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. MaxScale 2.3.0 increased this limit to 16777216 characters.

In versions 2.2.12 and earlier, the section names in the configuration files were limited to 49 characters. This limitation was increased to 1023 characters in MaxScale 2.2.13.

Multiple MaxScales on same server

Starting with MaxScale 2.4.0, on systems with Linux kernels 3.9 or newer due to the addition of SO_REUSEPORT support, it is possible for multiple MaxScale instances to listen on the same network port if the directories used by both instances are completely separate and there are no conflicts which can cause unexpected splitting of connections. This will only happen if users explicitly tell MaxScale to ignore the default directories and will not happen in normal use.

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.

MariaDB Default Values

MaxScale assumes that certain configuration parameters in MariaDB are set to their default values. These include but are not limited to:

• autocommit: Autocommit is enabled for all new connections.
• tx_read_only: Transactions use READ WRITE permissions by default.

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/MariaDB Protocol support (MariaDBClient)

• Compression is not included in the server handshake.

• If a KILL [CONNECTION] <ID> statement is executed, MaxScale will intercept it. If the ID matches a MaxScale session ID, it will be closed by sending modified KILL commands of the same type to all backend server to which the session in question is connected to. This results in behavior that is similar to how MariaDB does it. If the KILL CONNECTION USER <user> form is given, all connections with a matching username will be closed instead.

• MariaDB MaxScale does not support KILL QUERY ID <query_id> type statements. If a query by a query ID is to be killed, it needs to be done directly on the backend databases.

• Any KILL commands executed using a prepared statement are ignored by MaxScale. If any are executed, it is highly likely that the wrong connection ends up being killed.

• If a KILL connection kills a session that is connected to a readwritesplit service that has transaction_replay or delayed_retry enabled, it is possible that the query is retried even if the connection is killed. To avoid this, use KILL QUERY instead.

• A KILL on one service can cause a connection from another service to be closed even if it uses a different protocol.

• The change user command (COM_CHANGE_USER) only works with standard authentication.

• If a COM_CHANGE_USER succeeds on MaxScale yet fails on the server the session ends up in an inconsistent state. This can happen if the password of the target user is changed and MaxScale uses old user account data when processing the change user. In such a situation, MaxScale and server will disagree on the current user. This can affect e.g. reconnections.

Authenticator limitations

Limitations in the MySQL authenticator (MariaDBAuth)

• 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.

Filter limitations

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.

Tee filter limitations (tee)

The Tee filter does not support binary protocol prepared statements. The execution of a prepared statements through a service that uses the tee filter is not guaranteed to succeed on the service where the filter branches to as it does on the original service.

This possibility exists due to the fact that the binary protocol prepared statements are identified by a server-generated ID. The ID sent to the client from the main service is not guaranteed to be the same that is sent by the branch service.

Monitor limitations

A server can only be monitored by one monitor. Two or more monitors monitoring the same server is considered an error.

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

Refer to individual router documentation for a list of their limitations.