TL;DR – We encountered multiplexing issues with ProxySQL and after going around in circles for a while we found that the impact of mysql-auto_increment_delay_multiplex and mysql-connection_delay_multiplex_ms was not documented. Read the blog post why you should check these values in your configuration and check how your codebase handles last insert identifiers!
At my present company we are using a multi-layer ProxySQL setup to route our traffic to the appropriate database cluster’s primary or replica hosts. For this post it doesn’t matter whether you run a single or a multi-layer setup, have many backend servers configured or not, so I’ll simplify our architecture to a single ProxySQL layer where the application connects to all three proxies evenly:
The reason for having N+2 proxies is that this ensures us that we can retain high availability after failure of a single node. I’ve also added a fourth (canary) node and sent a small amount of traffic to that to see how any change would affect multiplexing.
One of the key features of ProxySQL is that it’s able to multiplex connections. In short: ProxySQL will attempt to send queries to your database servers over idle database connections. In effect queries of separate incoming connections can be sent to the master with as little backend connections as possible, lowering the connection overhead on your database servers. This could mean queries from the same incoming connection are being multiplexed over multiple backend connections. The limitation is that you can’t multiplex queries within a transaction and neither if you place locks, as this would require the connection to stick with the same backend connection. See also the ProxySQL multiplexing documentation on every condition where multiplexing will be (temporarily) disabled.
Problems encountered with high connection counts
After migration of a single database into multiple shards, we found that ProxySQL was hardly multiplexing connections at all and due to the increase in connections by adding a shard this wasn’t a scalable solution. The number of ingress connections is about 600 per shard, while the number egress connections stuck around 400 per shard. This meant that the ratio of ingress vs egress is about 66% and that’s not a good sign as ProxySQL is supposed to be multiplexing. A good and effective ratio would be more in the lines of 5%. We were certain ProxySQL was multiplexing on other hostgroups before as the ratio was far more favorable there.
Why is this a big issue then? For us having more than 2000 active connections frontend and about 1400 backend meant ProxySQL was using a lot of CPU time. On a 4 core machine we noticed our CPUs were over 70% busy all the time, which wouldn’t allow us to lose a single ProxySQL node anymore. To stay within safe margins we first upgraded all ProxySQL hosts to 8 cores and this kept the CPUs within the regions of 40% to 50%. Adding a new shard would increase the incoming connections with 600 and backend connections with 400. That would only allow us to add maybe one or two shards before we would no longer be able to lose a ProxySQL node again and upgrade once more. Adding more ProxySQL hosts would work, but as we have a multi-layer ProxySQL architecture we need to scale all layers to keep up with the incoming connections on the other layers as well. In other words: our sharded environment just faced a new challenge as scale out wasn’t an (easy) option anymore.
If you search for ProxySQL and Multiplexing, the top results will be the documentation of ProxySQL Multiplexing and a few blog posts describing the wonders of ProxySQL multiplexing. In the documentation a few conditions when multiplexing is disabled are described, so naturally we examined if our application was meeting these conditions. We examined the general log, query digest inside ProxySQL, tcpdumps and concluded it wasn’t meeting any of these conditions and thus theoretically multiplexing should be functioning. But as we already established with the high incoming and backend connection it clearly wasn’t. Our first dead end.
Investigating ProxySQL bug reports
Then we started to investigate bug reports of ProxySQL to see if anything matched there. At first we found issue #1897 where Multiplexing was erroneously was disabled and there were a few other solved issues that hinted we should try to upgrade. However upgrading a production environment without knowing the impact is never favorable, so instead we added a fourth ProxySQL host and sent 1% of the traffic towards it. This allowed us to easily upgrade the host to various ProxySQL versions and see if this would resolve the issue. Unfortunately we found none of the upgrades would resolve our multiplexing issues. Another dead end.
We did notice the set names metric in ProxySQL was increasing fast, so this lead us to issue #1019 where multi-layered ProxySQL can have issues with set names where the database has a different characterset as ProxySQL’s default characterset. This was the case with for us, but the documentation doesn’t mention set names influences multiplexing and the previous upgrade to 2.0.8 didn’t resolve our issues. At least we found out why the number of set names is increasing. Another dead end.
Then one of our developers pointed me towards ProxySQL issue #2339 in which a wjordan requests a documentation update on temporary conditions when multiplexing is disabled. He describes the mysql-auto_increment_delay_multiplex and mysql-connection_delay_multiplex_ms variables to be missing in this page. I totally ignored this when I searched through the ProxySQL issues as the title contained the word documentation. Insert face palm here.
The two variables basically are a workaround for issue #1048 (Problem with “LAST_INSERT_ID()” returning 0), issue #1421 (Forward SELECTs on LAST_INSERT_ID) and issue #1452 (select last_insert_id() as ID) and are bundled in issue #1828. Ironically issue #1828 is the source for issue #1897, so I went full circle there! Insert second facepalm here.
How ProxySQL delays multiplexing
So what do these two variables do then? First we have to understand the origin of the problem here: with multiplexing enabled, whenever an insert or update would cause an auto increment column to increase, the LAST_INSERT_ID() query would not be guaranteed to be run on the same connection or right after the insert happened. So either it would be proxied over another connection (returning a wrong identifier or 0) or it would be run out of order due to multiplexing (again returning a wrong identifier or 0). These two variables allow you to have the connection stop multiplexing for X-number of queries or X-number of milliseconds after an insert or update happened on an auto increment column.
But wait! How does ProxySQL detect an auto increment column increased then? That’s quite simple: on every successfully query the MySQL binary protocol returns an OK packet to the client. Whenever an insert or update would trigger an auto increment column this OK packet also returns the last inserted identifier. This is what triggers ProxySQL to stop multiplexing for X-number of queries or milliseconds.
But wait! Why did the majority of applications still function correctly before ProxySQL 1.4.14 then? That’s also quite simple: most programming languages are using the native MySQL drivers. For instance PHP PDO makes use of mysqlnd that uses the native MySQL C-driver. Just like ProxySQL, mysqlnd reads the OK packet and stores the last inserted identifier internally. So when you make use of the LastInsertId function in PDO, this will retrieve the value from mysqlnd internally using the function . In general you can assume database drivers never run a SELECT LAST_INSERT_ID() against a database. However you should be cautious with some ORMs, like Hybernate, that are actually depending on queries like this.
The default for mysql-auto_increment_delay_multiplex is 5 and mysql-connection_delay_multiplex_ms is 0. So whenever ProxySQL encounters an OK packet with last inserted identifier set it will disable multiplexing on the connection for 5 consecutive queries. This basically locks the frontend connection to the backend connection for at least 5 queries. This to allow an ORM (or application) to run a LAST_INSERT_ID() query on the same connection. New incoming connections will then have to use a different connection from the connection pool. Obviously whenever ProxySQL encounters another OK packet with last inserted identifier it will reset this connection again to 5 consecutive queries again.
Back to our own problem statement
So why did this happen only to the sharded databases then? The primary key of our main table in the sharded database contains a UUID and not an auto increment. There was, however, an auto increment column present on the table to facilitate a sequence number in our records. We clearly overlooked this in our investigation as well. There are various reasons why we currently have to keep this column, so it wasn’t feasible to remove the column for now.
Also our sharded databases have a read/write ratio of around 3:2. That means 2 out of 5 queries will cause an auto increment to trigger and thus lock the frontend connection to a backend connection for at least 5 consecutive queries! With that ratio we will hardly ever multiplex! Once we changed this variable on our canary ProxySQL we immediately saw a significant drop in backend connections on ProxySQL. First we set mysql-auto_increment_delay_multiplex to 0, which caused all 16 incoming connections to be proxied over an average of 0 connections! When we changed the value to 1 it averaged over 3 connections!
Before applying this to all other ProxySQL hosts, there was one more hurdle: both variables are global, so they apply to all hostgroups. Once we established our codebase never ran a LAST_INSERT_ID() anywhere we changed it on all other ProxySQL hosts. The difference is night and day:
And the CPU usage dropped to a more sane 30%:
There are clearly three problems with the introduction of mysql-auto_increment_delay_multiplex and mysql-connection_delay_multiplex_ms:
- Both variables are global variables. If you change them, they will apply to all ProxySQL hostgroups, servers, users and connections
- The default of mysql-auto_increment_delay_multiplex is set to 5. This means multiplexing will be disabled or less effective on any write-intensive workload that contains an auto increment column. This is regardless whether your application actually uses the LAST_INSERT_ID statement or not.
- With the introduction of both variables in version 1.4.14, the documentation on both variables was updated. However it’s impact on multiplexing has never been updated.
By the time this blogpost goes public I have already made a change in the documentation of ProxySQL on multiplexing. I’ve also created a feature request for ProxySQL for the ability to control the delay per server or per user. But after I submitted the feature request I realized that this might actually make it only worse: even more variables, configurations and query rules to check and it doesn’t fix the problem at it’s root. It would be much cleaner for ProxySQL to store the last inserted identifier returned by the OK packet in a variable bound to the frontend connection that made the request. This identifier can then be returned whenever that frontend connection has a query that contains the LAST_INSERT_ID function.