Almost Perfect Service Discovery and Failover with ProxySQL and Orchestrator

Almost Perfect
Service Discovery and Failover
with ProxySQL and Orchestrator
by Jean-François Gagné
and Art van Scheppingen
Presented at Percona Live Online, May 2021

Almost Perfect
Jean-François Gagné
System and MySQL Expert at HubSpot
jgagne AT hubspot DOT com / @jfg956

MySQL Service Discovery at MessageBird
(Service Discovery and Failover with ProxySQL and Orchestrator – PL May 2021)
3

Summary of part #1
• MySQL Primary High Availability
• Failover to a Replica War Story
• MySQL at MessageBird (Percona Server)
• MySQL Service Discovery at MessageBird (ProxySQL)
• Orchestrator integration and the Failover Process
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MySQL Primary High Availability [1 of 4]
Failing-over the primary to a replica is my favorite high availability method
• But it is not as easy as it sounds, and it is hard to automate well
• An example of complete failover solution in production:
https://github.blog/2018-06-20-mysql-high-availability-at-github/
The five considerations for primary high availability:
(https://jfg-mysql.blogspot.com/2019/02/mysql-master-high-availability-and-failover-more-thoughts.html)
• Plan how you are doing primary high availability
• Decide when you apply your plan (Failure Detection – FD)
• Tell the application about the primary change (Service Discovery – SD)
• Protect against the limit of FD and SD for avoiding split-brains (Fencing)
• Fix your data if something goes wrong
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Failure detection (FD) is the 1st part (and 1st challenge) of failing-over
• It is a very hard problem: partial failure, unreliable network, partitions, …
• It is impossible to be 100% sure of a failure, and confidence needs time
à quick FD is unreliable, relatively reliable FD implies longer downtime
Ø Quick FD for short downtime generates false positive
Repointing is the 2nd part of failing-over to a replica:
• Relatively easy with the right tools: GTID, Pseudo-GTID, Binlog Servers, …
• Complexity grows with the number of direct replicas of the primary
• Some software for repointing:
• Orchestrator, Ripple Binlog Server,
Replication Manager, MHA, Cluster Control, MaxScale
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What do I mean by repointing:
• In below configuration and when the primary fails,
once one of the replica as been chosen as the new primary
the other replica needs to be re-sourced (re-slaved) to the new primary
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Service Discovery (SD) is the 3rd part (and 2nd challenge) of failover:
• If centralized à SPOF; if distributed à impossible to update atomically
• SD will either introduce a bottleneck (including performance limits)
• Or it will be unreliable in some way (pointing to the wrong primary)
• Some ways to implement Service Discovery: DNS, ViP, Proxy, Zookeeper, …
http://code.openark.org/blog/mysql/mysql-master-discovery-methods-part-1-dns
Ø Unreliable FD and unreliable SD is a recipe for split-brains !
Protecting against split-brains (Fencing): Adv. Subject – not many solutions
(proxies and semi-synchronous replication might help)
Fixing your data in case of a split-brain: only you can know how to do this !
(tip on this in the war story) 8

Failover War Story [1 of 6]
Some infrastructure context:
• Service Discovery is DNS (and failure detector is Orchestrator)
• The databases are behind a firewall in two data centers
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Some infrastructure context:
• Service Discovery is DNS (and failure detector is Orchestrator)
• The databases are behind a firewall in two data centers
• And we have a failure of the firewall in the zone of the primary
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Things went as planned: failed-over from Zone1 to Zone2
• New primary in zone 2: stop replication, set it read-write, update DNS, …
• Everything was ok…
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Things went as planned: failed-over from Zone1 to Zone2
• New primary in zone 2: stop replication, set it read-write, update DNS, …
• Everything was ok… until the firewall came back up
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Once the firewall came back up, no detectable problems
• But some intuition made me checked the binary logs of the old primary
• And I found new transactions with timestamp after the firewall recovery
(and obviously this is after the failover to zone 2)
13

These new transactions are problematic:
• They are in the databases in zone 1, but not in zone 2
• They share common auto-increments with data in zone 2
• Luckily, there are only a few transactions, so easy to fix, but what happened ?
14

The infrastructure is a little more complicated than initially presented:
• There are web servers and local DNS behind the firewalls (fw)
• The DNS update of the failover did not reach zone 1 (because of the fw failure)
• When the firewall came back up, the web servers received traffic
and because the DNS was not yet updated, they wrote on the old primary
• Once updated (a few seconds later), writes went to the new primary in zone 2
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This war story was a decentralized Service Discovery causing problems
Remember that it is not a matter of “if” but “when” things will go wrong
Please share your war stories so we can learn from each-others’ experience
• GitHub has a MySQL public Post-Mortem (great of them to share this):
https://blog.github.com/2018-10-30-oct21-post-incident-analysis/
• I also have another MySQL Primary Failover war story in another talk:
https://www.usenix.org/conference/srecon19emea/presentation/gagne
Tip for easier data-reconciliation: use UUID instead of auto-increments
• But store UUID in an optimized way (in primary key order)
https://www.percona.com/blog/2014/12/19/store-uuid-optimized-way/
http://mysql.rjweb.org/doc.php/uuid 16

MySQL at MessageBird [1 of 2]
MessageBird is using MySQL 5.7 (more precisely Percona Server)
These are hosted in many Google Cloud Regions
There are three types of MySQL deployments
1. Multi-region primary:
replicas in many regions and primary potentially on many regions
2. Single-region primary with replicas in many regions
3. Primary and replicas all in a single region
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MySQL at MessageBird [2 of 2]
This is a multi-region primary (regions are color-coded)

MySQL Service Discovery at MessageBird [1 / 12]
Requirements for MySQL Service Discovery:
• Being able to route traffic to local replicas
• Embed some sort of fencing mechanism
This led to a multi-layer solution using ProxySQL:
• Three layers for multi-region primary:
1. Collect
2. Master-Gateway (mgw)
3. Fencing
For single-region, mgw and fencing are merged in local-fencing (locfen)
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The collect layer is a standard entry-point design
The fencing layer is a natural HA way to route
traffic to the primary (a single node would not be HA)
The master-gateway layer is the glue between
collect and fencing (more about this later in the talk)
The local-fencing layer is the mgw and the fencing layers
merged for single-region primary databases because
routing to a single region does not need the mgw glue
(three nodes for N+2 HA, more about this later in the talk)

The collect is the entry-point of the MySQL Service Discovery:
• It starts with a load-balancer sending traffic to ProxySQL
• We have at least 3 instances of ProxySQL for N+2 high availability
• From here, read-only traffic is sent directly to replicas
• Primary traffic (read-write) is sent to mgw or locfen
22

Routing from collect to locfen is either local or crossing a region boundary
For mgw, it is biased to local when a mgw is on the same region as collect
• ProxySQL routing is weight-based (no easy fallback routing)
23

The master-gateway is deployed on all regions potentially hosting a primary
The same way fencing (or locfen) is designed as small as possible
to reduce the update-scope of failover (to a single region) …
… mgw bounds the update-scope of moving the primary to another region to
a continent (in this case, the three mgw regions are in Europe)
(it avoids a Planet-Scale reconfiguration of collect on failover)
… and the way mgw routes traffic to fencing protects against writing to the
primary in case of network partitions
24

25
The mgw routing is as follow:
• If the primary is in a remote region, traffic is routed to fencing in that region
• If the primary is in the same region, traffic is routed to the other mgw
Ø No path to the primary not crossing a region boundary

No path to the primary that is not crossing a region boundary
• That might sound sub-optimal, but it is an interesting tradeoff
• It makes the best vs worse case round-trip ratio to the primary closer
Without crossing a region boundary:
• Best case (local access): ~1 ms round-trip to the primary
• Worse case (remote access): ~20 ms round-trip to the primary
Ø Ratio of 1 to 20
With crossing a region boundary in mgw:
• Best case (remote access): ~20 ms round-trip to the primary
• Worse case (local access): ~40 ms round-trip to the primary
• Even worse case (remote routed to mgw of the primary) 60 ms
Ø Ratio of 1 to 2 (3 in the worse case) 26

27
The worse case (which could be avoided with smarter collect routing)

Avoiding low round-trip variance over optimal best-case
• With region-remote primary accesses, a high latency is unavoidable:
when having 20 ms latency, having 40 or 60 should not be problematic
• Moving the average closer to the median avoids problems
• And in the case where most writes are local,
it avoids surprises when the database becomes remote
• And it prevents writing to the primary in case of a network partition
28

29
And therefore, I claim this design has interesting tradeoff
• But it might not fit everyone’s requirements

Why only two fencing nodes and three locfen nodes:
• I like high availability N+2
• This allows a single failure to not need an immediate fix
• If a locfen node fails on Friday evening, it can wait until Monday
• A failure of one of the two fencing node looks problematic
• But we can failover to another region having two healthy nodes
• And updating two ProxySQL in case of a failover is easier than three
(needing three locfen is something I dislike as it makes failover more fragile)
30

Failover
31
Failing-over is a multi-step process:
1. Detecting a failure
2. Fencing the primary: setting it as OFFLINE_HARD in ProxySQL
3. Regrouping replicas under the new primary
4. Waiting for replication to catch-up on the new primary
5. Making the new primary ready: stop replication, set writable, start HB, …
6. Updating ProxySQL to point to the new primary
7. Re-configure fencing and master-gateway if needed

Reconfiguring Fencing and MGW [1 of 6]
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Reacting to a network partition is a similar operation

Orchestrator integration
Orchestrator integration
• Pre-failover hook: fence the primary in fencing (or locfen)
• Post-failover hook: update fencing (or locfen) to the new primary
• List of ProxySQL nodes (fencing or locfen)
and the ProxySQL hostgroup are store in each databases
which make this information available to the Orchestrator hooks
38

MySQL Service Discovery at MessageBird
39

Almost Perfect
Art van Scheppingen
Senior Database Engineer at MessageBird
art.vanscheppingen AT messagebird DOT com

Has been successfully in production for over two years now
• Most of our workload is on single-region primary (e.g. locfen)
• We have one cluster on multi-region primary
ProxySQL has been very stable for us
• No big issues on 1.4.x, 2.0.x and 2.1.x
How does it work far?
41

Easier for devs to setup connections to primary
• Point connection to ip address
• No failover handling necessary
Easier for devs to scale out reads
• Point connection to the same ip address
• Uses a different user for RO
• We can differentiate reads (read-only, read-only replica-only, etc)
How does it work far?
42

Multiplexing in ProxySQL turned out to be tricky
Without Multiplexing:
• 1 on 1 number of connections between Collect and Locfen
• Application makes a connection for each shard (8 at the moment)
• Application only uses one actively
• High number of connections means high load
With multiplexing
• High number of connections on Collect
• Theoretically only 1/8th of the connections on Locfen
• Even lower due to less overhead on establishing connections
Mastering multiplexing [1 of 4]
43

Prior to multiplexing
44

We enabled multiplexing in our configuration but not much happened
ProxySQL disabled multiplexing due to an auto increment
• Bugreport on ORM that didn’t handle multiplexing well (Hibernate)
• Parsing of the OK packet for auto increments
• Whenever encountered: multiplexing is affected
• mysql-auto_increment_delay_multiplex set to 5 by default
• This means for 5 consecutive queries multiplexing is disabled
45

After mastering multiplexing
46

We tried reusing collect and locfen as much as possible
• Centralized configuration (hostgroups and users)
• Less complexity
Expansion was inevitable
• Noisy neighbors (hostgroups)
• Reducing risk
• Better tuning for certain workloads
• Easier for maintenance
• Cascading effect to other hostgroups
Currently running 3 vertical stacks of collect and locfen
Separation of stacks
47

Above 50% CPU usage ProxySQL will show increased latency
• Lower CPU usage by multiplexing
• Lower CPU usage by idle-threads
When a certain hostgroup in Locfen is under stress (1000x normal workload)
• CPU usage can get above 50%
• Latency will increase on all hostgroups
• Latency will cascade upstream to the collect layer
Separation of stacks: Noisy neighbors [1 of 2]
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Above 50% CPU usage ProxySQL will show increased latency
Separation of stacks: Noisy neighbors [2 of 2]
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For us ProxySQL scales up to about 12K of connections per host
• After this we will hit the limits of TCP
Our Collect layer reached 7.8K connections
• If one collect host fails two remain
• Two remaining hosts will have to do an additional 3.9K connections
• Very close to our 12K limit
• Replacing a failed host now becomes an emergency operation
Separation of stacks: Reducing risk [1 of 4]
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ProxySQL keeps count of connection errors (MySQL + TCP shared)
• Will shun a host if a backend becomes “less responsive” (e.g. high
load)
• Happens on hostgroups with any number of hosts
Hitting the limits of TCP
• Errors to locfen or MGW will increase
• Locfen and MGW backends will be shunned
• Established connections will also be closed
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ProxySQL keeps count of connection errors (MySQL + TCP shared)
• Will shun a host if a backend becomes “less responsive” (e.g. high
load)
• Happens on hostgroups with any number of hosts
Shunning a primary for 1 second will cause another torrent of connections
• Client gets a timeout and will reconnect immediately
• No available backend: new connection is “paused” up to 10 seconds
• After 1 second primary become available again
• ProxySQL has thousands of connections waiting
• Rinse and repeat…
Shunning a primary [1 of 4]
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How to detect shunned hosts?
• ProxySQL will log a shunned host in the proxysql log
• This includes server name, error rate and duration of shun
2020-06-11 12:01:39 MySQL_HostGroups_Manager.cpp:311:connect_error(): [ERROR] Shunning server x.x.x.x:3306 with 10 errors/sec. Shunning for 10 seconds
How do we get them in our graphs?
• ProxySQL log tailer
• Looks for: connection timeouts, shunned hosts, shunned due to
replication lag
• Exports metrics to Prometheus every minute
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Shunning a primary for 1 second will cause an avalanche connections
• Normal latency is 10ms to 50ms
• Added latency of 1 second will decrease application throughput
• Decreased application throughput means k8s scale up workers
• k8s scale up means more incoming connections
• Rinse and repeat…
How we dealt with this:
• During some incidents we throttle down workers
• Counter intuitive: throttling down works increases throughput
• Some application workers now have a fixed ceiling
57

Most ProxySQL tuning is done on a global level
Some examples:
• mysql-connection_delay_multiplex_ms
• mysql-free_connections_pct
• mysql-wait_timeout
Having a separate stack allows
• Fine tuned multiplexing configuration
• Earlier or later closing of connections
• Separate handling of (end) user connections
Separation of stacks: Better tuning
58

Maintenance on Collect is scary
• Draining from GLB gracefully is ”closing after X-minutes”
• Near capacity means we run a risk when performing maintenance
Maintenance on MGW/Fencing/Locfen is scary
• Draining a host takes ages to happen
• Aggressive reuse of connections by connection pool
• Connection timeout (wait_timeout) is 8 hours
• Some applications don’t handle closing of a connection well
Separation of stacks: maintenance
59

Warstory: instability on one cluster swiped out many others
The instable cluster
• MySQL back_log set too low
• TCP listen overflows on primary
• ProxySQL started to shun primary
The effect
• Continuous shunning happened
• TCP listen overflows started to happen on ProxySQL
• Affected stability on other hostgroups
Separation of stacks: cascading effect [1 of 4]
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Shunning of primary causes endless shunning loop
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Listen overflows on ProxySQL hosts make it even worse
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In effect Collect layer shuns Locfen layer hosts
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After a client-connection closes, the connection will be reused
• Collect → Locfen
• Locfen → database
ProxySQL resets connection to initial connection parameters
What if new connection doesn’t match settings (e.g. UTF8mb4 or CET timezone?)
Will the connection between Locfen → database also change?
Other quirks: Connection contamination
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Uneven distribution of connections/queries
• Weight influences the distribution of connections
• Reuse of existing connections is favored by ProxySQL
• Influenced by mysql-free_connections_pct
The variable mysql-free_connections_pct is a global variable
• Percentage of maximum allowed connections of a hostgroup
• Some hostgroups allow up to 3000 incoming connections
• 2% of 3000 is 60 connections, actual usage is 10 to 15
• More connections are kept open in connection pool than necessary
Other quirks: Uneven distribution
65

Thanks !
Almost Perfect Service Discovery and Failover with ProxySQL and Orchestrator
by Jean-François Gagné
and Art van Scheppingen
Presented at Percona Live Online, May 2021

Almost Perfect Service Discovery and Failover with ProxySQL and Orchestrator

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