The Ultimate Guide to High Availability with MariaDB
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Limitations and Known Issues within MariaDB MaxScale

Limitations and Known Issues within MariaDB MaxScale

This document lists known issues and limitations in MariaDB MaxScale and its plugins. Since limitations are related to specific plugins, this document is divided into several sections.

Configuration limitations

In versions 2.1.2 and earlier, the configuration files are limited to 1024 characters per line. This limitation was increased to 16384 characters in MaxScale 2.1.3.

Security limitiations

MariaDB 10.2

The parser of MaxScale correctly parses WITH statements, but fails to collect columns, functions and tables used in the SELECT defining the WITH clause.

Consequently, the database firewall will not block WITH statements where the SELECT of the WITH clause refers to forbidden columns.

Query Classification

Follow the MXS-1350 Jira issue to track the progress on this limitation.

XA transactions are not detected as transactions by MaxScale. This means that all XA commands will be treated as unknown commands and will be treated as operations that potentially modify the database (in the case of readwritesplit, the statements are routed to the master).

MaxScale will not track the XA transaction state which means that any SELECT queries done inside an XA transaction can be routed to servers that are not part of the XA transaction.

This limitation can be avoided on the client side by disabling autocommit before any XA transactions are done. The following example shows how a simple XA transaction is done via MaxScale by disabling autocommit for the duration of the XA transaction.

SET autocommit=0;
XA START 'MyXA';
INSERT INTO test.t1 VALUES(1);
XA END 'MyXA';
XA PREPARE 'MyXA';
XA COMMIT 'MyXA';
SET autocommit=1;

Prepared Statements

For its proper functioning, MaxScale needs in general to be aware of the transaction state and autocommit mode. In order to be that, MaxScale parses statements going through it.

However, if a transaction is commited or rolled back, or the autocommit mode is changed using a prepared statement, MaxScale will miss that and its internal state will be incorrect, until the transaction state or autocommit mode is changed using an explicit statement.

For instance, after the following sequence of commands, MaxScale will still think autocommit is on:

set autocommit=1
PREPARE hide_autocommit FROM "set autocommit=0"
EXECUTE hide_autocommit

To ensure that MaxScale functions properly, do not commit or rollback a transaction or change the autocommit mode using a prepared statement.

Protocol limitations

Limitations with MySQL Protocol support (MySQLClient)

Compression is not included in the MySQL server handshake.

Authenticator limitations

Limitations in the GSSAPI authenticator

Currently, MariaDB MaxScale only supports GSSAPI authentication when the backend connections use GSSAPI authentication. Client side GSSAPI authentication with a different backend authentication module is not supported.

Limitations in the MySQL authenticator (MySQLAuth)

  • 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 MariaDB 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 MariaDB MaxScale.

  • Only a subset of netmasks are supported for the Host-column in the mysql.user-table (and related tables). Specifically, if the Host is of the form base_ip/netmask, then the netmask must only contain the numbers 0 or 255. For example, a netmask of 255.255.255.0 is fine while 255.255.255.192 is not.

Filter limitations

Filters are not guaranteed to receive complete MySQL packets if they are used with the readconnroute router. This can be fixed by using the readwritesplit router.

Database Firewall limitations (dbfwfilter)

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

Monitor limitations

A server can only be monitored by one monitor. If multiple monitors monitor the same server, the state of the server is non-deterministic.

Limitations with Galera Cluster Monitoring (galeramon)

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.

Router limitations

Avrorouter limitations (avrorouter)

The avrorouter does not support the following data types, conversions or SQL statements:

The avrorouter does not do any crash recovery. This means that the avro files need to be truncated to valid block lengths before starting the avrorouter.

Binlog Checksums

The avrorouter does not support binlog checksums. They must must not be used in any of the binlogs that the avrorouter will process.

Follow MXS-1341 for progress on this issue.

Limitations in the connection router (readconnroute)

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

Sending of binary data with LOAD DATA LOCAL INFILE is not supported.

Limitations in the Read/Write Splitter (readwritesplit)

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

  • query is executed inside an open transaction
  • query is a prepared statement
  • 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();

JDBC Batched Statements

Readwritesplit does not support execution of JDBC batched statements with non-INSERT statements mixed in it. This is caused by the fact that readwritesplit expects that the protocol is idle before another command is sent.

Most clients conform to this expectation but some JDBC drivers send multiple requests without waiting for the protocol to be idle. If you are using the MariaDB Connector/J, add useBatchMultiSend=false to the JDBC connection string to disable batched statement execution.

Backend write timeout handling

The backend connections opened by readwritesplit will not be kept alive if they aren't used. To keep all of the connections alive, a session command must be periodically executed (for example SET @a = 1;).

If the backend server is configured with a low wait_timeout, it is possible that connections get closed during long sessions. It is recommended to set the wait_timeout to a high value and let MariaDB MaxScale handle the client timeouts. This can be achieved by using the connection_timeout parameter for the service.

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.

Limitations in client session handling

Some of the queries that a client sends are routed to all backends instead of just to one. 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 is executed in one of the slaves and fails, it may be due to replication lag rather than the database not existing. Thus, the same command may produce different result in different 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 used by the session.

The above-mentioned behavior can be partially controlled with the configuration parameter use_sql_variables_in:

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

Server-side session variables are handled similar to 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 MariaDB 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.

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

In a situation where a new database db is created, immediately after which a client executes USE db, it is possible that the command is routed to a slave before the CREATE DATABASE clause is replicated to all slaves. In this case a query may be executed in the wrong database. Similarly, if any response that ReadWriteSplit 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.

The preparation of a prepared statement is routed to all servers. The execution of a prepared statement is routed to the first available server or to the server pointed by a routing hint attached to the query.

Schemarouter limitations (schemarouter)

The schemarouter 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, 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 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.

  • The preparation of a prepared statement is routed to all servers. The execution of a prepared statement is routed to the first available server or to the server pointed by a routing hint attached to the query. In practice this means that prepared statements aren't supported by the schemarouter.

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