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.
The main limitation of SchemaRouter is that aside from session variable writes and some specific queries, a query can only target one server. This means that queries which depend on results from multiple servers give incorrect results. See Limitations for more information.
From 2.3.0 onwards, SchemaRouter is capable of limited table family sharding.
Table of Contents
- Routing Logic
- Router Parameters
- Table Family Sharding
- Router Options
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
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.
To check how databases and tables map to servers, execute the special query
SHOW SHARDS. The query does not support any modifiers such as
show shards; Database |Server | ---------|-------------| db1.t1 |MyServer1 | db1.t2 |MyServer1 | db2.t1 |MyServer2 |
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.
A PCRE2 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
Table Family Sharding
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, SchemaRouter is capable of routing queries to the right server, depending on which table is being addressed.
As an example, 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
tbl2. The query
SELECT * FROM db.tbl1 will be routed to server1 and the query
SELECT * FROM db.tbl2 will be routed to server2. As in the example queries, the table
names must be qualified with the database names for table-level sharding to work.
Specifically, the query series below is not supported.
USE db; SELECT * FROM tbl1; // May be routed to an incorrect backend if using table sharding.
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
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.
- Cross-database queries (e.g.
SELECT column FROM database1.table UNION select column FROM database2.table) are not properly supported. Such queries are routed either to the first explicit database in the query, the current database in use or to the first available database, depending on which succeeds.
Without a default database, queries without explicit databases that do not modify the session state will be routed to the first available server. This includes queries such as
CREATE DATABASE db1. Such queries should be done directly on the node or the router should be equipped with the hint filter and a routing hint should be used. Queries that modify the session state (e.g.
SET autocommit=1) will be routed to all servers regardless of the default database.
SELECT queries that modify session variables are not supported because uniform results can not be guaranteed. If such a query is executed, the behavior of the router is undefined. To work around this limitation, the query must be executed in separate parts.
If a query targets a database the SchemaRouter has not mapped to a server, the query will be routed to the first available server. This possibly returns an error about database rights instead of a missing database.
Prepared statement support is limited. PREPARE, EXECUTE and DEALLOCATE are routed to the correct backend if the statement is known and only requires one backend server. EXECUTE IMMEADIATE is not supported and is routed to the first available backend and may give wrong results. Similarly, preparing a statement from a variable (e.g.
PREPARE stmt FROM @a) is not supported and may be routed wrong.
SHOW DATABASESis handled by the router instead of routed to a server. The router only answers correctly to the basic version of the query. Any modifiers such as
SHOW TABLESis routed to the server with the current database. If using table-level sharding, the results will be incomplete. Similarly,
SHOW TABLES FROM db1is routed to the server with database
db1, ignoring table sharding. Use
SHOW SHARDSto get results from the router itself.
USE db1is routed to the server with
db1. If the database is divided to multiple servers, only one server will get the command.
Here is a small tutorial on how to set up a sharded database.