Readwritesplit

This document provides a short overview of the readwritesplit router module and its intended use case scenarios. It also displays all router configuration parameters with their descriptions. A list of current limitations of the module is included and use examples are provided.

Overview

The readwritesplit router is designed to increase the read-only processing capability of a cluster while maintaining consistency. This is achieved by splitting the query load into read and write queries. Read queries, which do not modify data, are spread across multiple nodes while all write queries will be sent to a single node.

The router is designed to be used with a traditional Master-Slave replication cluster. It automatically detects changes in the master server and will use the current master server of the cluster. With a Galera cluster, one can achieve a resilient setup and easy master failover by using one of the Galera nodes as a Write-Master node, where all write queries are routed, and spreading the read load over all the nodes.

Configuration

Readwritesplit router-specific settings are specified in the configuration file of MariaDB MaxScale in its specific section. The section can be freely named but the name is used later as a reference in a listener section.

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

Optional parameters

max_slave_connections

max_slave_connections sets the maximum number of slaves a router session uses at any moment. The default is to use all available slaves.

max_slave_connections=<max. number, or % of available slaves>

max_slave_replication_lag

max_slave_replication_lag specifies how many seconds a slave is allowed to be behind the master. If the lag is bigger than the configured value a slave can't be used for routing.

This feature is disabled by default.

max_slave_replication_lag=<allowed lag in seconds>

This applies to Master/Slave replication with MySQL monitor and detect_replication_lag=1 options set. max_slave_replication_lag must be greater than the monitor interval.

This option only affects Master-Slave clusters. Galera clusters do not have a concept of slave lag even if the application of write sets might have lag.

use_sql_variables_in

use_sql_variables_in specifies where should queries, which read session variable, be routed. The syntax for use_sql_variable_in is:

use_sql_variables_in=[master|all]

The default is to use SQL variables in all servers.

When value all is used, queries reading session variables can be routed to any available slave (depending on selection criteria). Queries modifying session variables are routed to all backend servers by default, excluding write queries with embedded session variable modifications, such as:

INSERT INTO test.t1 VALUES (@myid:=@myid+1)

In above-mentioned case the user-defined variable would only be updated in the master where the query would be routed to due to the INSERT statement.

Note: As of version 2.1 of MaxScale, all of the router options can also be defined as parameters. The values defined in router_options will have priority over the parameters.

[Splitter Service]
type=service
router=readwritesplit
servers=dbserv1, dbserv2, dbserv3
user=maxscale
passwd=96F99AA1315BDC3604B006F427DD9484
disable_sescmd_history=true
master_failure_mode=fail_on_write

Router options

router_options may include multiple readwritesplit-specific options. All the options are parameter-value pairs. All parameters listed in this section must be configured as a value in router_options.

Multiple options can be defined as a comma-separated list of parameter-value pairs.

router_options=<option>,<option>

For example, to set slave_selection_criteria and disable_sescmd_history, write

router_options=slave_selection_criteria=LEAST_GLOBAL_CONNECTIONS,disable_sescmd_history=true

slave_selection_criteria

This option controls how the readwritesplit router chooses the slaves it connects to and how the load balancing is done. The default behavior is to route read queries to the slave server with the lowest amount of ongoing queries i.e. LEAST_CURRENT_OPERATIONS.

The option syntax:

router_options=slave_selection_criteria=<criteria>

Where <criteria> is one of the following values.

  • LEAST_GLOBAL_CONNECTIONS, the slave with least connections from MariaDB MaxScale
  • LEAST_ROUTER_CONNECTIONS, the slave with least connections from this service
  • LEAST_BEHIND_MASTER, the slave with smallest replication lag
  • LEAST_CURRENT_OPERATIONS (default), the slave with least active operations

The LEAST_GLOBAL_CONNECTIONS and LEAST_ROUTER_CONNECTIONS use the connections from MariaDB MaxScale to the server, not the amount of connections reported by the server itself.

LEAST_BEHIND_MASTER does not take server weights into account when choosing a server.

Interaction Between slave_selection_criteria and max_slave_connections

Depending on the value of max_slave_connections, the slave selection criteria behave in different ways. Here are a few example cases of how the different criteria work with different amounts of slave connections.

  • With slave_selection_criteria=LEAST_GLOBAL_CONNECTIONS and max_slave_connections=1, each session picks one slave and one master

  • With slave_selection_criteria=LEAST_CURRENT_OPERATIONS and max_slave_connections=100%, each session picks one master and as many slaves as possible

  • With slave_selection_criteria=LEAST_CURRENT_OPERATIONS each read is load balanced based on how many queries are active on a particular slave

  • With slave_selection_criteria=LEAST_GLOBAL_CONNECTIONS each read is sent to the slave with the least amount of connections

max_sescmd_history

max_sescmd_history sets a limit on how many session commands each session can execute before the session command history is disabled. The default is an unlimited number of session commands.

# Set a limit on the session command history
router_options=max_sescmd_history=1500

When a limitation is set, it effectively creates a cap on the session's memory consumption. This might be useful if connection pooling is used and the sessions use large amounts of session commands.

disable_sescmd_history

This option disables the session command history. This way no history is stored and if a slave server fails, the router will not try to replace the failed slave. Disabling session command history will allow long-lived connections without causing a constant growth in the memory consumption.

This option is only intended to be enabled if the value of max_slave_connections is lowered below the default value. This will allow a failed slave to be replaced with a standby slave server.

In versions 2.0 and older, the session command history is enabled by default. Starting with version 2.1, the session command history is disabled by default.

# Disable the session command history
router_options=disable_sescmd_history=true

master_accept_reads

master_accept_reads allows the master server to be used for reads. This is a useful option to enable if you are using a small number of servers and wish to use the master for reads as well.

By default, no reads are sent to the master.

# Use the master for reads
router_options=master_accept_reads=true

strict_multi_stmt

When a client executes a multi-statement query, all queries after that will be routed to the master to guarantee a consistent session state. This behavior can be controlled with the strict_multi_stmt router option. This option is enabled by default.

If set to false, queries are routed normally after a multi-statement query.

Warning: this can cause false data to be read from the slaves if the multi-statement query modifies the session state. Only disable the strict mode if you know that no changes to the session state will be made inside the multi-statement queries.

# Disable strict multi-statement mode
router_options=strict_multi_stmt=false

strict_sp_calls

Similar to strict_multi_stmt, this option allows all queries after a CALL operation on a stored procedure to be routed to the master. This option is disabled by default and was added in MaxScale 2.1.9.

All warnings and restrictions that apply to strict_multi_stmt also apply to strict_sp_calls.

master_failure_mode

This option controls how the failure of a master server is handled. By default, the router will close the client connection as soon as the master is lost.

The following table describes the values for this option and how they treat the loss of a master server.

Value Description
fail_instantly When the failure of the master server is detected, the connection will be closed immediately.
fail_on_write The client connection is closed if a write query is received when no master is available.
error_on_write If no master is available and a write query is received, an error is returned stating that the connection is in read-only mode.

These also apply to new sessions created after the master has failed. This means that in fail_on_write or error_on_write mode, connections are accepted as long as slave servers are available.

Note: If master_failure_mode is set to error_on_write and the connection to the master is lost, clients will not be able to execute write queries without reconnecting to MariaDB MaxScale once a new master is available.

retry_failed_reads

This option controls whether autocommit selects are retried in case of failure. This option is enabled by default.

When a simple autocommit select is being executed outside of a transaction and the slave server where the query is being executed fails, readwritesplit can retry the read on a replacement server. This makes the failure of a slave transparent to the client.

Routing hints

The readwritesplit router supports routing hints. For a detailed guide on hint syntax and functionality, please read this document.

Note: Routing hints will always have the highest priority when a routing decision is made. This means that it is possible to cause inconsistencies in the session state and the actual data in the database by adding routing hints to DDL/DML statements which are then directed to slave servers. Only use routing hints when you are sure that they can cause no harm.

Limitations

For a list of readwritesplit limitations, please read the Limitations document.

Examples

Examples of the readwritesplit router in use can be found in the Tutorials folder.

Readwritesplit routing decisions

Here is a small explanation which shows what kinds of queries are routed to which type of server.

Routing to Master

Routing to master is important for data consistency and because majority of writes are written to binlog and thus become replicated to slaves.

The following operations are routed to master:

  • write statements,
  • all statements within an open transaction,
  • stored procedure calls
  • user-defined function calls
  • DDL statements (DROP|CREATE|ALTER TABLE … etc.)
  • EXECUTE (prepared) statements
  • all statements using temporary tables

In addition to these, if the readwritesplit service is configured with the max_slave_replication_lag parameter, and if all slaves suffer from too much replication lag, then statements will be routed to the Master. (There might be other similar configuration parameters in the future which limit the number of statements that will be routed to slaves.)

Routing to Slaves

The ability to route some statements to slaves is important because it also decreases the load targeted to master. Moreover, it is possible to have multiple slaves to share the load in contrast to single master.

Queries which can be routed to slaves must be auto committed and belong to one of the following group:

  • read-only database queries,
  • read-only queries to system, or user-defined variables,
  • SHOW statements
  • system function calls.

Routing to every session backend

A third class of statements includes those which modify session data, such as session system variables, user-defined variables, the default database, etc. We call them session commands, and they must be replicated as they affect the future results of read and write operations. They must be executed on all servers that could execute statements on behalf of this client.

Session commands include for example:

  • SET statements
  • USE<dbname>
  • system/user-defined variable assignments embedded in read-only statements, such as SELECT (@myvar := 5)
  • PREPARE statements
  • QUIT, PING, STMT RESET, CHANGE USER, etc. commands

NOTE: if variable assignment is embedded in a write statement it is routed to Master only. For example, INSERT INTO t1 values(@myvar:=5, 7) would be routed to Master only.

The router stores all of the executed session commands so that in case of a slave failure, a replacement slave can be chosen and the session command history can be repeated on that new slave. This means that the router stores each executed session command for the duration of the session. Applications that use long-running sessions might cause MariaDB MaxScale to consume a growing amount of memory unless the sessions are closed. This can be solved by setting a connection timeout on the application side.

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