Readconnroute

This document provides an overview of the readconnroute router module and its intended use case scenarios. It also displays all router configuration parameters with their descriptions.

Overview

The readconnroute router provides simple and lightweight load balancing across a set of servers. The router can also be configured to balance connections based on a weighting parameter defined in the server's section.

Note that *readconnroute balances connections and not statements. When a client connects, the router selects a server based upon the router configuration and current server load, but the single created connection is fixed and will not be changed for the duration of the session. If the connection between MaxScale and the server breaks, the connection cannot be re-established and the session will be closed. The fact that the server is fixed when the client connects also means that routing hints are ignored.

Warning: readconnroute will not prevent writes from being done even if you define router_options=slave. The client application is responsible for making sure that it only performs read-only queries in such cases. readconnroute is simple by design: it selects a server for each client connection and routes all queries there. If something more complex is required, the readwritesplit router is usually the right choice.

Configuration

For more details about the standard service parameters, refer to the Configuration Guide.

router_options

• Type: enum_mask
• Mandatory: No
• Dynamic: Yes
• Values: master, slave, synced, running
• Default: running

router_options can contain a comma separated list of valid server roles. These roles are used as the valid types of servers the router will form connections to when new sessions are created.

Examples:

router_options=slave
router_options=master,slave

Here is a list of all possible values for the router_options.

If no router_options parameter is configured in the service definition, the router will use the default value of running. This means that it will load balance connections across all running servers defined in the servers parameter of the service.

When a connection is being created and the candidate server is being chosen, the list of servers is processed in from first entry to last. This means that if two servers with equal weight and status are found, the one that's listed first in the servers parameter for the service is chosen.

master_accept_reads

• Type: boolean
• Mandatory: No
• Dynamic: Yes
• Default: true

This option can be used to prevent queries from being sent to the current primary. If router_options does not contain master, the readconnroute instance is usually meant for reading. Setting master_accept_reads=false excludes the primary from server selection (and thus from receiving reads).

If router_options contains master, the setting of master_accept_reads has no effect.

By default master_accept_reads=true.

max_replication_lag

• Type: duration
• Mandatory: No
• Dynamic: Yes
• Default: 0s

The maximum acceptable replication lag. The value is in seconds and is specified as documented here. The default value is 0s, which means that the lag is ignored.

The replication lag of a server must be less than the configured value in order for it to be used for routing. To configure the router to not allow any lag, use the smallest duration larger than 0, that is, max_replication_lag=1s.

Examples

The most common use for the readconnroute is to provide either a read or write port for an application. This provides a more lightweight routing solution than the more complex readwritesplit router but requires the application to be able to use distinct write and read ports.

To configure a read-only service that tolerates primary failures, we first need to add a new section into the configuration file.

[Read-Service]
type=service
router=readconnroute
servers=replica1,replica2,replica3
router_options=slave

Here the router_options designates replicas as the only valid server type. With this configuration, the queries are load balanced across the replica servers.

For more complex examples of the readconnroute router, take a look at the examples in the Tutorials folder.

Router Diagnostics

The router_diagnostics output for readconnroute has the following fields.

• queries: Number of queries executed through this service.

Limitations

• Sending of binary data with LOAD DATA LOCAL INFILE is not supported.

• The router will never reconnect to the server it initially connected to.