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 limitations

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

Transaction Boundary Detection

If a module in MaxScale requires tracking of transaction boundaries but does not require query classification, a custom parser is used to detect them. Currently the only situation in which this parser is used is when a readconnroute service uses the cache filter.

The custom parser detects a subset of the full SQL syntax used to start transactions. This means that more complex statements will not be fully parsed and will cause the transaction state to not match the real state on the database. For example, SET @my_var = (SELECT 1), autocommit = 0 is not parsed by the custom parser and causes the autocommit modification to not be noticed.

XA Transactions

MaxScale will treat statements executed after XA START and before XA END as if they were executed in a normal read-write transaction started with START TRANSACTION. This means that only XA transactions in the ACTIVE state will be routed as transactions and all statements after XA END are routed normally.

XA transactions and normal transactions are mutually exclusive in MariaDB. This means that a START TRANSACTION command will fail if the connection already has an open XA transaction. MaxScale currently only inspects the SQL and deduces the transaction state from that. If a transaction fails to start due to an open XA transaction, the state in MaxScale and in MariaDB can be different and MaxScale will keep routing statements as if they were inside of a transaction. However, as this is an unlikely scenario, usually no action needs to be taken.

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 committed 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

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.

ETL Limitations

The ETL feature in MaxScale always uses the MariaDB Connector/ODBC driver to perform the data loading into MariaDB. The recommended minimum version of the connector is 3.1.18. Older versions of the driver suffer from problems that may manifest as crashes or memory leaks. The driver must be installed on the system in order for the ETL feature to work.

The data loading into MariaDB is done with autocommit, unique_checks and foreign_key_checks disabled inside of a single transaction. This is done to leverage the optimizations done for InnoDB that allows faster insertions into empty tables. When loading data into MariaDB versions 10.5 or older, this can translate into long rollback times in case the ETL operation fails.

ETL Limitations with PostgreSQL as the Source

For ETL operations that migrate data from PostgreSQL, we recommend using the official PostgreSQL ODBC driver. Use of other PostgreSQL ODBC drivers is possible but not recommended: correct configuration of the driver is necessary to prevent the driver from consuming too much memory.

Limitations in Automatic SQL Generation

• Triggers on tables are not migrated automatically.

• Check constraints are defined using the native PostgreSQL syntax. Incompatibilities must be manually fixed.

• All indexes specific to PostgreSQL will be converted into normal indexes in MariaDB.

• The GEOMETRY type is assumed to be the type provided by PostGIS. It is converted into a MariaDB GEOMETRY type and is extracted using ST_AsText.

ETL Limitations with Generic ODBC Targets

It is the responsibility of the end-user to correctly configure the ODBC driver. Some drivers read the whole resultset into memory by default which will result in MaxScale running out of memory

• ETL operations that operate on more than one catalog are not supported.