# 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 examples of the router's use 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 from listener section.

For more details about the standard service parameters, refer to the [Configuration Guide](../Getting-Started/Configuration-Guide.md).

## 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 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.
Please note max_slave_replication_lag must be greater than 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). Note, that 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 query would be routed due to `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
```

### `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](../Reference/Hint-Syntax.md) 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](../About/Limitations.md) document.

## Examples

Examples of the readwritesplit router in use can be found in the [Tutorials](../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, and
* 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 *Slave*s 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, and
* 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, so 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.