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

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.

In this mode, the router measures replica lag using the Seconds_Behind_Master column from 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

  1. 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
    
  2. Restart the MaxScale instance.

    On operating systems that use systemd:

    $ sudo systemctl restart maxscale
    

    On systems that use Upstart:

    $ sudo service maxscale restart