster). 

While I had extensively covered the first, my last serious work on GR, goes back to some lab version in the past years. 
Given the decision Oracle made to declare it GA, and the Percona decision to provide some level of support to GR, I decide it was time for me to take a look at it again.
A lot of reviews were already done covering different topics. I saw articles about GR and performance, GR and basic functionalities (or lack of it like automatic node provisioning), GR and ProxySQL and so on.

But one question was coming up over and over in my mind. If GR and InnoDB cluster has to work as alternative to other (virtually) synchronous replication mechanism, what change or shift our customers must consider if they want move from one to the other.
In solutions using Galera, like Percona Xtradb Cluster (PXC), there is a main concept to which all of us must refer to. The cluster is data-centric, which at the end brings us to what matters, which is the data and its state, that must be exactly the same on each node at a given time (commit/apply). To guarantee this PXC and others use a set of data validation and FlowControl that at the end will make possible to the cluster dataset to be consistent on each node, respecting the main principle (be data-centric).
Immediate application of this principle is that an application can query ANY node in the PXC and be sure to get the same data, or to write and know that the data will be visible on all node (virtually) at the same time.
Last but not list, if a node is not consistent with the others, it will be excluded and must be rebuild, or inconsistency fix, before joining back.

Not a minor thing if you think carefully, and a very useful thing to have because allow you to transparently split write/read operations, or failover from one node to another with lees troubles, and more.

When I thought to GR (or InnoDB Cluster), I put myself in the customer shoes, and I asked to the other myself: “Aside all the other things we know (see above) what is the real impact of moving form PXC to GR/Innodb-Cluster for my application? Because when you mention me that GR is still using (basically) replication with binlogs and relaylog, also if there is a Flow-Control mechanism an alarm bell started to ring in my mind.”

My Answer was: “Let us do a POC, and see what is really going on in that terms.”

{autotoc enabled=yes}

The POC

Given that I setup a simple set of servers using GR, with a very basic application performing writes on a single Writer node, and (eventually) reads on the other nodes.Schema definition can be found here, mainly I had use the 4 tables from my windmills test suite, nothing special, or weird of specifically design for GR. Actually I had use this test a lot for PXC in the past, so was a perfect fit.

Test definition

The application will do very simple work, and I had thought to test 4 main cases:

• One thread performing one insert at each transaction.
• One thread performing 50 batched inserts at each transaction.
• 8 threads performing one insert to each transaction.
• 8 threads performing 50 batched inserts at each transaction.

As you can see nothing crazy, a quite simple operation.
Then I decide to test it using the following 4 conditions on the servers:

• Single slave worker FC as default
• Single slave worker FC set to 25
• 8 slave workers FC as default
• 8 slave workers FC set to 25

Again nothing weird or strange from my point of view.I had used 4 nodes:

• Gr1 Writer
• Gr2 Reader
• Gr3 Reader minimal latency (~10ms)
• Gr4 Reader minimal latency (~10ms)

Finally, I had to be sure I measure the lag in a way that allow me to reference it in a consistent way on all nodes.
I think we can safely say that the incoming GTID (last_ Received_transaction_set from replication_connection_status) is for sure the last apply on the master a slave node can know about.
This because network delay can prevent the last one to really be "received".
The other point of reference is the GTID_EXECUTED which refers to the latest GTID processed on the Node itself.

The closest Query that can track the distance will be:

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select @last_exec:=SUBSTRING_INDEX(SUBSTRING_INDEX(SUBSTRING_INDEX( @@global.GTID_EXECUTED,':',-2),':',1),'-',-1) last_executed;

select  @last_rec:=SUBSTRING_INDEX(SUBSTRING_INDEX(SUBSTRING_INDEX( Received_transaction_set,':',-2),':',1),'-',-1) last_received

FROM performance_schema.replication_connection_status WHERE Channel_name = 'group_replication_applier'; select (@last_rec - @last_exec) as real_lag

Or in case of a single worker

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select @last_exec:=SUBSTRING_INDEX(SUBSTRING_INDEX( @@global.GTID_EXECUTED,':',-1),'-',-1) last_executed;

select  @last_rec:=SUBSTRING_INDEX(SUBSTRING_INDEX(Received_transaction_set,':',-1),'-',-1) last_received 

FROM performance_schema.replication_connection_status WHERE Channel_name = 'group_replication_applier'; select (@last_rec - @last_exec) as real_lag;

The result will be something like this:

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+---------------+
| last_executed |
+---------------+
| 23607         |
+---------------+
 
+---------------+
| last_received |
 
+---------------+
| 23607         |
+---------------+
 
+----------+
| real_lag |
 
+----------+
|        0 |
+----------+

The whole set of tests can be found here, with all the commands you may need to run the application (you can find it here), and replicate the tests.
I will focus on the results, or this blogpost will be far too long, but I invite you to see the details.

The results

Efficiency on Writer by execution time & Rows/secHere using the raw data from the tests (excel spreadsheet available here) I was interested to identify if and how the writer is affected by the use of GR and FC.

Reviewing the Graph we can see that the Writer has a linear increase of the execution time (when using default FC), with the increase of the load, nothing really concerning and all in all expected also if the load is light, we will see after that the volume of rows at the end justify the execution time.

Different scenario if we use FC, the execution time increase significantly in both cases (single  worker/multiple workers). In the worth case (8 threads, 50 inserts batch) it becomes 4 time higher the same load without FC.
What happen to the inserted rows? In the application I trace the rows inserted/sec, as such is easy to see what is going on there as well.

We can see that the Writer with FC activated is able to insert less than 1/3 of the rows it can process without FC.We can definitely say, that FC has a significant impact on the Writer performance.
To clarify let see this graph:

Without FC the Writer is able to process a high volume of rows in a limited number of time (results from test 8 workers; 8 threads; 50 insert batch).
While with FC the situation changes drastically, the Writer will take a long time processing a significant smaller amount of rows/sec. In short performance will drop significantly.

But, hey I can be ok with that if this means to have a consistent data-set cross all the nodes.
At the end also PXC and similar, pay a significant price in performance to match the data-centric Principle.

Ok let see what happen on the other nodes.

Entries Lag

Well, the scenario is not so good.

When NOT using FC, the nodes lag behind the writer significantly. Remember that by default the FC in GR is set to 25000 entries, I mean 25K of entries!!!

So what happens here is that as as soon as I put some salt (see load) on the Writer the slave nodes will start to lag.
When using the default single worker, that will be significant, while when using multiple workers, we will see that the lag will mainly happen only on the node(s) with a minimal (10ms network latency).
Sad thing is that is not really going down respect to the single thread worker, indicating the simple minimal latency of 10ms was enough to affect the replication.

Well time to activate the FC and have no lag.

Unfortunately, this is not going to be the case. As we can see the lag in case of single worker remain high also in Gr2 (154 entries).
While when using multiple workers, the Gr3/4 nodes are able to perform much better, and lag is significantly less, but still high ~1k entries.

It is important to remember that at this time the Writer is processing 1/3 or less of the rows it is normally able to do. It is important to note, that I had set 25 to the entry limit in the FC, and never the less the Gr3 (and Gr4) nodes are lagging more than 1K entries behind.

To clarify, let check the two graphs below:

Using the Writer (Master) as baseline in entry #N, without FC, the nodes (slaves) using GR, will start to lag behind the writer, in a significant way also with light load. The distance in this POC was from very minimal with 58 entries, up to 3849 entries in the case of higher load.

Using FC the Writer (Master) will diverge less, as expected, but also if it will have a significant drop in performance (1/3 or less), the Nodes will lag anyhow, worse case up to 1363 entries.Need to underline that we have not further way (I am aware of) to tune the lag and prevent it to happen.
Which means an application cannot transparently split Writes/Reads and expect consistency. The gap will be too high

A graph that tell us a story

I was using PMM to keep an eye on the nodes while doing the tests. And one of the graph was really telling me that GR has still some “limits” if we want to consider it as the replication mechanism for a cluster.

This graph shows the MySQL Queries executed on all the 4 nodes, in the 8-50 threads-batch with FC test.As you can see the Gr1 (the writer) is the first one to takeoff, followed by Gr2, the nodes Gr3 and Gr4 will require a bit more, given the binlog transmission (and 10ms delay), once the data is there, they match (inconsistently) the Gr2 node, this is an effect of the FC asking the master to slow down. But as seen previously, the nodes will never be able to match the Writer. At the end, when the load test is over, the nodes will continue to process the queue for additional ~130 sec. Considering that the whole load takes 420 sec on the Writer, this means that 1/3 of the total time on the Writer is spent AFTER on the slave to sync.

The above graph shows the same test without FC, is interesting to see how the Writer was going above the 300 Queries/sec while G2 stay around 200 and Gr3/4 far below. The writer was able to process the whole load in ~120 seconds instead 420, while Gr3/4 continue to process the load for additional ~360 seconds.This means that without FC set the Nodes will lag around 360 seconds behind the Master, while with FC set to 25, they will lag 130 seconds.

A significant gap.

Conclusions

Going back to the origin, and the reason why I was looking to this POC.
I, as customer of myself, think that my application(s) will not be a good fit for GR, given I have set PXC to scale out the reads, and be able to efficiently move my writer to another when in need to.
GR while based on a very interesting concept it is still based on asynchronous replication (as my colleague Kenny said). I am sure it could make sense in many other cases, but it cannot be compare to solutions that are based on virtually synchronous replication; and it still requires a lot of refinement.

On the other hand, all the applications that can afford to have a significant gap between writer and readers, will probably be fine with that.But I raise another question … was not standard replication already covering that?Reviewing the Oracle documentations (https://dev.mysql.com/doc/refman/5.7/en/group-replication-background.html) I can see why GR as part of the InnoDB cluster, can help me in improving HA, when comparing it to standard replication.But I also think it is important to understand that GR (and derived solutions like InnoDB cluster) are not comparable or a replacement of data-centric solutions as PXC. At least up to now.

Good MySQL to everyone.

References

https://dev.mysql.com/doc/refman/5.7/en/group-replication.html

https://dev.mysql.com/doc/refman/5.7/en/mysql-innodb-cluster-userguide.html

http://lefred.be/content/mysql-group-replication-understanding-flow-control/

https://dev.mysql.com/worklog/task/?id=9838

Overview

I love ProxySQL, I think it is a great component for expanding architecture flexibility and HA, but not all what shine is gold.

Let me make clear that I only want to set the expectations right, and avoid to sell carbon for gold. Carbon has it's own use, gold has another. 

First of all let me clarify what is mirroring for me (and hope most of you).

Then we need to cover the basic of how ProxySQL manage the (I cannot say mirroring) traffic dispatch.

ProxySQL receive a connection from the application, and through it we can have a simple SELECT or a more complex transaction. ProxySQL will get each query, pass it to the QueryProcessor, process the query, identify if the query is mirrored, duplicate the whole mysql session ProxySQL internal object and associate it to a mirror queue, which refer to a mirror threads pool.

If the pool is free (has an available active slot in the concurrent active threads set) then the query is processed right away, if not it will stay in the queue. If the queue is full, the query is lost.

Whatever is returned from the query goes to /dev/null, as such no result set is passed back to client.

The whole process is not free for a server, actually if you will check the CPU utilization you will see that the “mirroring” in ProxySQL will actually double the CPU utilization. Meaning that also the traffic on server A will be impacted because resource contention.

Summarizing ProxySQL will:

• Send the query for execution in different order
• Completely ignore any transaction isolation
• Have different number of query executed on B respect to A
• Add significant load on the server resources

Comparing this with the point and expectations I mention in the reasoning at the end of this article, it is quite clear to me that at the moment we cannot consider ProxySQL as a valid mechanism to duplicate consistent load from server A to server B.

Personally, I don’t think that ProxySQL development Team (Rene’ :D), should waste his time on fixing this part, there are so many other things to cover and improve on ProxySQL.

After having work extensively with ProxySQL and have done deep QA on mirroring, I think that either we keep it as basic blind traffic dispatcher or a full re-conceptualization is required.

Setup

But once we have clarified that, we can still see ProxySQL “traffic dispatch” (cannot say mirroring really) as a very interesting feature, that may result useful in many ways, especially because it is so easy to setup.

The following is the result of tests I had performed, which should help in setting correct expectations.

Tests were simple, load data in a PXC cluster and use ProxySQL to replicate the load on a MySQL master-slave environment.

Machines for MySQL/PXC where VM with CentOS 7, 4 CPU 3 GB RAM, attached storage.

Machine for ProxySQL VM CentOS 7, 8 CPU 8GB RAM.

Why I choose to give ProxySQL such higher  volume of resources?

I knew in advance I may need to play a bit with a couple of settings requiring more memory and CPU cycles and I want to be sure I don’t get any problem from ProxySQL in relation to CPU and Memory.

The application that I was using to add load is a Java application I develop to perform my tests. App is https://github.com/Tusamarco/blogs/blob/master/stresstool_base_app.tar.gz, the whole set I had used to do the test is here

I had used 4 different tables,

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+------------------+
| Tables_in_mirror |
+------------------+
| mirtabAUTOINC    |
| mirtabMID        |
| mirtabMIDPart    |
| mirtabMIDUUID    |

For full table definition see here

ProxySQL setup

Ok so let start.

First setup ProxySQL

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DELETE FROM mysql_servers WHERE hostgroup_id IN (500,501,700,701);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.0.5',500,3306,60000,400);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.0.5',501,3306,100,400);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.0.21',501,3306,20000,400);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.0.231',501,3306,20000,400);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.0.7',700,3306,1,400);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.0.7',701,3306,1,400);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.0.25',701,3306,1,400);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight,max_connections) VALUES ('192.168.0.43',701,3306,1,400);
LOAD MYSQL SERVERS TO RUNTIME; SAVE MYSQL SERVERS TO DISK;

DELETE FROM mysql_users WHERE username='load_RW';
INSERT INTO mysql_users (username,password,active,default_hostgroup,default_schema,transaction_persistent) VALUES ('load_RW','test',1,500,'test',1);
LOAD MYSQL USERS TO RUNTIME;SAVE MYSQL USERS TO DISK;

DELETE FROM mysql_query_rules WHERE rule_id=202;
INSERT INTO mysql_query_rules (rule_id,username,destination_hostgroup,mirror_hostgroup,active,retries,apply) VALUES(202,'load_RW',500,700,1,3,1);
LOAD MYSQL QUERY RULES TO RUNTIME;SAVE MYSQL QUERY RULES TO DISK

Tests results

Now the action. 

Test1

The first test is mainly a simple functional test during which I insert records using 1 single thread in PXC and in MySQL.

No surprise here I have 3000 loops and at the end I have 3000 records on both platforms.

To have a baseline we can see that ProxySQL CPU utilization is quite low

AT the same time the number of "questions" against PXC and MySQL very similar:
PXC

MySQL

The other two metrics we want to keep an eye on are Mirror_concurrency and Mirror_queue_length this two refer respectively to mysql-mirror_max_concurrency and mysql-mirror_max_queue_length.

The two new variables and metrics were introduced in ProxySQL 1.4.0 with the intent to control and manage the load ProxySQL generate internally related to the mirroring feature.

In this case as you can see we have a max of 3 concurrent connections and 0 queue entry, all good.

Now that we have a baseline, and that we know at functional level "it works" let see what happen increasing the load.

Test 2.

Scope of the test was to identify how ProxySQL will behave with standard configuration and increasing load.

It comes up that as soon as ProxySQL has a little bit more load, it will start to loose some query along the way.

Executing 3000 loop for 40 threads, insert only will result in 120,000 rows inserted in all the 4 tables in PXC but the table in the Secondary (mirrored) platform will only have a variable number between 101,359 and 104,072. Showing consistent lost of data.

Reviewing the insight and comparing the connections running in PXC and the secondary we can see that, as expected, the PXC number of connections is scaling serving the number of incoming requests, while the connections on the Secondary are limited by the default value of mysql-mirror_max_concurrency=16.

Is also interesting to notice that to process the queue of transaction existing in ProxySQL the connection on the Secondary persist longer than the connection in PXC.

As we can see above the queue as an evident bell curve reaching the 6K entries which is quite below the mysql-mirror_max_queue_length limit (32K). Yet the queries were drop by ProxySQL, which indicate the queue is not really enough to accommodate the pending work.

CPU wise ProxySQL as expected take a bit more cycles, but nothing crazy and the overhead for the simple mirroring queue processing can be see when the main load stops around 12:47.

Another interesting graph to keep an eye on is the one describing the executed command inside PXC and the Secondary:

PXC

Secondary

As you can see the traffic on the Secondary was significantly less 669 average than PXC 1.17K. Then it spikes when the main load on the PXC node terminates.

In short it is quite clear that ProxySQL is not able here to scale following the traffic existing in PXC and actually loosing significant amount of data on the Secondary.

Doubling the load in the Test3 show the same behavior, having ProxySQL reach his limit for the traffic duplication.

But can this be optimized?

Of course yes, this is what the mysql-mirror_max_concurrency is for, lets see what is going to happen if I increase the value from 16 to 100 just to make it crazy high.

Test 4

2 app node writing.

I am jumping the description of test 3 because is mainly the same of Test 2 with more load.

The first thing that is coming to the attention is that both PXC and secondary report same number of rows in the tables (240,000). That is a first good win.

Second the number of running connections:

Lines now are much closer and the queue just drop to few entries.

Commands executed in PXC:

And commands executed in the Secondary:

Average execution report the same value, and very similar trend.

Finally, what was the CPU cost and effect?

PXC:

Secondary:

As expected some difference in the CPU usage distribution exists, but the trend is consistent between the two nodes and with the operations.

The ProxySQL CPU utilization is definitely higher than before:

But absolutely manageable, and still reflecting the initial distribution.

Finally what about CRUD now?
So far I had only tested the insert operation, but what happen if we run a full CRUD set of tests?

Test 7 Crud

First of all, let us review the executed commands, in PXC

And Secondary

While in appearance we have very similar workload, selects aside the behavior will significantly diverge.
This because in the Secondary the different operations are not encapsulated by the transaction and executed as they are received.  We can see significant difference in update and delete operations between the two.

Also the threads in execution will show a different picture between the two platform.

PXC

Secondary

It appears quite clear that PXC is constantly having more running threads and more connections.

Never the less both platform process similar total number of questions.

PXC

Secondary

Both having an average around 1.17K/second questions.

This is also another indication of how much the behavior is impacted when we have concurrent operation but no respect of the isolation or execution order.

Different behavior that is also clear reviewing the CPU utilization.

PXC

Secondary

Conclusions

To close this long article, I want to go back to the start.

We cannot consider the mirror function in ProxySQL as a real mirroring, but more a traffic redirection.

Use ProxySQL with this approach, is still partially effective in testing the load and the effect it may have on a secondary platform.
As we know data consistency is not guarantee in this scenario, and Selects, Updates and Delete are affected by this given the different data-set and result-set they will manage.

Given that, the server behaviors will change between original and mirror, if not in the quantity in the quality.

I am convinced that when we need a tool able to test our production load on a different or new platform, we will do better to look to something else, like Query Playback recently reviewed and significantly patch by DropBox (https://github.com/Percona-Lab/query-playback).

At the end ProxySQL is already a cool tool and if it doesn't cover this, well I can live with that, I am interested to have it working as it should and does in many other functionalities.

Acknowledgments

As usual to Rene' who had work on fixing and introducing new functionalities associated to the mirror, like the queue and concurrency control.

To the Percona team who develop PMM, all the graphs here (except 3) comes from PMM, and some of them are my customization.

If you met one of the (few) MySQL consultants around the globe and asked him/her to review your queries and/or schemas, I am sure that he/she would tell you something regarding the importance of good primary key(s) design. Especially in the case of InnoDB, I’m sure they started to explain to you about index merges and page splits. These two notions are closely related to performance, and you should take this relationship into consideration when designing any index (not just PKs).

That may sound like mumbo jumbo to you, and you may be right. This is not easy stuff, especially when talking about internals. This is not something you deal with on a regular basis, and often you don’t want to deal with it at all.

But sometimes it’s a necessity. If so, this article is for you.

In this article, I want to shed some light in explaining some of the most unclear, behind the scenes operations in InnoDB: page index creation, page merging and page splitting.

In Innodb all data is an index. You’ve probably heard that as well right? But what exactly does that mean?

File-Table Components

Let's say you have MySQL installed, the latest 5.7 version (Percona Server for MySQL, right?  ), and you have a table named wmills in the schema windmills. In the data directory (normally /var/lib/mysql/) you will see that it contains:

data/  

windmills/

     wmills.ibd

     wmills.frm

This is because the parameter innodb_file_per_table is set to 1 since MySQL 5.6. With that setting, each table in your schema is represented by one file (or many files if the table is partitioned).

What is important here is that the physical container is a file named wmills.ibd. This file is broken up into and contains N number of segments. Each segment is associated with an index.

While a file’s dimensions do not shrink with row-deletions, a segment itself can grow or shrink in relation to a sub-element named extent. An extent can only exist inside a segment and has a fixed dimension of 1MB (in the case of default page size). A page is a sub-element of an extent and has a default size of 16KB.

Given that, an extent can contain a maximum of 64 pages. A page can contain two to N number of rows. The number of rows a page can contain is related to the size of the row, as defined by your table schema. There is a rule within InnoDB that says, at minimum, two rows must fit into a page. Therefore, we have a row-size limit of 8000 bytes.

If you think this sounds like Matryoshka dolls, you are right! An image might help:

InnoDB uses B-trees to organize your data inside pages across extents, within segments.

Roots, Branches, and Leaves

Each page (leaf) contains 2-N rows(s) organized by the primary key. The tree has special pages to manage the different branch(es). These are known as internal nodes (INodes).

This image is just an example, and is not indicative of the real-world output below.

Let’s see the details:

ROOT NODE #3: 4 records, 68 bytes
 NODE POINTER RECORD ≥ (id=2) → #197
 INTERNAL NODE #197: 464 records, 7888 bytes
 NODE POINTER RECORD ≥ (id=2) → #5
 LEAF NODE #5: 57 records, 7524 bytes
 RECORD: (id=2) → (uuid="884e471c-0e82-11e7-8bf6-08002734ed50", millid=139, kwatts_s=1956, date="2017-05-01", lo

Below is the table structure:

CREATE TABLE `wmills` (
  `id` bigint(11) NOT NULL AUTO_INCREMENT,
  `uuid` char(36) COLLATE utf8_bin NOT NULL,
  `millid` smallint(6) NOT NULL,
  `kwatts_s` int(11) NOT NULL,
  `date` date NOT NULL,
  `location` varchar(50) COLLATE utf8_bin DEFAULT NULL,
  `active` tinyint(2) NOT NULL DEFAULT '1',
  `time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `strrecordtype` char(3) COLLATE utf8_bin NOT NULL,
  PRIMARY KEY (`id`),
  KEY `IDX_millid` (`millid`)
) ENGINE=InnoDB;

All styles of B-trees have a point of entry known as the root node. We’ve identified that here as page #3. The root page contains information such as index ID, number of INodes, etc. INode pages contain information about the pages themselves, their value ranges, etc. Finally, we have the leaf nodes, which is where we can find our data. In this example, we can see that leaf node #5 has 57 records for a total of 7524 bytes. Below that line is a record, and you can see the row data.

The concept here is that while you organize your data in tables and rows, InnoDB organizes it in branches, pages, and records. It is very important to keep in mind that InnoDB does not work on a single row basis. InnoDB always operates on pages. Once a page is loaded, it will then scan the page for the requested row/record.

Is that clear up to now? Good. Let’s continue.

Page Internals

A page can be empty or fully filled (100%). The row-records will be organized by PK. For example, if your table is using an AUTO_INCREMENT, you will have the sequence ID = 1, 2, 3, 4, etc.

A page also has another important attribute: MERGE_THRESHOLD. The default value of this parameter is 50% of the page, and it plays a very important role in InnoDB merge activity:

While you insert data, the page is filled up sequentially if the incoming record can be accommodated inside the page.

When a page is full, the next record will be inserted into the NEXT page:

Given the nature of B-trees, the structure is browsable not only top-down following the branches, but also horizontally across the leaf nodes. This is because each leaf node page has a pointer to the page that contains the NEXT record value in the sequence.

For example, Page #5 has a reference to the next page, Page #6. Page #6 has references backward to the previous page (Page #5) and a forward to the next page (Page #7).

This mechanism of a linked list allows for fast, in-order scans (i.e., Range Scans). As mentioned before, this is what happens when you are inserting and have a PK based on AUTO_INCREMENT. But what happens if I start to delete values?

Page Merging

When you delete a record, the record is not physically deleted. Instead, it flags the record as deleted and the space it used becomes reclaimable.

When a page has received enough deletes to match the MERGE_THRESHOLD (50% of the page size by default), InnoDB starts to look to the closest pages (NEXT and PREVIOUS) to see if there is any chance to optimize the space utilization by merging the two pages.

In this example, Page #6 is utilizing less than half of its space. Page #5 received many deletes and is also now less than 50% used. From InnoDB’s perspective, they are mergeable:

The merge operation results in Page #5 containing its previous data plus the data from Page #6. Page #6 becomes an empty page, usable for new data.

The same process also happens when we update a record and the size of the new record brings the page below the threshold.

The rule is: Merges happen on delete and update operations involving close linked pages.

If a merge operation is successful, the index_page_merge_successful metric in INFORMATION_SCHEMA.INNODB_METRICS is incremented.

Page Splits

As mentioned above, a page can be filled up to 100%. When this happens, the next page takes new records.

But what if we have the following situation?

Page #10 doesn’t have enough space to accommodate the new (or updated) record. Following the next page logic, the record should go on Page #11. However:

Page #11 is also full, and data cannot be inserted out of order. So what can be done?

Remember the linked list we spoke about? At this moment Page #10 has Prev=9 and Next=11.

What InnoDB will do is (simplifying):

1. Create a new page
2. Identify where the original page (Page #10) can be split (at the record level)
3. Move records
4. Redefine the page relationships

A new Page #12 is created:

Page #11 stays as it is. The thing that changes is the relationship between the pages:

• Page #10 will have Prev=9 and Next=12
• Page #12 Prev=10 and Next=11
• Page #11 Prev=12 and Next=13

The path of the B-tree still sees consistency since it is following a logical organization. However, physically the page is located out of order, and in most cases in a different extent.

As a rule we can say: Page splits happens on Insert or Update, and cause page dislocation (in many cases on different extents).

InnoDB tracks the number of page splits in INFORMATION_SCHEMA.INNODB_METRICS. Look for index_page_splits and index_page_reorg_attempts/successful metrics.

Once the split page is created, the only way to move back is to have the created page drop below the merge threshold. When that happens, InnoDB moves the data from the split page with a merge operation.

The other way is to reorganize the data by OPTIMIZE the table. This can be a very heavy and long process, but often is the only way to recover from a situation where too many pages are located in sparse extents.

Another aspect to keep in mind is that during merge and split operations, InnoDB acquires an x-latch to the index tree. On a busy system, this can easily become a source of concern. This can cause index latch contention. If no merges and splits (aka writes) touch only a single page, this is called an “optimistic” update in InnoDB, and the latch is only taken in S. Merges and splits are called “pessimistic” updates, and take the latch in X.

My Primary Key

A good Primary Key (PK) is not only important for retrieving data, but also correctly distributing the data inside the extents while writing (which is also relevant in the case of split and merge operations).

In the first case, I have a simple auto-increment. In the second my PK is based on an ID (1-200 range) and an auto-increment value. In my third, I have the same ID (1-200 range) but associate with a UUID.

When inserting, InnoDB must add pages. This is read as a SPLIT operation:

The behavior is quite different depending on the kind of Primary Key I use.

The first two cases will have more “compact” data distribution. This means they will also have better space utilization, while the semi-random nature of the UUID will cause a significant “sparse” page distribution (causing a higher number of pages and related split operations).

In the case of merges, the number of attempts to merge is even more different by PK type.

On Insert-Update-Delete operations, auto-increment has less page merge attempts and 9.45% less of a success ratio than the other two types. The PK with UUID (on the side other of the spectrum) has a higher number of merge attempts, but at the same time also a significantly higher success ratio at 22.34%, given that the “sparse” distribution left many pages partially empty.

The PK values with similar numbers also come from a secondary index.

Conclusion

MySQL/InnoDB constantly performs these operations, and you have very limited visibility of them. But they can bite you, and bite hard, especially if using a spindle storage VS SSD (which have different issues, by the way).

The sad story is there is also very little we can do to optimize this on the server side using parameters or some other magic. But the good news is there is A LOT that can be done at design time.

Use a proper Primary Key and design a secondary index, keeping in mind that you shouldn’t abuse of them. Plan proper maintenance windows on the tables that you know will have very high levels of inserts/deletes/updates.

This is an important point to keep in mind. In InnoDB you cannot have fragmented records, but you can have a nightmare at the page-extent level. Ignoring table maintenance will cause more work at the IO level, memory and InnoDB buffer pool.

You must rebuild some tables at regular intervals. Use whatever tricks it requires, including partitioning and external tools (pt-osc). Do not let a table to become gigantic and fully fragmented.

Wasting disk space? Need to load three pages instead one to retrieve the record set you need? Each search causes significantly more reads?
That’s your fault; there is no excuse for being sloppy!

Happy MySQL to everyone!

Acknowledgments

Laurynas Biveinis: who had the time and patience to explain some internals to me.

Jeremy Cole: for his project InnoDB_ruby (that I use constantly).

During the last few months we had a lot of opportunities to present and discuss about a very powerful tool that will become more and more used in the architectures supporting MySQL, ProxySQL.

ProxySQL is becoming every day more flexible, solid, performant and used (http://www.proxysql.com/ and recent http://www.proxysql.com/compare).

This is it, the tool is a winner in comparing it with similar ones, and we all need to have a clear(er) idea on how integrate it in our architectures in order to achieve the best results.

The first to keep in mind is that ProxySQL is not natively supporting any high availability solution, in short we can setup a cluster of MySQL(s) and achieve 4 or even 5 nines of HA, but if we include ProxySQL, as it is, and as single block, our HA will include a single point of failure (SPOF) that will drag us down in case of crash.

To solve this, the most common solution so far had be to setup ProxySQL as part of a tile architecture, where Application/ProxySQL where deploy together.

This is a good solution for some cases, and for sure it reduce the network hops, but what it may be less than practical when our architecture has a very high number of tiles.
Say 100 or 400 application servers, not so unusual nowadays. 
In that case managing the ProxySQL will be challenging, but most problematic it will be the fact that ProxySQL must perform several checks on the destination servers (MySQL), and if we have 400 instance of ProxySQL we will end up keeping our databases busy just because the checks.

In short ... is not a smart move.

Another possible approach, used so far was to have two layers of ProxySQL, one close to the application, another in the middle to finally connect to the database.

I personally don't like this approach for many reasons, but the most relevants are that this approach create additional complexity in the management of the platform, and it adds network hops.

So what can be done?

I like to have things simple, I love the KISS principle, I love to have things simple and because I am lazy I love to reuse the wheel instead re-invent things that someone else had already invent.

Last thing I like to have my customers not depending from me or any other colleague, once I am done, and gone, they must be able to manage their things, understand their things, fix their things by themselves.

Anyhow as said I like simple things. So my point here is the following:  

• excluding the cases where a tile (application/ProxySQL) make sense;  
• or when in the cloud and tools like ELB (Elastic load balancer) exist; 
• or on architecture already including a balancer.

What I can use for the remaining cases?

The answer comes with existing solutions and combining existing blocks, KeepAlived + ProxySQl + MySQL.

For KeepAlived explanation visit http://www.keepalived.org/.

Short description
"Keepalived is a routing software written in C. The main goal of this project is to provide simple and robust facilities for loadbalancing and high-availability to Linux system and Linux based infrastructures. Loadbalancing framework relies on well-known and widely used Linux Virtual Server (IPVS) kernel module providing Layer4 loadbalancing. Keepalived implements a set of checkers to dynamically and adaptively maintain and manage loadbalanced server pool according their health. On the other hand high-availability is achieved by VRRP protocol. VRRP is a fundamental brick for router failover. In addition, Keepalived implements a set of hooks to the VRRP finite state machine providing low-level and high-speed protocol interactions. Keepalived frameworks can be used independently or all together to provide resilient infrastructures."

Bingo! this is exactly what we need for our ProxySQL setup.

Below I will show how to setup: 

• Simple solution base on a single VIP 
• More complex solution using multiple VIPs 
• Even more complex solution using virtual VIPs and virtual servers.

Just remind that what we want to achieve is to prevent ProxySQL to become a SPOF, that's it. 

While achieving that we need to reduce as much as possible the network hops and keep the solution SIMPLE.

Another important concept to keep in mind is that ProxySQL (re)start take place in less then a second. 

This means that if it crash and it can be restarted by the angel process, having it doing so and recovery the service is much more efficient than to have any kind of failover mechanism to take place.

As such whenever you plan your solution keep in mind the ~1 second time of ProxySQL restart as base line.

Ready?

Let's go.

Setup

Choose 3 machines that will host the combination of Keepalive and ProxySQL.

In the following example I will use 3 machines for ProxySQL and Keepalived and 3 hosting PXC, but you can have the Keepalived+ProxySQL whenever you like also on the same PXC box.

For the following examples we will have:

PXC
node1 192.168.0.5 galera1h1n5
node2 192.168.0.21 galera2h2n21
node3 192.168.0.231 galera1h3n31
 
ProxySQL-Keepalived
test1 192.168.0.11
test2 192.168.0.12
test3 192.168.0.235
 
VIP 192.168.0.88 /89/90
 

To check I will use this table, please create it in your MySQL server:

DROP TABLE  test.`testtable2`;
 CREATE TABLE test.`testtable2` (
  `autoInc` bigint(11) NOT NULL AUTO_INCREMENT,
  `a` varchar(100) COLLATE utf8_bin NOT NULL,
  `b` varchar(100) COLLATE utf8_bin NOT NULL,
  `host` varchar(100) COLLATE utf8_bin NOT NULL,
  `userhost` varchar(100) COLLATE utf8_bin NOT NULL,
  PRIMARY KEY (`autoInc`)
) ENGINE=InnoDB ROW_FORMAT=DYNAMIC;

And this bash TEST command to use later

while [ 1 ];do export mydate=$(date +'%Y-%m-%d %H:%M:%S.%6N');

mysql --defaults-extra-file=./my.cnf -h 192.168.0.88 -P 3311  --skip-column-names  

-b -e "BEGIN;set @userHost='a';select concat(user,'_', host) into @userHost from information_schema.processlist  where user = 'load_RW' limit 1;insert into test.testtable2 values(NULL,'$mydate',SYSDATE(6),@@hostname,@userHost);commit;select * from test.testtable2 order by 1 DESC limit 1" ;

sleep 1;done

1. Install ProxySQL (refer to https://github.com/sysown/proxysql/wiki#installation)
2. Install Keepalived (yum install keepalived; apt-get install keepalived)
3. Setup ProxySQL users and servers

Once you have your ProxySQL up (run the same on all ProxySQL nodes, it is much simpler), connect to the Admin interface and:

DELETE FROM mysql_replication_hostgroups WHERE writer_hostgroup=500 ;
DELETE FROM mysql_servers WHERE hostgroup_id IN (500,501);
 
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight) VALUES ('192.168.0.5',500,3306,1000000000);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight) VALUES ('192.168.0.5',501,3306,100);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight) VALUES ('192.168.0.21',500,3306,1000000);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight) VALUES ('192.168.0.21',501,3306,1000000000);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight) VALUES ('192.168.0.231',500,3306,100);
INSERT INTO mysql_servers (hostname,hostgroup_id,port,weight) VALUES ('192.168.0.231',501,3306,1000000000);
LOAD MYSQL SERVERS TO RUNTIME; SAVE MYSQL SERVERS TO DISK;
 
DELETE FROM mysql_users WHERE username='load_RW';
INSERT INTO mysql_users (username,password,active,default_hostgroup,default_schema,transaction_persistent) VALUES ('load_RW','test',1,500,'test',1);
LOAD MYSQL USERS TO RUNTIME;SAVE MYSQL USERS TO DISK;
 
DELETE FROM mysql_query_rules WHERE rule_id IN (200,201);
INSERT INTO mysql_query_rules (rule_id,username,destination_hostgroup,active,retries,match_digest,apply) VALUES(200,'load_RW',501,1,3,'^SELECT.*FOR UPDATE',1);
INSERT INTO mysql_query_rules (rule_id,username,destination_hostgroup,active,retries,match_digest,apply) VALUES(201,'load_RW',501,1,3,'^SELECT ',1); 
 
LOAD MYSQL QUERY RULES TO RUNTIME;SAVE MYSQL QUERY RULES TO DISK;

Create a my.cnf file in your default dir with

[mysql]
user=load_RW
password=test

Simple Setup using a single VIP 3 ProxySQL 3 Galera nodes

First setup the keepalive configuration file (/etc/keepalived/keepalived.conf):

global_defs {
  # Keepalived process identifier
  lvs_id proxy_HA
}
# Script used to check if Proxy is running
vrrp_script check_proxy {
  script "killall -0 proxysql"
  interval 2
  weight 2
}
# Virtual interface
# The priority specifies the order in which the assigned interface to take over in a failover
vrrp_instance VI_01 {
  state MASTER 
  interface em1
  virtual_router_id 51
  priority <calculate on the WEIGHT for each node>
 
  # The virtual ip address shared between the two loadbalancers
  virtual_ipaddress {
    192.168.0.88 dev em1
  }
  track_script {
    check_proxy
  }
}
 

Given the above and given I want to have test1 as main priority will be set as:

test1 = 101
test2 = 100
test3 = 99 

Modify the config in each node following the above values and (re)start keepalived.

If all is set correctly in the system log of the TEST1 machine you will see:

Jan 10 17:56:56 mysqlt1 systemd: Started LVS and VRRP High Availability Monitor.
Jan 10 17:56:56 mysqlt1 Keepalived_healthcheckers[6183]: Configuration is using : 6436 Bytes
Jan 10 17:56:56 mysqlt1 Keepalived_healthcheckers[6183]: Using LinkWatch kernel netlink reflector...
Jan 10 17:56:56 mysqlt1 Keepalived_vrrp[6184]: Configuration is using : 63090 Bytes
Jan 10 17:56:56 mysqlt1 Keepalived_vrrp[6184]: Using LinkWatch kernel netlink reflector...
Jan 10 17:56:56 mysqlt1 Keepalived_vrrp[6184]: VRRP sockpool: [ifindex(2), proto(112), unicast(0), fd(10,11)]
Jan 10 17:56:56 mysqlt1 Keepalived_vrrp[6184]: VRRP_Script(check_proxy) succeeded
Jan 10 17:56:57 mysqlt1 Keepalived_vrrp[6184]: VRRP_Instance(VI_01) Transition to MASTER STATE
Jan 10 17:56:57 mysqlt1 Keepalived_vrrp[6184]: VRRP_Instance(VI_01) Received lower prio advert, forcing new election
Jan 10 17:56:57 mysqlt1 Keepalived_vrrp[6184]: VRRP_Instance(VI_01) Received higher prio advert
Jan 10 17:56:57 mysqlt1 Keepalived_vrrp[6184]: VRRP_Instance(VI_01) Entering BACKUP STATE
Jan 10 17:56:58 mysqlt1 Keepalived_vrrp[6184]: VRRP_Instance(VI_01) forcing a new MASTER election
...
Jan 10 17:57:00 mysqlt1 Keepalived_vrrp[6184]: VRRP_Instance(VI_01) Transition to MASTER STATE
Jan 10 17:57:01 mysqlt1 Keepalived_vrrp[6184]: VRRP_Instance(VI_01) Entering MASTER STATE <-- MASTER
Jan 10 17:57:01 mysqlt1 Keepalived_vrrp[6184]: VRRP_Instance(VI_01) setting protocol VIPs.
Jan 10 17:57:01 mysqlt1 Keepalived_healthcheckers[6183]: Netlink reflector reports IP 192.168.0.88 added
Jan 10 17:57:01 mysqlt1 avahi-daemon[937]: Registering new address record for 192.168.0.88 on em1.IPv4.
Jan 10 17:57:01 mysqlt1 Keepalived_vrrp[6184]: VRRP_Instance(VI_01) Sending gratuitous ARPs on em1 for 192.168.0.88
 

While in the other two:

Jan 10 17:56:59 mysqlt2 Keepalived_vrrp[13107]: VRRP_Instance(VI_01) Entering BACKUP STATE <--- 

Which means node is there as ... :D Backup.

Now is time to test our connection to our ProxySQL pool.

From an application node or just from your laptop.

Open 3 terminals and in each one:

 watch -n 1 'mysql -h <IP OF THE REAL PROXY (test1|test2|test3)> -P 3310 -uadmin -padmin -t -e "select * from stats_mysql_connection_pool where hostgroup in (500,501,9500,9501) order by hostgroup,srv_host ;" -e " select srv_host,command,avg(time_ms), count(ThreadID) from stats_mysql_processlist group by srv_host,command;" -e "select * from stats_mysql_commands_counters where  Total_Time_us > 0;"'

You will see that unless you are already sending queries to proxies, you have the Proxies just doing nothing.
Time to start the test bash as I indicate above.
If everything is working correctly you will see the bash command reporting this:

+----+----------------------------+----------------------------+-------------+----------------------------+
| 49 | 2017-01-10 18:12:07.739152 | 2017-01-10 18:12:07.733282 | galera1h1n5 | load_RW_192.168.0.11:33273 |
+----+----------------------------+----------------------------+-------------+----------------------------+
  ID    execution time in the bash   exec time inside mysql     node hostname   user and where the connection is coming from

The other 3 running bash commands will show that ONLY the ProxySQL in TEST1 is currently getting/serving requests, because is the one with the VIP.

Like:

+-----------+---------------+----------+--------+----------+----------+--------+---------+---------+-----------------+-----------------+------------+
| hostgroup | srv_host      | srv_port | status | ConnUsed | ConnFree | ConnOK | ConnERR | Queries | Bytes_data_sent | Bytes_data_recv | Latency_ms |
+-----------+---------------+----------+--------+----------+----------+--------+---------+---------+-----------------+-----------------+------------+
| 500       | 192.168.0.21  | 3306     | ONLINE | 0        | 0        | 0      | 0       | 0       | 0               | 0               | 629        |
| 500       | 192.168.0.231 | 3306     | ONLINE | 0        | 0        | 0      | 0       | 0       | 0               | 0               | 510        |
| 500       | 192.168.0.5   | 3306     | ONLINE | 0        | 0        | 3      | 0       | 18      | 882             | 303             | 502        |
| 501       | 192.168.0.21  | 3306     | ONLINE | 0        | 0        | 0      | 0       | 0       | 0               | 0               | 629        |
| 501       | 192.168.0.231 | 3306     | ONLINE | 0        | 0        | 0      | 0       | 0       | 0               | 0               | 510        |
| 501       | 192.168.0.5   | 3306     | ONLINE | 0        | 0        | 0      | 0       | 0       | 0               | 0               | 502        |
+-----------+---------------+----------+--------+----------+----------+--------+---------+---------+-----------------+-----------------+------------+
+---------+---------------+-----------+-----------+-----------+---------+---------+----------+----------+-----------+-----------+--------+--------+---------+----------+
| Command | Total_Time_us | Total_cnt | cnt_100us | cnt_500us | cnt_1ms | cnt_5ms | cnt_10ms | cnt_50ms | cnt_100ms | cnt_500ms | cnt_1s | cnt_5s | cnt_10s | cnt_INFs |
+---------+---------------+-----------+-----------+-----------+---------+---------+----------+----------+-----------+-----------+--------+--------+---------+----------+
| BEGIN   | 9051          | 3         | 0         | 0         | 0       | 3       | 0        | 0        | 0         | 0         | 0      | 0      | 0       | 0        |
| COMMIT  | 47853         | 3         | 0         | 0         | 0       | 0       | 0        | 3        | 0         | 0         | 0      | 0      | 0       | 0        |
| INSERT  | 3032          | 3         | 0         | 0         | 1       | 2       | 0        | 0        | 0         | 0         | 0      | 0      | 0       | 0        |
| SELECT  | 8216          | 9         | 3         | 0         | 3       | 3       | 0        | 0        | 0         | 0         | 0      | 0      | 0       | 0        |
| SET     | 2154          | 3         | 0         | 0         | 3       | 0       | 0        | 0        | 0         | 0         | 0      | 0      | 0       | 0        |
+---------+---------------+-----------+-----------+-----------+---------+---------+----------+----------+-----------+-----------+--------+--------+---------+----------+

So nothing special right, all as expected. 

Time to see if the failover-failback works along the chain.

Let us kill the ProxySQL on TEST1 while the test bash command is running.

killall -9 proxysql 

Here is what you will get:

+----+----------------------------+----------------------------+-------------+----------------------------+
| 91 | 2017-01-10 18:19:06.188233 | 2017-01-10 18:19:06.183327 | galera1h1n5 | load_RW_192.168.0.11:33964 |
+----+----------------------------+----------------------------+-------------+----------------------------+
ERROR 2003 (HY000): Can't connect to MySQL server on '192.168.0.88' (111)
+----+----------------------------+----------------------------+-------------+----------------------------+
| 94 | 2017-01-10 18:19:08.250093 | 2017-01-10 18:19:11.250927 | galera1h1n5 | load_RW_192.168.0.12:39635 | <-- note 
+----+----------------------------+----------------------------+-------------+----------------------------+
 

the source had change  but not the PXC node.

If you check the system log for TEST1:

Jan 10 18:19:06 mysqlt1 Keepalived_vrrp[6184]: VRRP_Script(check_proxy) failed
Jan 10 18:19:07 mysqlt1 Keepalived_vrrp[6184]: VRRP_Instance(VI_01) Received higher prio advert
Jan 10 18:19:07 mysqlt1 Keepalived_vrrp[6184]: VRRP_Instance(VI_01) Entering BACKUP STATE
Jan 10 18:19:07 mysqlt1 Keepalived_vrrp[6184]: VRRP_Instance(VI_01) removing protocol VIPs.
Jan 10 18:19:07 mysqlt1 Keepalived_healthcheckers[6183]: Netlink reflector reports IP 192.168.0.88 removed

While on TEST2

Jan 10 18:19:08 mysqlt2 Keepalived_vrrp[13107]: VRRP_Instance(VI_01) Transition to MASTER STATE
Jan 10 18:19:09 mysqlt2 Keepalived_vrrp[13107]: VRRP_Instance(VI_01) Entering MASTER STATE
Jan 10 18:19:09 mysqlt2 Keepalived_vrrp[13107]: VRRP_Instance(VI_01) setting protocol VIPs.
Jan 10 18:19:09 mysqlt2 Keepalived_healthcheckers[13106]: Netlink reflector reports IP 192.168.0.88 added
Jan 10 18:19:09 mysqlt2 Keepalived_vrrp[13107]: VRRP_Instance(VI_01) Sending gratuitous ARPs on em1 for 192.168.0.88

Simple ... and elegant. No need to re-invent the wheel and works smooth.

The total time for the recovery given the ProxySQL crash had be of 5.06 seconds,

considering the wider window ( last application start, last recovery in PXC  2017-01-10 18:19:06.188233|2017-01-10 18:19:11.250927)

As such the worse scenario, keeping in mind we run the check for the ProxySQL every 2 seconds (real recover max window 5-2=3 sec).

OK what about fail-back?

Let us restart the proxysql service:

/etc/init.d/proxysql start (or systemctl)

Here the output:

+-----+----------------------------+----------------------------+-------------+----------------------------+
| 403 | 2017-01-10 18:29:34.550304 | 2017-01-10 18:29:34.545970 | galera1h1n5 | load_RW_192.168.0.12:40330 |
+-----+----------------------------+----------------------------+-------------+----------------------------+
+-----+----------------------------+----------------------------+-------------+----------------------------+
| 406 | 2017-01-10 18:29:35.597984 | 2017-01-10 18:29:38.599496 | galera1h1n5 | load_RW_192.168.0.11:34640 |
+-----+----------------------------+----------------------------+-------------+----------------------------+

Worse recovery time = 4.04 seconds of which 2 of delay because the check interval.

Of course the test is running every second and is running one single operation, as such the impact is minimal (no error in fail-back), and recovery longer.

But I think I have made clear the concept here. 

Let see another thing... is the failover working as expected? Test1 -> 2 -> 3 ??

Let us kill 1 - 2 and see:

Kill Test1 :
+-----+----------------------------+----------------------------+-------------+----------------------------+
| 448 | 2017-01-10 18:35:43.092878 | 2017-01-10 18:35:43.086484 | galera1h1n5 | load_RW_192.168.0.11:35240 |
+-----+----------------------------+----------------------------+-------------+----------------------------+
+-----+----------------------------+----------------------------+-------------+----------------------------+
| 451 | 2017-01-10 18:35:47.188307 | 2017-01-10 18:35:50.191465 | galera1h1n5 | load_RW_192.168.0.12:40935 |
+-----+----------------------------+----------------------------+-------------+----------------------------+
...
Kill Test2
+-----+----------------------------+----------------------------+-------------+----------------------------+
| 463 | 2017-01-10 18:35:54.379280 | 2017-01-10 18:35:54.373331 | galera1h1n5 | load_RW_192.168.0.12:40948 |
+-----+----------------------------+----------------------------+-------------+----------------------------+
+-----+----------------------------+----------------------------+-------------+-----------------------------+
| 466 | 2017-01-10 18:36:08.603754 | 2017-01-10 18:36:09.602075 | galera1h1n5 | load_RW_192.168.0.235:33268 |
+-----+----------------------------+----------------------------+-------------+-----------------------------+

This image is where you should be at the end:

In this case given I have done one kill immediately after the other, Keepalived had take a bit more in failing over, but still it did correctly and following the planned chain.

Fail-back as smooth as usual:

1
2
3
4
5
6

+-----+----------------------------+----------------------------+-------------+-----------------------------+
| 502 | 2017-01-10 18:39:18.749687 | 2017-01-10 18:39:18.749688 | galera1h1n5 | load_RW_192.168.0.235:33738 |
+-----+----------------------------+----------------------------+-------------+-----------------------------+
+-----+----------------------------+----------------------------+-------------+----------------------------+
| 505 | 2017-01-10 18:39:19.794888 | 2017-01-10 18:39:22.800800 | galera1h1n5 | load_RW_192.168.0.11:35476 |
+-----+----------------------------+----------------------------+-------------+----------------------------+

Let us see now another case.

The case above is nice and simple, but as a cavet.

I can access only one ProxySQL a time, which may be good or not.

In any case it may be nice to have the possibility to choose, and with Keepalived you can. 

We can actually set an X number of VIP and associate them to each test box.

The result will be that each server hosting ProxySQL will also host a VIP, and will be eventually able to fail-over to any of the other two servers.

Failing-over/Back will be fully managed by Keepalived, checking as we did before if ProxySQL is running.
Example of configuration for one node can be the one below:

global_defs {
  # Keepalived process identifier
  lvs_id proxy_HA
}
# Script used to check if Proxy is running
vrrp_script check_proxy {
  script "killall -0 proxysql"
  interval 2
  weight 3
}
 
# Virtual interface 1
# The priority specifies the order in which the assigned interface to take over in a failover
vrrp_instance VI_01 {
  state MASTER
  interface em1
  virtual_router_id 51
  priority 102
 
  # The virtual ip address shared between the two loadbalancers
  virtual_ipaddress {
    192.168.0.88 dev em1
  }
  track_script {
    check_proxy
  }
}
 
# Virtual interface 2
# The priority specifies the order in which the assigned interface to take over in a failover
vrrp_instance VI_02 {
  state MASTER
  interface em1
  virtual_router_id 52
  priority 100
 
  # The virtual ip address shared between the two loadbalancers
  virtual_ipaddress {
    192.168.0.89 dev em1
  }
  track_script {
    check_proxy
  }
}
 
# Virtual interface 3
# The priority specifies the order in which the assigned interface to take over in a failover
vrrp_instance VI_03 {
  state MASTER
  interface em1
  virtual_router_id 53
  priority 99
 
  # The virtual ip address shared between the two loadbalancers
  virtual_ipaddress {
    192.168.0.90 dev em1
  }
  track_script {
    check_proxy
  }
}

The tricky part in this case is to play with the PRIORITY for each VIP and each server such that you will NOT assign the same ip twice.

The whole set of configs can be found here

Performing the check with the test bash as above we have:

Test 1 crash
+-----+----------------------------+----------------------------+-------------+----------------------------+
| 422 | 2017-01-11 18:30:14.411668 | 2017-01-11 18:30:14.344009 | galera1h1n5 | load_RW_192.168.0.11:55962 |
+-----+----------------------------+----------------------------+-------------+----------------------------+
ERROR 2003 (HY000): Can't connect to MySQL server on '192.168.0.88' (111)
+-----+----------------------------+----------------------------+-------------+----------------------------+
| 426 | 2017-01-11 18:30:18.531279 | 2017-01-11 18:30:21.473536 | galera1h1n5 | load_RW_192.168.0.12:49728 | <-- new server
+-----+----------------------------+----------------------------+-------------+----------------------------+
....
Test 2 crash
+-----+----------------------------+----------------------------+-------------+----------------------------+
| 450 | 2017-01-11 18:30:27.885213 | 2017-01-11 18:30:27.819432 | galera1h1n5 | load_RW_192.168.0.12:49745 |
+-----+----------------------------+----------------------------+-------------+----------------------------+
ERROR 2003 (HY000): Can't connect to MySQL server on '192.168.0.88' (111)
+-----+----------------------------+----------------------------+-------------+-----------------------------+
| 454 | 2017-01-11 18:30:30.971708 | 2017-01-11 18:30:37.916263 | galera1h1n5 | load_RW_192.168.0.235:33336 | <-- new server
+-----+----------------------------+----------------------------+-------------+-----------------------------+
 

Final state of IPs on Test3:

enp0s8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 08:00:27:c2:16:3f brd ff:ff:ff:ff:ff:ff
    inet 192.168.0.235/24 brd 192.168.0.255 scope global enp0s8   <-- Real IP
       valid_lft forever preferred_lft forever
    inet 192.168.0.90/32 scope global enp0s8    <--- VIP 3
       valid_lft forever preferred_lft forever
    inet 192.168.0.89/32 scope global enp0s8    <--- VIP 2
       valid_lft forever preferred_lft forever
    inet 192.168.0.88/32 scope global enp0s8    <--- VIP 1
       valid_lft forever preferred_lft forever
    inet6 fe80::a00:27ff:fec2:163f/64 scope link 
       valid_lft forever preferred_lft forever
 

And this is the image:

Recovery times:

  test 1 crash = 7.06 sec (worse case scenario)
  test 2 crash = 10.03 sec (worse case scenario)

Conclusions

In this example I had just use a test that checks the process, but a check can be anything reporting 0|1, the limit is define only from what you need.

The times for the failover can be significant shorter, reducing the check time and considering only the time taken to move the VIP, I had prefer to show the worse case scenario considering an application with a second interval, but that is a pessimistic view of what normally happens with real traffic. 

I was looking for a simple, simple simple way to add HA to ProxySQL, something that can be easily integrate with automation and that is actually also well established and maintained.

In my opinion using Keepalived is a good solution because it match all the above expectations. 

Implementing a set of ProxySQL and have Keepalived manage the failover between them is pretty easy, but you can expand the usage (and the complexity) if you need, counting on tools that are already part of the Linux stack, no need to re-invent the wheel with crazy mechanism.

If you want to have fun doing crazy things... at least start from something that helps you to go beyond the basiscs. 

For instance I was also playing a bit with keepalived and virtual server, creating set of redundant Proxysql with load balancers and ... .. but this is another story (blog).

Great MySQL & ProxySQL to all!