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 clients have comments disabled by default and
they need to be enabled by passing the
-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
# 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>]
Route to primary
-- maxscale route to master
master value in a routing hint will route the query to a primary server. This
can be used to direct read queries to a primary server for a up-to-date result
with no replication lag.
Route to replica
-- maxscale route to slave
slave value will route the query to a replica 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 replica server.
Route to named server
-- maxscale route to server <server name>
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
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
-- 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.
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>
The hintfilter supports routing hints in prepared statements for both the
EXECUTE SQL commands as well as the binary protocol prepared
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
mariadb_stmt_prepare and then executed with
mariadb_stmt_execute the result is always returned from the primary:
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 prepared statements (i.e. the
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
PREPARE is routed normally and will be routed to all servers. The
EXECUTE will be routed to the primary as a result of it having the
SELECT queries to primary
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 password=maxpwd filters=Hint [Hint] type=filter module=hintfilter
Behind MariaDB MaxScale is a primary server and a replica server. If there is replication lag between the primary and the replica, read queries sent to the replica 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 primary. The following SELECT query would normally be routed to the replica but with the added routing hint it will be routed to the primary. This way we can do an INSERT and a SELECT right after it and still get up-to-date data.