This filter adds routing hints to a service. The filter has no parameters.
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
mysql command line client has comments disabled by default and they need to be
enabled by passing the
-c option. Most, if not all, connectors keep all
comments intact in executed queries.
For comment types, use either
-- (notice the whitespace after the double
# 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.
All hints must start with the
-- 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>]
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. A
slave value will route the query to a slave
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.
These control the behavior and affect the routing decisions made by the router.
-- maxscale <param>=<value>
Currently the only accepted parameter is
max_slave_replication_lag. This will
route the query to a server with lower replication lag then what is defined in
the hint value.
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>
SELECT queries to master
In this example, MariaDB MaxScale is configured with the readwritesplit router and the hint filter.
[ReadWriteService] type=service router=readwritesplit servers=server1,server2 user=maxuser passwd=maxpwd filters=Hint [Hint] type=filter module=hintfilter
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.