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 mysql, information_schema,performance_schema, sys that are never treated as duplicates.
If duplicate tables are expected, use the parameter to controls which
duplicate tables are allowed. To disable the duplicate database detection, useignore_tables_regex=.*.
Schemarouter compares table and database names case-insensitively. This means
that the tables test.t1 and 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 for more information.
From 2.3.0 onwards, SchemaRouter is capable of limited table family sharding.
Changes in Version 6
The auth_all_servers parameter is no longer automatically enabled by the
schemarouter. To retain the old behavior that was present in 2.5, explicitly
define auth_all_servers=true for all schemarouter services.
Routing Logic
If a command modifies the session state by modifying any session or user
variables, the query is routed to all nodes. These statements include SET
statements 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_DB command, 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.
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 (BEGIN, COMMIT
and 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 INFILE commands 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 querySHOW SHARDS. The query does not support any modifiers such as LIKE.
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
updated.
Database Mapping
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 refresh_interval parameter.
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.
Configuration
Here is an example configuration of the schemarouter:
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 databaseshard would be routed to the server where the grants were given.
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.
Router Parameters
ignore_tables
Type: stringlist
Mandatory: No
Dynamic: Yes
Default: ""
List of full table names (e.g. db1.t1) to ignore when checking for duplicate
tables. By default no tables are ignored.
This parameter was once called ignore_databases.
ignore_tables_regex
Type:
Mandatory: No
Dynamic: No
Default: ""
A
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 db1 and db2,
and all tables starting with "t" in db3.
This parameter was once called ignore_databases_regex.
max_sescmd_history
This parameter has been moved to
in MaxScale 6.0.
disable_sescmd_history
This parameter has been moved to
in MaxScale 6.0.
refresh_databases
Type:
Mandatory: No
Dynamic: No
Default: false
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.
refresh_interval
Type:
Mandatory: No
Dynamic: Yes
Default: 300s
The minimum interval between database map refreshes in seconds. The default
value is 300 seconds.
The interval is specified as documented . 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
table tbl2. The query SELECT * FROM db.tbl1 will be routed to server1 and the querySELECT * 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.
Router Diagnostics
The router_diagnostics output for a schemarouter service contains the
following fields.
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.
Limitations
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 (BEGIN, COMMIT, ROLLBACK
Examples
is a small tutorial on how
to set up a sharded database.
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 targets a table or a database that is present on all nodes
(e.g. information_schema) and the connection is using a default database,
the query is routed based on the default database. This makes it possible to
control where queries that do match a specifc node are routed. If the
connection is not using a default database, the query is routed based solely
on the tables it contains.
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.
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.
), all non-table
CREATE
commands (
CREATE DATABASE
,
CREATE SEQUENCE
) as well as any
SELECT
statements that do not directly refer to a table.
CREATE
commands 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=0
to start a transaction and
SET autocommit=1
to commit it,
otherwise routing hints are required to correctly route the transaction control
commands.
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 DATABASES is 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 LIKE are
ignored.
SHOW TABLES is routed to the server with the current database. If using table-level
sharding, the results will be incomplete. Similarly, SHOW TABLES FROM db1 is routed to
the server with database db1, ignoring table sharding. Use SHOW SHARDS to get results
from the router itself.
USE db1 is routed to the server with db1. If the database is divided to multiple
servers, only one server will get the command.
# 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'@'%';