Limitations and Known Issues within MaxScale

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Limitations and Known Issues within MaxScale

The purpose of this documentation is to provide a central location that will document known issues and limitations within the MaxScale product and the plugins that form part of that product. Since limitations may related to specific plugins or to MaxScale as a whole this document is divided into a number of sections, the purpose of which are to isolate the limitations to the components which illustrate them.

Limitations in the MaxScale core

This section describes the limitations that are common to all configuration of plugins with MaxScale.

Limitations with MySQL Protocol support

Compression is not included in MySQL server handshake

Limitations with Galera Cluster Monitoring

The default master selection is based only on MIN(wsrep_local_index). This can be influenced with the server priority mechanic described in the Galera Monitor manual.

Limitations in the connection router

  • If Master changes (ie. new Master promotion) during current connection the router cannot check the change.

  • Sending of LONGBLOB data is not supported

Limitations in the Read/Write Splitter

Read queries are routed to the master server in the following situations:

  • if they are executed inside an open transaction

  • in case of prepared statement execution

  • statement includes a stored procedure, or an UDF call

  • if there are multiple statements inside one query e.g. INSERT INTO ... ; SELECT LAST_INSERT_ID();

Limitations in multi-statement handling

When a multi-statement query is executed through the readwritesplit router, it will always be routed to the master. With the default configuration, all queries after a multi-statement query will be routed to the master to prevent possible reads of false data.

You can override this behavior with the strict_multi_stmt=false router option. In this mode, the multi-statement queries will still be routed to the master but individual statements are routed normally. If you use multi-statements and you know they don't modify the session state in any relevant way, you can disable this option for better performance.

For more information, read the ReadWriteSplit router documentation.

Galera Cluster variables, such as @@wsrep_node_name, are not resolved by the embedded MariaDB parser. This usually means that the query will be routed to the master.

Limitations in client session handling

Some of the queries that client sends are routed to all backends instead of sending them just to one of server. These queries include USE <db name> and SET autocommit=0 among many others. Readwritesplit sends a copy of these queries to each backend server and forwards the master's reply to the client. Below is a list of MySQL commands which are classified as session commands :

COM_INIT_DB (USE <db name> creates this)

COM_CHANGE_USER

COM_STMT_CLOSE

COM_STMT_SEND_LONG_DATA

COM_STMT_RESET

COM_STMT_PREPARE

COM_QUIT (no response, session is closed)

COM_REFRESH

COM_DEBUG

COM_PING

SQLCOM_CHANGE_DB (USE ... statements)

SQLCOM_DEALLOCATE_PREPARE

SQLCOM_PREPARE

SQLCOM_SET_OPTION

SELECT ..INTO variable|OUTFILE|DUMPFILE

SET autocommit=1|0

There is a possibility for misbehavior; if USE mytable was executed in one of the slaves and it failed, it may be due to replication lag rather than the fact it didn’t exist. Thus the same command may end up with different result among backend servers. The slaves which fail to execute a session command will be dropped from the active list of slaves for this session to guarantee a consistent session state across all the servers that are in use by the session.

The above-mentioned behavior can be partially controller with the use_sql_variables_in configuration parameter.

use_sql_variables_in=[master|all] (default: all)

Server-side session variables are called as SQL variables. If "master" is set, SQL variables are read and written in master only. Autocommit values and prepared statements are routed to all nodes always.

WARNING

If a SELECT query modifies a user variable when the use_sql_variables_in parameter is set to all, it will not be routed and the client will receive an error. A log message is written into the log further explaining the reason for the error. Here is an example use of a SELECT query which modifies a user variable and how MaxScale responds to it.

MySQL [(none)]> set @id=1;
Query OK, 0 rows affected (0.00 sec)

MySQL [(none)]> SELECT @id := @id + 1 FROM test.t1;
ERROR 1064 (42000): Routing query to backend failed. See the error log for further details.

You allow user variable modification in SELECT queries by setting the value of use_sql_variables_in to master. This will route all queries that use user variables to the master.

Examples of session command limitations

If a new database "db" was created and client executes “USE db” and it is routed to a slave before the CREATE DATABASE clause is replicated to all slaves, there is a risk of executing a query in the wrong database. Similarly, if any response that RWSplit sends back to the client differ from that of the master, there is a risk for misbehavior. To prevent this, any failures in session command execution are treated as fatal errors and all connections by the session to that particular slave server will be closed. In addition, the server will not used again for routing for the duration of the session.

The most likely reasons are related to replication lag but it could be possible that a slave fails to execute something because of some non-fatal, temporary failure, while the execution of the same command succeeds in other backends.

  • MaxScale can not manage authentication that uses wildcard matching in hostnames in the mysql.user table of the backend database. The only wildcards that can be used are in IP address entries.

  • MySQL old style passwords are not supported. MySQL versions 4.1 and newer use a new authentication protocol which does not support pre-4.1 style passwords.

  • When users have different passwords based on the host from which they connect MaxScale is unable to determine which password it should use to connect to the backend database. This results in failed connections and unusable usernames in MaxScale.

Schemarouter limitations

The schemarouter router currently has some limitations due to the nature of the sharding implementation and the way the session variables are detected and routed. Here is a list of the current limitations.

  • Cross-database queries (e.g. SELECT column FROM database1.table UNION select column FROM database2.table) are not supported and are routed either to the first explicit database in the query, the current database in use or to the first available database, if none of the previous conditions are met.

  • Without a default database, queries without explicit databases that do not modify the session state will be routed the first available server. This means that, for example when creating a new database, 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 currently 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 hasn't 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.

Dbfwfilter limitations

The Database Firewall filter does not support multi-statements or prepared statements. Using them will result in an error being sent to the client.

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