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.
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.
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.
max_slave_connections sets the maximum number of slaves a router session
uses at any moment. The default is to use at most 255 slave connections per
client connection. In older versions the default was to use all available slaves
with no limit.
max_slave_connections=<max. number, or % of available slaves>
For example, if you have configured MaxScale with one master and three slaves
max_slave_connections=2, for each client connection a connection to
the master and two slave connections would be opened. The read query load
balancing is then done between these two slaves and writes are sent to the
By tuning this parameter, you can control how dynamic the load balancing is at
the cost of extra created connections. With a lower value of
max_slave_connections, less connections per session are created and the set of
possible slave servers is smaller. With a higher value in
max_slave_connections, more connections are created which requires more
resources but load balancing will almost always give the best single query
response time and performance. Longer sessions are less affected by a high
max_slave_connections as the relative cost of opening a connection is lower.
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 specifies where should queries, which read session
variable, be routed. The syntax for
The default is to use SQL variables in all servers.
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
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
Send keepalive pings to backend servers. This feature was introduced in MaxScale 2.2.0 and is disabled by default.
The parameter value is the interval in seconds between each keepalive ping. A
keepalive ping will be sent to a backend server if the connection is idle and it
has not been used within
n seconds where
n is greater than or equal to the
value of connection_keepalive. The keepalive pings are only sent when the
client executes a query.
This functionality allows the readwritesplit module to keep all backend connections alive even if they are not used. This is a common problem if the backend servers have a low wait_timeout value and the client connections live for a long time.
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
Multiple options can be defined as a comma-separated list of parameter-value pairs.
For example, to set
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.
The option syntax:
<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
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 Weights and
The following formula is used to calculate a score for a server when the
weightby parameter is defined.
score = x / w
x is the absolute value of the chosen metric (queries, connections) and
w is the weight of the server. The value of
w is the relative weight
of the server in relation to all the servers configured for the
service. The server with the highest score that fulfills all other
criteria is chosen as the target server.
Read the configuration guide for a more detailed example on how the weights are calculated.
LEAST_CURRENT_OPERATIONS, the metric is number of active queries on
the candidate server, for
LEAST_ROUTER_CONNECTIONS it is the number of open connections and for
LEAST_BEHIND_MASTER it is the number of seconds a server is behind the
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.
max_slave_connections=1, each session picks one slave and one master
max_slave_connections=100%, each session picks one master and as many slaves as possible
slave_selection_criteria=LEAST_CURRENT_OPERATIONSeach read is load balanced based on how many queries are active on a particular slave
slave_selection_criteria=LEAST_GLOBAL_CONNECTIONSeach read is sent to the slave with the least amount of connections
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.
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 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
This option is disabled by default since MaxScale 2.2.1. In older versions, this option was enabled by default.
When a client executes a multi-statement query, it will be treated as if it were a DML statement and routed to the master. If the option is enabled, all queries after a multi-statement query will be routed to the master to guarantee a consistent session state.
If the feature is disabled, queries are routed normally after a multi-statement query.
Warning: Enable the strict mode only if you know that the clients will send statements that cause inconsistencies in the session state.
# Enable strict multi-statement mode router_options=strict_multi_stmt=true
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
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.
|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.
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.
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.
For a list of readwritesplit limitations, please read the Limitations document.
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 (
ALTER TABLE… etc.)
- 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,
- 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:
- system/user-defined variable assignments embedded in read-only statements, such
SELECT (@myvar := 5)
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.