MaxScale and Xpand Tutorial
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.