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Documentation/About/Limitations.md
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# Limitations and Known Issues within MaxScale # 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. 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 ## Limitations in the MaxScale core
This section describes the limitations that are common to all configuration of plugins with MaxScale. This section describes the limitations that are common to all
configuration of plugins with MaxScale.
## Limitations with MySQL Protocol support ## Limitations with MySQL Protocol support
@ -12,11 +18,14 @@ Compression is not included in MySQL server handshake
## Limitations with Galera Cluster Monitoring ## 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](../Monitors/Galera-Monitor.md) manual. 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](../Monitors/Galera-Monitor.md) manual.
## Limitations in the connection router ## Limitations in the connection router
* If Master changes (ie. new Master promotion) during current connection the router cannot check the change * If Master changes (ie. new Master promotion) during current connection
the router cannot check the change.
* Sending of LONGBLOB data is not supported * Sending of LONGBLOB data is not supported
@ -30,31 +39,39 @@ Read queries are routed to the master server in the following situations:
* statement includes a stored procedure, or an UDF call * 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();` * if there are multiple statements inside one query e.g.
`INSERT INTO ... ; SELECT LAST_INSERT_ID();`
### Limitations in multi-statement handling ### Limitations in multi-statement handling
When a multi-statement query is executed through the readwritesplit router, it will always When a multi-statement query is executed through the readwritesplit
be routed to the master. With the default configuration, all queries after a router, it will always be routed to the master. With the default
multi-statement query will be routed to the master to prevent possible reads of configuration, all queries after a multi-statement query will be routed to
false data. the master to prevent possible reads of false data.
You can override this behavior with the `strict_multi_stmt=false` You can override this behavior with the `strict_multi_stmt=false` router
router option. In this mode, the multi-statement queries will still be routed option. In this mode, the multi-statement queries will still be routed to
to the master but individual statements are routed normally. If you use 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 multi-statements and you know they don't modify the session state in any
relevant way, you can disable this option for better performance. relevant way, you can disable this option for better performance.
For more information, read the [ReadWriteSplit](../Routers/ReadWriteSplit.md) router documentation. For more information, read the
[ReadWriteSplit](../Routers/ReadWriteSplit.md) router documentation.
### Parsing related limitations ### Parsing related limitations
Galera Cluster variables, such as @@wsrep_node_name, are not resolved by 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. the embedded MariaDB parser. This usually means that the query will be
routed to the master.
### Limitations in client session handling ### 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 : 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_INIT_DB (USE <db name> creates this)
@ -87,18 +104,27 @@ SQLCOM_SET_OPTION
SELECT ..INTO variable|OUTFILE|DUMPFILE SELECT ..INTO variable|OUTFILE|DUMPFILE
SET autocommit=1|0 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. 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. The above-mentioned behavior can be partially controller with the
`use_sql_variables_in` configuration parameter.
``` ```
use_sql_variables_in=[master|all] (default: all) 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. 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** **WARNING**
@ -116,15 +142,26 @@ MySQL [(none)]> SELECT @id := @id + 1 FROM test.t1;
ERROR 1064 (42000): Routing query to backend failed. See the error log for further details. 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 You allow user variable modification in SELECT queries by setting the
`use_sql_variables_in` to `master`. This will route all queries that use user value of `use_sql_variables_in` to `master`. This will route all queries
variables to the master. that use user variables to the master.
#### Examples of session command limitations #### Examples of session command limitations
If new database "db" was created and client executes “USE db” and it is routed to slave before the CREATE DATABASE clause is replicated to all slaves there is a risk of executing query in 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. If new database "db" was created and client executes “USE db” and it is
routed to slave before the CREATE DATABASE clause is replicated to all
slaves there is a risk of executing query in 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.
Most imaginable 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 execution of same command succeeds in other backends. Most imaginable 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 execution of same command succeeds in
other backends.
## Authentication Related Limitations ## Authentication Related Limitations
@ -146,10 +183,11 @@ The schemarouter router currently has some limitations due to the nature of
the sharding implementation and the way the session variables are detected the sharding implementation and the way the session variables are detected
and routed. Here is a list of the current limitations. 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`) * Cross-database queries (e.g.
are not supported and are routed either to the first explicit database in the query, `SELECT column FROM database1.table UNION select column FROM database2.table`)
the current database in use or to the first available database, are not supported and are routed either to the first explicit database
if none of the previous conditions are met. 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 * Without a default database, queries without explicit databases that do not
modify the session state will be routed the first available server. This modify the session state will be routed the first available server. This