MaxScale and Xpand Tutorial

Since version 2.4, MaxScale has built-in support for Xpand. This tutorial explains how to setup MaxScale in front of a Xpand cluster.

There is no Xpand specific router, but both the readconnroute and the readwritesplit routers can be used.

Xpand and Readconnroute

With readconnroute you get simple connection based routing, where each new connection is created (by default) to the Xpand node with the least amount of existing connections. That is, with readconnroute the behaviour will be very similar to the behaviour when HAProxy is used as the Xpand load balancer.

Bootstrap servers

The Xpand monitor is capable of autonomously figuring out the cluster configuration, but in order to get going there must be at least one server-section referring to a node in the Xpand cluster.

[Bootstrap-1]
type=server
address=IP-OF-NODE
port=3306
protocol=MySQLBackend

That server defintion will be used by the monitor in order to connect to the Xpand cluster. There can be more than one such "bootstrap" definition to cater for the case that the node used as a bootstrap server is down when MaxScale starts.

NOTE These bootstrap servers should only be referred to from the Xpand monitor configuration, but never from a service.

Monitor

In the Xpand monitor section, the bootstrap servers are referred to in the same way as "ordinary" servers are referred to in other monitors.

[Xpand]
type=monitor
module=xpandmon
servers=Bootstrap-1
user=USER
password=PASSWORD

The bootstrap servers are only used for connecting to the Xpand cluster; thereafter the Xpand monitor will dynamically find out the cluster configuration.

The discovered cluster configuration will be stored (the ips and ports of the Xpand nodes) and upon subsequent restarts the Xpand monitor will use that information if the bootstrap servers happen to be unavailable.

With the configuration above maxctrl list servers might output the following:

┌───────────────────┬──────────────┬──────┬─────────────┬─────────────────┬──────┐
│ Server            │ Address      │ Port │ Connections │ State           │ GTID │
├───────────────────┼──────────────┼──────┼─────────────┼─────────────────┼──────┤
│ @@Xpand:node-7    │ 10.2.224.102 │ 3306 │ 0           │ Master, Running │      │
├───────────────────┼──────────────┼──────┼─────────────┼─────────────────┼──────┤
│ @@Xpand:node-8    │ 10.2.224.103 │ 3306 │ 0           │ Master, Running │      │
├───────────────────┼──────────────┼──────┼─────────────┼─────────────────┼──────┤
│ @@Xpand:node-6    │ 10.2.224.101 │ 3306 │ 0           │ Master, Running │      │
├───────────────────┼──────────────┼──────┼─────────────┼─────────────────┼──────┤
│ Bootstrap-1       │ 10.2.224.101 │ 3306 │ 0           │ Master, Running │      │
└───────────────────┴──────────────┴──────┴─────────────┴─────────────────┴──────┘

All servers whose name start with @@ have been detected dynamically.

Note that the address 10.2.224.101 appears twice; once for Bootstrap-1 and another time for @@Xpand:node-6. The Xpand monitor will create a dynamic server instance for all nodes in the Xpand cluster; also for the ones used in bootstrap server sections.

Service

The service is specified as follows:

[Xpand-Service]
type=service
router=readconnroute
user=USER
password=PASSWORD
cluster=Xpand

Note that the service does not list any specific servers, but instead refers, using the argument cluster, to the Xpand monitor.

In practice this means that the service will use the servers of the monitor named Xpand and in the case of a Xpand monitor those servers will be the ones that the monitor has detected dynamically. That is, when setup like this, the service will automatically adjust to any changes taking place in the Xpand cluster.

NOTE There is no need to specify any router_options, but the default router_options=running provides the desired behaviour. In particular do not specify router_options=master as that will cause only a single node to be used.

Listener

To complete the configuration, a listener must be specified.

[Xpand-Service-Listener]
type=listener
service=Xpand-Service
protocol=MariaDBClient
port=4008

Xpand and Readwritesplit

The primary purpose of the router readwritesplit is to split statements between one master and multiple slaves. In the case of Xpand, all servers will be masters, but readwritesplit may still be the right choise.

Namely, as readwritesplit is transaction aware and capable of replaying transactions, it can be used for hiding certain events taking place in Xpand from the clients that use it.

For instance, whenever a node is removed from or added to a Xpand cluster there will be a group change, which is visible to a client as a transaction rollback. However, if readwritesplit is used and transaction replay is enabled, then MaxScale may be able to hide the group change so that the client only detects a slight delay.

Apart from the service section, the configuration when using readwritesplit is identical to the readconnroute configuration described above.

Service

The service is specified as follows:

[Xpand-Service]
type=service
router=readwritesplit
user=maxscale
password=maxscale
cluster=Xpand
transaction_replay=true
slave_selection_criteria=LEAST_GLOBAL_CONNECTIONS

With this configuration, subject to the boundary conditions of transaction replaying, a client will neither notice group change events nor the disappearance of the very node the client is connected to. In that latter case, MaxScale will simply connect to another node and replay the current transaction (if one is active). For detailed information about the transaction replay functionality, please refer to the readwritesplit documentation.

NOTE It is vital to have slave_selection_criteria=LEAST_GLOBAL_CONNECTIONS, as otherwise connections will not be distributed evenly across all Xpand nodes.

As a rule of thumb, use readwritesplit if it is important that changes taking place in the cluster configuration are hidden from the applications, otherwise use readconnroute.