MaxScale 21.06 Hintfilter


This filter adds routing hints to a service. The filter has no parameters.

Hint Syntax

Note: If a query has more than one comment only the first comment is processed. Always place any MaxScale related comments first before any other comments that might appear in the query.

Comments and comment types

The client connection will need to have comments enabled. For example the mariadb and mysql command line clients have comments disabled by default and they need to be enabled by passing the --comments or -c option to it. Most, if not all, connectors keep all comments intact in executed queries.

# The --comments flag is needed for the command line client
mariadb --comments -u my-user -psecret -e "SELECT @@hostname -- maxscale route to server db1"

For comment types, use either -- (notice the whitespace after the double hyphen) or # after the semicolon or /* ... */ before the semicolon.

Inline comment blocks, i.e. /* .. */, do not require a whitespace character after the start tag or before the end tag but adding the whitespace is advised.

Hint body

All hints must start with the maxscale tag.

-- maxscale <hint body>

The hints have two types, ones that define a server type and others that contain name-value pairs.

Routing destination hints

These hints will instruct the router to route a query to a certain type of a server.

-- maxscale route to [master | slave | server <server name>]

Route to master

-- maxscale route to master

A master value in a routing hint will route the query to a master server. This can be used to direct read queries to a master server for a up-to-date result with no replication lag.

Route to slave

-- maxscale route to slave

A slave value will route the query to a slave server. Please note that the hints will override any decisions taken by the routers which means that it is possible to force writes to a slave server.

Route to named server

-- maxscale route to server <server name>

A server value will route the query to a named server. The value of <server name> needs to be the same as the server section name in maxscale.cnf. If the server is not used by the service, the hint is ignored.

Route to last used server

-- maxscale route to last

A last value will route the query to the server that processed the last query. This hint can be used to force certain queries to be grouped to the same server.

Name-value hints

-- maxscale <param>=<value>

These control the behavior and affect the routing decisions made by the router. Currently the only accepted parameter is the readwritesplit parameter max_slave_replication_lag. This will route the query to a server with a lower replication lag than this parameter's value.

Hint stack

Hints can be either single-use hints, which makes them affect only one query, or named hints, which can be pushed on and off a stack of active hints.

Defining named hints:

-- maxscale <hint name> prepare <hint content>

Pushing a hint onto the stack:

-- maxscale <hint name> begin

Popping the topmost hint off the stack:

-- maxscale end

You can define and activate a hint in a single command using the following:

-- maxscale <hint name> begin <hint content>

You can also push anonymous hints onto the stack which are only used as long as they are on the stack:

-- maxscale begin <hint content>

Prepared Statements

The hintfilter supports routing hints in prepared statements for both the PREPARE and EXECUTE SQL commands as well as the binary protocol prepared statements.

Binary Protocol

With binary protocol prepared statements, a routing hint in the prepared statement is applied to the execution of the statement but not the preparation of it. The preparation of the statement is routed normally and is sent to all servers.

For example, when the following prepared statement is prepared with the MariaDB Connector-C function mariadb_stmt_prepare and then executed with mariadb_stmt_execute the result is always returned from the master:

SELECT user FROM accounts WHERE id = ? -- maxscale route to master

Support for binary protocol prepared statements was added in MaxScale 6.0 (MXS-2838).

The protocol commands that the routing hints are applied to are:


Support for direct execution of prepared statements was added in MaxScale 6.2.0. For example the MariaDB Connector-C uses direct execution when mariadb_stmt_execute_direct is used.

Text Protocol

Text protocol prepared statements (i.e. the PREPARE and EXECUTE SQL commands) behave differently. If a PREPARE command has a routing hint, it will be routed according to the routing hint. Any subsequent EXECUTE command will not be affected by the routing hint in the PREPARE statement. This means they must have their own routing hints.

The following example is the recommended method of executing text protocol prepared statements with hints:

PREPARE my_ps FROM 'SELECT user FROM accounts WHERE id = ?';
EXECUTE my_ps USING 123; -- maxscale route to master

The PREPARE is routed normally and will be routed to all servers. The EXECUTE will be routed to the master as a result of it having the route to master hint.


Routing SELECT queries to master

In this example, MariaDB MaxScale is configured with the readwritesplit router and the hint filter.



Behind MariaDB MaxScale is a master server and a slave server. If there is replication lag between the master and the slave, read queries sent to the slave might return old data. To guarantee up-to-date data, we can add a routing hint to the query.

INSERT INTO table1 VALUES ("John","Doe",1);
SELECT * from table1; -- maxscale route to master

The first INSERT query will be routed to the master. The following SELECT query would normally be routed to the slave but with the added routing hint it will be routed to the master. This way we can do an INSERT and a SELECT right after it and still get up-to-date data.


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