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.
By default the SchemaRouter assumes that each database and table is only located on one server. If it finds the same database or table on multiple servers, it will close the session with the following error:
ERROR 5000 (DUPDB): Error: duplicate tables found on two different shards.
The exception to this rule are the system tables
sys that are never treated as duplicates.
If duplicate tables are expected, use the
ignore_tables_regex parameter to controls which
duplicate tables are allowed. To disable the duplicate database detection, use
Schemarouter compares table and database names case-insensitively. This means
that the tables
test.T1 are assumed to refer to the same table.
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.
Changes in Version 6
auth_all_serversparameter is no longer automatically enabled by the schemarouter. To retain the old behavior that was present in 2.5, explicitly define
auth_all_servers=truefor all schemarouter services.
- Changes in Version 6
- Routing Logic
- Router Parameters
- Table Family Sharding
- Router Diagnostics
If a command modifies the session state by modifying any session or user variables, the query is routed to all nodes. These statements include
SETstatements as well as any other statements that modify the behavior of the client.
If a client changes the default database after connecting, either with a
USE <db>query or a
COM_INIT_DBcommand, the query is routed to all servers that contain the database. This same logic applies when a client connects with a default database: the default database is set only on servers that actually contain it.
If a query targets one or more tables that the schemarouter has discovered during the database mapping phase, the query is only routed if a server is found that contains all of the tables that the query uses. If no such server is found, the query is routed to the server that was previously used or to the first available backend if none have been used. If a query uses a table but doesn't define the database it is in, it is assumed to be located on the default database of the connection.
If a query uses a table that is unknown to the schemarouter or executes a command that doesn't target a table, the query is routed to a server contains the current active default database. If the connection does not have a default database, the query is routed to the backend that was last used or to the first available backend if none have been used. If the query contains a routing hint that directs it to a server, the query is routed there.
This means that all administrative commands, replication related command as well as certain transaction control statements (XA transaction) are routed to the first available server in certain cases. To avoid problems, use routing hints to direct where these statements should go.
Starting with MaxScale 6.4.5, transaction control commands (
ROLLBACK) are routed to all nodes. Older versions of MaxScale routed the queries to the first available backend. This means that cross-shard transactions are technically possible but, without external synchronization, the transactions are not guaranteed to be globally consistent.
LOAD DATA LOCAL INFILEcommands are routed to the first available server that contains the tables listed in the query.
Custom SQL Commands
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 |
The schemarouter will also intercept the
SHOW DATABASES command and generate
it based on its internal data. This means that newly created databases will not
show up immediately and will only be visible when the cached data has been
The schemarouter maps each of the servers to know where each database and table
is located. As each user has access to a different set of tables and databases,
the result is unique to the username and the set of servers that the service
uses. These results are cached by the schemarouter. The lifetime of the cached
result is controlled by the
When a server needs to be mapped, the schemarouter will route a query to each of the servers using the client's credentials. While this query is being executed, all other sessions that would otherwise share the cached result will wait for the update to complete. This waiting functionality was added in MaxScale 2.4.19, older versions did not wait for existing updates to finish and would perform parallel database mapping queries.
Here is an example configuration of the schemarouter:
[Shard-Router] type=service router=schemarouter servers=server1,server2 user=myuser password=mypwd
The module generates the list of databases based on the servers parameter using the connecting client's credentials. The user and password 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 full table names (e.g. db1.t1) to ignore when checking for duplicate tables. By default no tables are ignored.
A PCRE2 regular expression that is matched against database names when checking for duplicate databases. By default no tables are ignored.
The following configuration ignores duplicate tables in the databases
and all tables starting with "t" in
[Shard-Router] type=service router=schemarouter servers=server1,server2 user=myuser password=mypwd ignore_tables_regex=^db1|^db2|^db3\.t
This parameter has been removed in MaxScale 6.0. It is no longer needed after the fix to MXS-2793 made it possible to correctly store the database location information.
This parameter has been removed in MaxScale 6.0, use ignore_tables instead.
This parameter has been removed in MaxScale 6.0, use ignore_tables_regex instead.
This parameter has been moved to the MaxScale core in MaxScale 6.0.
This parameter has been moved to the MaxScale core in MaxScale 6.0.
Enable database map refreshing mid-session. These are triggered by a failure to
change the database i.e.
USE ... queries. This feature is disabled by default.
Before MaxScale 6.2.0, this parameter did nothing. Starting with the 6.2.0 release of MaxScale this parameter now works again but it is disabled by default to retain the same behavior as in older releases.
The minimum interval between database map refreshes in seconds. The default value is 300 seconds.
The interval is specified as documented here. If no explicit unit is provided, the value is interpreted as seconds in MaxScale 2.4. In subsequent versions a value without a unit may be rejected. Note that since the granularity of the intervaltimeout is seconds, a timeout specified in milliseconds will be rejected, even if the duration is longer than a second.
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.
router_diagnostics output for a schemarouter service contains the
queries: Number of queries executed through this service.
sescmd_percentage: The percentage of queries that were session commands.
longest_sescmd_chain: The largest amount of session commands executed by one client session.
times_sescmd_limit_exceeded: Number of times the session command history limit was exceeded.
longest_session: The longest client session in seconds.
shortest_session: The shortest client session in seconds.
average_session: The average client session duration in seconds.
shard_map_hits: Cache hits for the shard map cache.
shard_map_misses: Cache misses for the shard map cache.
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 that do not use fully qualified table names and which do not modify the session state (e.g.
SELECT * FROM t1) will be routed to the first available server. This includes queries such as explicit transaction commands (
ROLLBACK), all non-table
CREATE SEQUENCE) as well as any
SELECTstatements that do not directly refer to a table.
CREATEcommands 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. For explicit transactions, the recommended way is to use
SET autocommit=0to start a transaction and
SET autocommit=1to commit it, otherwise routing hints are required to correctly route the transaction control commands. MXS-4467 changed the routing of transaction control commands to route them to all servers used by the schemarouter.
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.