The SchemaRouter provides an easy and manageable sharding solution by building a single logical database server from multiple separate ones. Each database is shown to the client and queries targeting unique databases are routed to their respective servers. In addition to providing simple database-based sharding, the schemarouter also enables cross-node session variable usage by routing all queries that modify the session to all nodes.
From 2.3.0 onwards, the SchemaRouter is capable of table family sharding, in addition to being capable of sharding databases.
- Routing Logic
- Configuration
- Router Parameters
- Table Family Sharding
- Router Options
- Limitations
- Examples
If a command line client is used, i.e. mysql, and a direct connection to
the database is initialized without a default database, the router starts
with no default server where the queries are routed. This means that each
query that doesn't specify a database is routed to the first available
server.
If a USE <database> query is executed or a default database is defined
when connecting to MariaDB MaxScale, all queries without explicitly stated
databases will be routed to the server which has this database. If multiple
servers have the same database and the user connecting to MariaDB MaxScale
has rights to all of them, the database is associated to the first server
that responds when the databases are mapped. In practice this means that
query results will always come from a single server but the data might not
always be from the same node.
In almost all the cases these can be avoided by proper server configuration and the databases are always mapped to the same servers. More on configuration in the next chapter.
Here is an example configuration of the schemarouter:
[Shard-Router]
type=service
router=schemarouter
servers=server1,server2
user=myuser
passwd=mypwd
The module generates the list of databases based on the servers parameter using the connecting client's credentials. The user and passwd parameters define the credentials that are used to fetch the authentication data from the database servers. The credentials used only require the same grants as mentioned in the configuration documentation.
The list of databases is built by sending a SHOW DATABASES query to all the servers. This requires the user to have at least USAGE and SELECT grants on the databases that need be sharded.
If you are connecting directly to a database or have different users on some
of the servers, you need to get the authentication data from all the
servers. You can control this with the auth_all_servers parameter. With
this parameter, MariaDB MaxScale forms a union of all the users and their
grants from all the servers. By default, the schemarouter will fetch the
authentication data from all servers.
For example, if two servers have the database shard and the following
rights are granted only on one server, all queries targeting the database
shard would be routed to the server where the grants were given.
# Execute this on both servers
CREATE USER 'john'@'%' IDENTIFIED BY 'password';
# Execute this only on the server where you want the queries to go
GRANT SELECT,USAGE ON shard.* TO 'john'@'%';
This would in effect allow the user 'john' to only see the database 'shard' on this server. Take notice that these grants are matched against MariaDB MaxScale's hostname instead of the client's hostname. Only user authentication uses the client's hostname and all other grants use MariaDB MaxScale's hostname.
List of databases to ignore when checking for duplicate databases.
Regular expression that is matched against database names when checking for duplicate databases.
The name of a server in MaxScale which will be used as the preferred server when a database is found on more than one server. If a database exists on two servers, of which neither is the server referred by this parameter, the server that replies first will be assigned as the location of the database.
This parameter allows deterministic conflict resolution when a sharded cluster has a central database server and one or more sharded databases spread across multiple servers which replicate from the central database server.
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.
[Shard-Router]
type=service
router=schemarouter
servers=server1,server2
user=myuser
passwd=mypwd
refresh_databases=true
refresh_interval=60
This functionality was introduced in 2.3.0.
If the same database exists on multiple servers, but the database contains different tables in each server, the SchemaRouter is capable of transparently routing queries to the right server, depending on which table is being addressed.
For instance, suppose the database db exists on servers server1 and
server2, but that the database on server1 contains the table tbl1 and
on server2 contains the table tbl2.
In that case, the query
SELECT * FROM db.tbl1
will be routed to server1 and the query
SELECT * FROM db.tbl2
will be routed to server2.
Note: Router options for the Schemarouter were deprecated in MaxScale 2.1.
The following options are options for the router_options parameter of the
service. Multiple router options are given as a comma separated list of key
value pairs.
Set a limit on the number of session modifying commands a session can execute. This sets an effective cap on the memory consumption of the session.
Disable the session command history. This will prevent growing memory consumption of a long-running session and allows pooled connections to MariaDB MaxScale to be used. The drawback of this is the fact that if a server goes down, the session state will not be consistent anymore.
Enable database map refreshing mid-session. These are triggered by a failure to
change the database i.e. USE ... queries.
The minimum interval between database map refreshes in seconds.
For a list of schemarouter limitations, please read the Limitations document.
Here is a small tutorial on how to set up a sharded database.