# MySQL Clustering: 2020 Edition

Database servers are traditionally a pain. None of that has changed. However, some things are somewhat less of a pain than they were before. Getting MySQL to cluster is one of those things, though it involves deep, badly-documented magic. This post is intended for the average system administrator that knows their way around UNIX, but not necessarily DB systems. I won’t spend time explaining things like how to download and verify a file, but will try to explain what’s going on behind the curtain.

## Prior Work

First, let’s talk about how it used to be. Let’s talk about galera cluster.

Galera cluster is not a part of upstream mysql. A patch it required did become a part of MariaDB 10.1, but that doesn’t apply to the other components necessarily. You would configure approximately ~7 variables (including funny things like telling it to use rsync), custom-assign addresses, in config files. Then you would use a custom command galera_new_cluster on the bootstrap node. This would show a bunch of variables (starting with wsrep) that you can use to diagnose anything going wrong. Then you start the other nodes, and call it there. Replication data was unencrypted.

## MySQL Shell

This isn’t about mysql(1). This is actually about mysqlsh(1). It’s a new tool by upstream MySQL, not present in MariaDB (which has deviated now) or anything else. And it is made of pure black magic.

It connects to a MySQL server over either the classic (3306) or X (33060) protocol, and presents a prompt. This prompt comes in three styles:

1. JavaScript (default)

2. Python

3. SQL

The SQL prompt (accessible with \sql) is basically mysql(1) with tab-completion. We are interested in the JavaScript prompt, which allows performing administrative tasks over those protocols. Basically all of our interactions with mysql should likely be over MySQL Shell.

## The Clustering

First up, we need to install and have MySQL set up on all of our nodes (3, for the purposes of demonstration). We’ll call them "s1", "s2" and "s3". This MySQL installation has to be MySQL, not MariaDB, and should be of version 8.0.17 or higher. Then, create an administration user (doesn’t need to be root, but give it GRANT OPTION and such, I’ll call it "admin"). Once it’s set up and started, we can begin.

### Configuring Instances

From any host that can access port 3306 of all our nodes, we can start mysqlsh. We’ll connect to node 1, though we won’t use that connection right away. mysqlsh admin@s1:3306

This drops us into the JavaScript console. Now, MySQL’s clustering feature (called "InnoDB Cluster") is based on MySQL’s Group Replication feature. This feature has prerequisites, but we don’t have to worry about those. Recent MySQL has support for setting options programmatically and having them persist in the data directory. The options needed can change from version to version, and that’s where MySQL Shell will help us.

There is a global variable in the JS prompt called "dba". As you might imagine, it’s meant to help with DataBase Administration. It has just the function: dba.configureInstance('admin@s1:3306')

This should tell you that there are some missing options, and offer to make them for you, after asking for a password. If you go through it, it’ll stop and be happy. By default it will create a cluster administrator account for you. This is mostly fine, but if you want to have more control, you can set it yourself, like this: dba.configureInstance('admin@s1:3306', {clusterAdmin: "'innodb'@'%'"}). Repeat this for every instance.

Now, all of our nodes are ready to become cluster members.

### Creating the Cluster

You thought there was more to it? As much as the documentation tries to convince you otherwise, we’re already almost done!

This is the part where we use that connection to a specific node. If you just started mysqlsh without other arguments, use \connect to change that - \connect admin@s1:3306. This node will be our bootstrap node.

Now we can create the cluster, with the dedicated function! var cluster = dba.createCluster("myCluster"). You can call it whatever you want though. There are also some options you might potentially want to use (they work the same way as in the example from the last section). These options are:

ipWhitelist

set this if you know exactly what IPs your instances will have. It defaults to "AUTOMATIC", which just checks what private IPs your server has. An example value would be "192.168.1.0/24,10.0.0.1".

memberSslMode

set this if you want to disable SSL communications between the nodes for replication purposes. The defaults are to autodetect, and with the MySQL version I recommended it should automatically work. You can check in the status screen later.

multiPrimary

A boolean to enable multi-primary mode. Set to true if you REALLY REALLY know what you’re doing.

What this does in the background is a couple of things. It bootstraps the cluster, sure, but it also creates metadata stuff (you can later analyze what databases are present) that will be very important for MySQL Router.

Now, we can add additional nodes. This is done using, you guessed it, yet another function! This function needs to be run against the cluster object we created in the previous section. cluster.addInstance("admin@s2:3306"), and admin@s3:3306.

 Note If you took a break and need to get it back, you can call var cluster = dba.getCluster() while connected to any node that is a member (well, just our seed node right now).

The only relevant option here is ipWhitelist. Same story as when we created the cluster. It will ask you for a replication method. I strongly recommend just using the default ("Clone") method, especially if you’re bootstrapping a new cluster.

### Verifying Cluster Status

Well, actually, we’re done now. You can check your cluster’s status using cluster.status().

One important thing to note here is that some nodes are R/O, while one is R/W. This is because we are in what is called "single-primary" mode that MySQL InnoDB Cluster defaults to. In single primary mode, there is one node that accepts writes, while the other nodes are readers. They communicate using paxos to check up on one another, automatically recover from tolerable failures, and elect new primaries.

### MySQL Router

You might then ask - how would the clients function? The clients clearly need some information to know where to send their writes (at the very least). This is where MySQL Router comes in. It is a thin application that knows how to read the InnoDB Cluster metadata, and then proxy requests over. MySQL recommends simply deploying it on application servers, to minimize latency. This is what the process looks like in general, though.

Technically, MySQL Router doesn’t really need to talk to InnoDB - it can just perform the job of a simple reverse proxy. You can look at the docs on that here. However, since we have an InnoDB Cluster now, it’s much easier.

Once it is installed, we simply need to run mysqlrouter --bootstrap admin@s1:3306 --user mysqlrouter (assuming "mysqlrouter" is the user that will run it). You can examine the configuration file it creates for itself (under /etc/mysqlrouter, most likely). There are a few options that may be interesting to us. By default, mysqlrouter will create an unprivileged account for itself that it will remember, but we can use a specific one using --account if we have something particular in mind.

 Note the --account option is only available in MySQL Router version 8.0.19 and above.

Once it’s set up, we now have 4 ports. 6446 and 6447 are the classic protocol ports, while 64460 and 64470 are the X protocol ports. The 60? ports are used for reads and writes, which means they’re limited to a single destination server in single primary mode. The 70? ports are used for reads only, and will route to all of the possible (online) servers.

You can also see it as a metadata consumer in cluster.status()!

## Debugging

A few things can go wrong during this process, but most of them should be fairly obvious. For instance, if the error message says "s1 is 127.0.0.1", your distro has deployed itself in such a way that it refers to itself as localhost. This is great and all, but you should probably remove that for the purposes of initial setup. Another possible issue is an issue with IPs (e.g if you don’t use "%" as the hostname, while not fully realizing how the traffic might flow).

## Multi-Primary Mode

I mentioned it before, so one may be wondering what this mode is. The short version is that it makes all nodes equivalent - they can all be used for writes - they are all primaries. There are several tradeoffs though.

The first, and lesser one, is that the cluster now becomes eventually (instead of strongly) consistent. I am of the opinion that for the absolute vast majority of cases, this is fine (and even desirable), though. Still, if it’s a problem, the absolute newest versions of MySQL have settings to increase the consistency guarantees, at the expense of performance.

The second problem is that multiple features are no longer valid. For example, you can no longer have foreign key constraints that cascade. The subject is fairly lengthy, so refer to the docs as to what limitations are in place.

If you’re certain that you want to use multi-primary mode, you can just switch to it with this function. cluster.switchToMultiPrimaryMode()

## Recovering From Complete Outage

It happens to everyone. Lightning struck down the whole city’s power supply for 4 days straight or something. Now your cluster can’t recover, since it went fully down. Just connect to one of the nodes with MySQL Shell, and then call this function. Yes, there is literally a function you use like this: dba.rebootClusterFromCompleteOutage(). No, really.