Selecting Replica Servers with MaxScale's Read/Write Split Router

This page is part of MariaDB's Documentation.

The parent of this page is: Read/Write Split Router

Topics on this page:

Overview
Using Adaptive Routing
Using Least Behind Primary
Setting the Replica Selection Criteria

Overview

The Read/Write Split Router (readwritesplit) load balances read-only queries between one or more replica servers. It selects a replica server to execute a query using criteria configured by the slave_selection_criteria parameter.

Criterion	Description
`ADAPTIVE_ROUTING`	Selects using average response times
`LEAST_BEHIND_MASTER`	Selects based on replication lag
`LEAST_CURRENT_OPERATIONS`	Selects based on number of active operations (the default)
`LEAST_GLOBAL_CONNECTIONS`	Selects based on number of connections from MariaDB MaxScale
`LEAST_ROUTER_CONNECTIONS`	Selects based on number of connections from the service

Using Adaptive Routing

The Read/Write Split Router (readwritesplit) uses adaptive routing when the slave_selection_criteria parameter is set to ADAPTIVE_ROUTING.

In this mode, the router measures average server response times. When the router routes queries, it compares the response times of the different replica servers. It favors the faster servers for most queries, while still guaranteeing some traffic on the slowest servers.

Using Least Behind Primary

The Read/Write Split Router (readwritesplit) uses the replica server that is least behind the primary server when the slave_selection_criteria parameter is set to LEAST_BEHIND_MASTER. This mode is only compatible with MariaDB replication MariaDB replication.

In this mode, the router measures replica lag using the Seconds_Behind_Master column from SHOW REPLICA STATUS SHOW REPLICA STATUS. The replica server that has the lowest value is considered to be the least behind the primary server.

Setting the Replica Selection Criteria

Set the replica selection criteria by configuring the slave_selection_criteria parameter for the Read/Write Split Router in maxscale.cnf:

[split-router]
type                     = service
router                   = readwritesplit
...
slave_selection_criteria = LEAST_GLOBAL_CONNECTIONS

Restart the MaxScale instance.
```
$ sudo systemctl restart maxscale
```