This document provides a short overview of the readwritesplit router module and its intended use case scenarios. It also displays all router configuration parameters with their descriptions. A list of current limitations of the module is included and use examples are provided.
- Interaction with servers in Maintenance and Draining state
- Router Diagnostics
- Server Ranks
- Routing hints
- Readwritesplit routing decisions
The readwritesplit router is designed to increase the read-only processing capability of a cluster while maintaining consistency. This is achieved by splitting the query load into read and write queries. Read queries, which do not modify data, are spread across multiple nodes while all write queries will be sent to a single node.
The router is designed to be used with a traditional Master-Slave replication cluster. It automatically detects changes in the master server and will use the current master server of the cluster. With a Galera cluster, one can achieve a resilient setup and easy master failover by using one of the Galera nodes as a Write-Master node, where all write queries are routed, and spreading the read load over all the nodes.
Interaction with servers in
When a server that readwritesplit uses is put into maintenance mode, any ongoing requests are allowed to finish before the connection is closed. If the server that is put into maintenance mode is a master, open transaction are allowed to complete before the connection is closed. Note that this means neither idle session nor long-running transactions will be closed by readwritesplit. To forcefully close the connections, use the following command:
maxctrl set server <server> maintenance --force
If a server is put into the
Draining state while a connection is open, the
connection will be used normally. Whenever a new connection needs to be created,
whether that be due to a network error or when a new session being opened, only
servers that are neither
Drained will be used.
Readwritesplit router-specific settings are specified in the configuration file of MariaDB MaxScale in its specific section. The section can be freely named but the name is used later as a reference in a listener section.
For more details about the standard service parameters, refer to the Configuration Guide.
Starting with 2.3, all router parameters can be configured at runtime. Use
maxctrl alter service to modify them. The changed configuration will only be
taken into use by new sessions.
max_slave_connections sets the maximum number of slaves a router session uses
at any moment. The default is to use at most 255 slave connections per client
connection. In older versions the default was to use all available slaves with
For MaxScale 2.5.12 and newer, the minimum value is 0.
For MaxScale versions 2.5.11 and older, the minimum value is 1. These versions suffer from a bug (MXS-3536) that causes the parameter to accept any values but only function when a value greated than one was given.
Starting with MaxScale 2.5.0, the use of percentage values in
max_slave_connections is deprecated. The support for percentages will be
removed in a future release.
For example, if you have configured MaxScale with one master and three slaves
max_slave_connections=2, for each client connection a connection to
the master and two slave connections would be opened. The read query load
balancing is then done between these two slaves and writes are sent to the
By tuning this parameter, you can control how dynamic the load balancing is at
the cost of extra created connections. With a lower value of
max_slave_connections, less connections per session are created and the set of
possible slave servers is smaller. With a higher value in
max_slave_connections, more connections are created which requires more
resources but load balancing will almost always give the best single query
response time and performance. Longer sessions are less affected by a high
max_slave_connections as the relative cost of opening a connection is lower.
This parameter controls how many slave connections each new session starts
with. The default value is 255 which is the same as the default value of
In contrast to
slave_connections serves as a
soft limit on how many slave connections are created. The number of slave
connections can exceed
slave_connections if the load balancing algorithm
finds an unconnected slave server better than all other slaves.
Setting this parameter to 1 allows faster connection creation and improved
resource usage due to the smaller amount of initial backend
connections. It is recommended to use
slave_connections=1 when the
lifetime of the client connections is short.
Specify how many seconds a slave is allowed to be behind the master. The lag of a slave must be less than the configured value in order for it to be used for routing. If set to 0 (the default value), the feature is disabled.
In MaxScale 2.5.0, the slave lag must be less than
whereas in older versions the slave lag had to be less than or equal to
max_slave_replication_lag. This means that in MaxScale 2.5.0 it is possible to
max_slave_replication_lag=1, that all slaves must be up to date
in order for them to be used for routing.
Note that this feature does not guarantee that writes done on the master are
visible for reads done on the slave. This is mainly due to the method of
replication lag measurement. For a feature that guarantees this, refer to
The lag is specified as documented here. If no explicit unit is provided, the value is interpreted as seconds in MaxScale 2.4. In subsequent versions a value without a unit may be rejected. Note that since the granularity of the lag is seconds, a lag specified in milliseconds will be rejected, even if the duration is longer than a second.
The Readwritesplit-router does not detect the replication lag itself. A monitor such as the MariaDB-monitor for a Master/Slave-cluster is required. This option only affects Master-Slave clusters. Galera clusters do not have a concept of slave lag even if the application of write sets might have lag. When a server is disqualified from routing because of replication lag, a warning is logged. Similarly, when the server has caught up enough to be a valid routing target, another warning is logged. These messages are only logged when a query is being routed and the replication state changes.
use_sql_variables_in specifies where should queries, which read session
variable, be routed. The syntax for
The default is to use SQL variables in all servers.
all is used, queries reading session variables can be routed to any
available slave (depending on selection criteria). Queries modifying session
variables are routed to all backend servers by default, excluding write queries
with embedded session variable modifications, such as:
INSERT INTO test.t1 VALUES (@myid:=@myid+1)
In above-mentioned case the user-defined variable would only be updated in the
master where the query would be routed to due to the
[Splitter-Service] type=service router=readwritesplit servers=dbserv1, dbserv2, dbserv3 user=maxscale password=96F99AA1315BDC3604B006F427DD9484 disable_sescmd_history=true master_failure_mode=fail_on_write
Note: This parameter has been moved into the MaxScale core. For the
current documentation, read the
section in the configuration guide.
Send keepalive pings to backend servers. This feature was introduced in MaxScale 2.2.0. The default value is 300 seconds starting with 2.3.2 and for older versions the feature was disabled by default. This parameter was converted into a service parameter in MaxScale 2.5.0.
Allow the master server to change mid-session. This feature was introduced in
MaxScale 2.3.0 and is disabled by default. This feature requires that
disable_sescmd_history is not used.
When a readwritesplit session starts, it will pick a master server as the current master server of that session. By default, when this master server is lost or changes to another server, the connection will be closed.
master_reconnection is enabled, readwritesplit can sometimes recover a
lost connection to the master server. This largely depends on the value of
master_failure_mode=fail_instantly, the master server is only allowed to
change to another server. This change must happen without a loss of the master
master_failure_mode=fail_on_write, the loss of the master server is no
longer a fatal error: if a replacement master server appears before any write
queries are received, readwritesplit will transparently reconnect to the new
In both cases the change in the master server can only take place if
prune_sescmd_history is enabled or
max_sescmd_history has not yet
been exceeded and the session does not have an open transaction.
The recommended configuration is to use
master_failure_mode=fail_on_write. This provides improved fault tolerance
without any risk to the consistency of the database.
This option controls how the readwritesplit router chooses the slaves it
connects to and how the load balancing is done. The default behavior is to route
read queries to the slave server with the lowest amount of ongoing queries i.e.
The option syntax:
<criteria> is one of the following values.
LEAST_GLOBAL_CONNECTIONS, the slave with least connections from MariaDB MaxScale
LEAST_ROUTER_CONNECTIONS, the slave with least connections from this service
LEAST_BEHIND_MASTER, the slave with smallest replication lag
LEAST_CURRENT_OPERATIONS(default), the slave with least active operations
ADAPTIVE_ROUTING, based on server average response times. See below.
LEAST_ROUTER_CONNECTIONS use the
connections from MariaDB MaxScale to the server, not the amount of connections
reported by the server itself.
ADAPTIVE_ROUTING do not take server weights into account
when choosing a server.
ADAPTIVE_ROUTING Measures average server response times. The server averages
are used as proxies of server load conditions. At selection time the averages
are copied and modified to favor faster servers, while at the same time
guaranteeing at lest some traffic to the slowest servers. The server selection
is probabilistic based on roulette wheel selection.
Starting with MaxScale versions 2.5.29, 6.4.11, 22.08.9, 23.02.5 and 23.08.1,
lowercase versions of the values are also accepted. For example,
slave_selection_criteria=least_current_operations are both accepted as valid
Server Weights and
NOTE: Server Weights have been deprecated in MaxScale 2.3 and will be removed at a later time.
The following formula is used to calculate a score for a server when the
weightby parameter is defined.
score = x / w
x is the absolute value of the chosen metric (queries, connections) and
w is the weight of the server. The value of
w is the relative weight
of the server in relation to all the servers configured for the
service. The server with the highest score that fulfills all other
criteria is chosen as the target server.
Read the configuration guide for a more detailed example on how the weights are calculated.
LEAST_CURRENT_OPERATIONS, the metric is number of active queries on
the candidate server, for
LEAST_ROUTER_CONNECTIONS it is the number of open connections and for
LEAST_BEHIND_MASTER it is the number of seconds a server is behind the
Depending on the value of
max_slave_connections, the slave selection criteria
behave in different ways. Here are a few example cases of how the different
criteria work with different amounts of slave connections.
max_slave_connections=1, each session picks one slave and one master
max_slave_connections=100%, each session picks one master and as many slaves as possible
slave_selection_criteria=LEAST_CURRENT_OPERATIONSeach read is load balanced based on how many queries are active on a particular slave
slave_selection_criteria=LEAST_GLOBAL_CONNECTIONSeach read is sent to the slave with the least amount of connections
max_sescmd_history sets a limit on how many distinct session commands each
session can execute before the session command history is disabled. The default
is 50 session commands starting with MaxScale 2.3.0. In older versions, the
session command history was disabled by default.
# Set a limit on the session command history max_sescmd_history=1500
When a session command is executed for the first time, it is stored in memory. Any subsequent executions of the same command are stored as references to the original command. By storing references instead of copies of the data, the amount of memory used is reduced.
If you have long-running sessions which change the session state often, increase the value of this parameter if server reconnections fail due to disabled session command history.
When a limitation is set, it effectively creates a cap on the session's memory consumption. This might be useful if connection pooling is used and the sessions use large amounts of session commands.
This option disables the session command history. This way no history is stored and if a slave server fails, the router will not try to replace the failed slave. Disabling session command history will allow long-lived connections without causing a constant growth in the memory consumption.
This option is only intended to be enabled if the value of
max_slave_connections is lowered below the default value. This will allow a
failed slave to be replaced with a standby slave server.
In versions 2.0 and older, the session command history is enabled by default. In version 2.1 and 2.2, the session command history is disabled by default. In 2.3 and newer versions, the session command is enabled but it is limited to a default of 50 session commands after which the history is disabled.
# Disable the session command history disable_sescmd_history=true
This option prunes the session command history when it exceeds the value
max_sescmd_history. When this option is enabled, only a set
number of statements are stored in the history. This limits the per-session
memory use while still allowing safe reconnections. This parameter was added in
MaxScale 2.3.4 and is disabled by default.
This parameter is intended to be used with pooled connections that remain in use for a very long time. Most connection pool implementations do not reset the session state and instead re-initialize it with new values. This causes the session command history to grow at roughly a constant rate for the lifetime of the pooled connection.
Each client-side session that uses a pooled connection only executes a finite amount of session commands. By retaining a shorter history that encompasses all session commands the individual clients execute, the session state of a pooled connection can be accurately recreated on another server.
If the session command history pruning is enabled, there is a theoretical possibility that upon server reconnection the session states of the connections are inconsistent. This can only happen if the length of the stored history is shorter than the list of relevant statements that affect the session state. In practice the default value of 50 session commands is a fairly reasonable value and the risk of inconsistent session state is relatively low.
In case the default history length is too short for safe pruning, set the value
max_sescmd_history to the total number of commands that affect the session
state plus a safety margin of 10. The safety margin reserves some extra space
for new commands that might be executed due to changes in the client side
master_accept_reads allows the master server to be used for reads. This is
a useful option to enable if you are using a small number of servers and wish to
use the master for reads as well.
By default, no reads are sent to the master as long as there is a valid slave
server available. If no slaves are available, reads are sent to the master
regardless of the value of
# Use the master for reads master_accept_reads=true
This option is disabled by default since MaxScale 2.2.1. In older versions, this option was enabled by default.
When a client executes a multi-statement query, it will be treated as if it were a DML statement and routed to the master. If the option is enabled, all queries after a multi-statement query will be routed to the master to guarantee a consistent session state.
If the feature is disabled, queries are routed normally after a multi-statement query.
Warning: Enable the strict mode only if you know that the clients will send statements that cause inconsistencies in the session state.
# Enable strict multi-statement mode strict_multi_stmt=true
strict_multi_stmt, this option allows all queries after a CALL
operation on a stored procedure to be routed to the master. This option is
disabled by default and was added in MaxScale 2.1.9.
All warnings and restrictions that apply to
strict_multi_stmt also apply to
This option controls how the failure of a master server is handled. By default, the router will close the client connection as soon as the master is lost.
The following table describes the values for this option and how they treat the loss of a master server.
|fail_instantly||When the failure of the master server is detected, the connection will be closed immediately.|
|fail_on_write||The client connection is closed if a write query is received when no master is available.|
|error_on_write||If no master is available and a write query is received, an error is returned stating that the connection is in read-only mode.|
These also apply to new sessions created after the master has failed. This means
error_on_write mode, connections are accepted as
long as slave servers are available.
When configured with
error_on_write, sessions that are idle
will not be closed even if all backend connections for that session have
failed. This is done in the hopes that before the next query from the idle
session arrives, a reconnection to one of the slaves is made. However, this can
leave idle connections around unless the client application actively closes
them. To prevent this, use the
master_failure_mode is set to
error_on_write and the connection
to the master is lost, by default, clients will not be able to execute write
queries without reconnecting to MariaDB MaxScale once a new master is
master_reconnection is enabled, the
session can recover if one of the slaves is promoted as the master.
This option controls whether autocommit selects are retried in case of failure. This option is enabled by default.
When a simple autocommit select is being executed outside of a transaction and the slave server where the query is being executed fails, readwritesplit can retry the read on a replacement server. This makes the failure of a slave transparent to the client.
Retry queries over a period of time. This parameter takes a boolean value, was added in Maxscale 2.3.0 and is disabled by default.
When this feature is enabled, a failure to route a query due to a connection problem will not immediately result in an error. The routing of the query is delayed until either a valid candidate server is available or the retry timeout is reached. If a candidate server becomes available before the timeout is reached, the query is routed normally and no connection error is returned. If no candidates are found and the timeout is exceeded, the router returns to normal behavior and returns an error.
When combined with the
master_reconnection parameter, failures of writes done
outside of transactions can be hidden from the client connection. This allows a
master to be replaced while a write is in progress.
The delayed query retrying mode in readwritesplit does not do any sort of duplicate write detection. To prevent accidental data duplication, it is highly recommended to tune the monitor timeouts to values that produce accurate results.
Duplicate execution of a statement can occur if the connection to the server is
lost or the server crashes but the server comes back up before the timeout for
the retrying is exceeded. At this point, if the server managed to read the
client's statement, it will be executed. For this reason, it is recommended to
delayed_retry when the possibility of duplicate statement
execution is an acceptable risk.
The duration to wait until an error is returned to the client when
delayed_retry is enabled. The default value is 10 seconds.
The timeout is specified as documented here. If no explicit unit is provided, the value is interpreted as seconds in MaxScale 2.4. In subsequent versions a value without a unit may be rejected. Note that since the granularity of the timeout is seconds, a timeout specified in milliseconds will be rejected, even if the duration is longer than a second.
Replay interrupted transactions. This parameter was added in MaxScale 2.3.0 and
is disabled by default. Enabling this parameter enables both
master_reconnection and sets
overriding any configured values for these parameters.
When the server where the transaction is in progress fails, readwritesplit can migrate the transaction to a replacement server. This can completely hide the failure of a master node without any visible effects to the client.
If no replacement node becomes available before the timeout controlled by
delayed_retry_timeout is exceeded, the client connection is closed.
Please refer to the Transaction Replay Limitations section for a more detailed explanation of what should and should not be done with transaction replay.
The limit on transaction size for transaction replay in bytes. Any transaction
that exceeds this limit will not be replayed. The default value is 1 MiB. This
limit applies at a session level which means that the total peak memory
consumption can be
transaction_replay_max_size times the number of client
The amount of memory needed to store a particular transaction will be slightly larger than the length in bytes of the SQL used in the transaction. If the limit is ever exceeded, a message will be logged at the info level.
Read the configuration guide for more details on size type parameters in MaxScale.
The upper limit on how many times a transaction replay is attempted before giving up. The default value is 5.
A transaction replay failure can happen if the server where the transaction is being replayed fails while the replay is in progress. In practice this parameter controls how many server and network failures a single transaction replay tolerates. If a transaction is replayed successfully, the counter for failed attempts is reset.
Enable automatic retrying of transactions that end up in a deadlock. This parameter was added in MaxScale 2.4.6 and the feature is disabled by default. MaxScale versions from 2.4.0 to 2.4.5 always tried to replay deadlocked transactions.
If this feature is enabled and a transaction returns a deadlock error
SQLSTATE 40001: Deadlock found when trying to get lock; try restarting transaction),
the transaction is automatically retried. If the retrying of the transaction
results in another deadlock error, it is retried until it either succeeds or a
transaction checksum error is encountered.
Enable optimistic transaction execution. This parameter controls whether normal
START TRANSACTION or
BEGIN) are load balanced across
slaves. This feature is disabled by default and enabling it implicitly enables
When this mode is enabled, all transactions are first attempted on slave servers. If the transaction contains no statements that modify data, it is completed on the slave. If the transaction contains statements that modify data, it is rolled back on the slave server and restarted on the master. The rollback is initiated the moment a data modifying statement is intercepted by readwritesplit so only read-only statements are executed on slave servers.
transaction_replay and transactions that are replayed, if the results
returned by the master server are not identical to the ones returned by the
slave up to the point where the first data modifying statement was executed, the
connection is closed. If the execution of ROLLBACK statement on the slave fails,
the connection to that slave is closed.
All limitations that apply to
transaction_replay also apply to
Enable causal reads. This parameter is disabled by default and was introduced in MaxScale 2.3.0.
If a client connection modifies the database and
causal_reads is enabled, any
subsequent reads performed on slave servers will be done in a manner that
prevents replication lag from affecting the results. This only applies to the
modifications done by the client itself.
Note: This feature requires MariaDB 10.2.16 or newer to function. In
addition to this, the
session_track_system_variables parameter must include
last_gtid in its list of tracked system variables.
The possible values for this parameter are:
Read causality is disabled.
Writes are locally visible. Writes are guaranteed to be visible only to the connection that does it. Unrelated modifications done by other connections are not visible. This mode improves read scalability at the cost of latency and reduces the overall load placed on the master server without breaking causality guarantees.
Writes are globally visible. If one connection writes a value, all connections to the same service will see it. In general this mode is slower than the
localmode due to the extra synchronization it has to do. This guarantees global happens-before ordering of reads when all transactions are inside a single GTID domain.This mode gives similar benefits as the
localmode in that it improves read scalability at the cost of latency.
With MaxScale versions 2.5.14 and older, multi-domain use of causal_reads could cause non-causal reads to occur. Starting with MaxScale 2.5.15, this was fixed and all the GTID coordinates are passed alongside all requests which makes multi-domain GTIDs safe to use. However, this does mean that the GTID coordinates will never be reset: if replication is reset and and GTID coordinates go "backwards", readwritesplit will not consider these as being newer than the ones already stored. To reset the stored GTID coordinates in readwritesplit, MaxScale must be restarted.
This mode is similar to the
localmode where it will only affect the connection that does the write but where the
localmode waits for a slave server to catch up, the
fastmode will only use servers that are known to have replicated the write. This means that if no slave has replicated the write, the master where the write was done will be used. The value of
causal_reads_timeoutis ignored in this mode. Currently the replication state is only updated by the mariadbmon monitor whenever the servers are monitored. This means that a smaller
monitor_intervalprovides faster replication state updates and possibly better overall usage of servers.
This mode is the inverse of the
localmode in the sense that it improves read latency at the cost of read scalability while still retaining the causality guarantees for reads. This functionality can also be considered an improved version of the functionality that the CCRFilter module provides.
Before MaxScale 2.5.0, the
causal_reads parameter was a boolean
parameter. False values translated to
none and true values translated to
local. The use of boolean parameters is deprecated but still accepted in
A practical example can be given by the following set of SQL commands executed
INSERT INTO test.t1 (id) VALUES (1); SELECT * FROM test.t1 WHERE id = 1;
As the statements are not executed inside a transaction, from the load balancers point of view, the latter statement can be routed to a slave server. The problem with this is that if the value that was inserted on the master has not yet replicated to the server where the SELECT statement is being performed, it can appear as if the value we just inserted is not there.
By prefixing these types of SELECT statements with a command that guarantees consistent results for the reads, read scalability can be improved without sacrificing consistency.
The set of example SQL above will be translated by MaxScale into the following statements.
INSERT INTO test.t1 (id) VALUES (1); SET @maxscale_secret_variable=( SELECT CASE WHEN MASTER_GTID_WAIT('0-3000-8', 10) = 0 THEN 1 ELSE (SELECT 1 FROM INFORMATION_SCHEMA.ENGINES) END); SELECT * FROM test.t1 WHERE id = 1;
SET command will synchronize the slave to a certain logical point in
the replication stream (see
for more details).
If the slave has not caught up to the master within the configured time, it will be retried on the master. In MaxScale 2.3.0 an error was returned to the client when the slave timed out.
Starting with MaxScale 2.5.17, a failed causal read inside of a read-only
transaction started with
START TRANSACTION READ ONLY will return the following
Error: 1792 SQLSTATE: 25006 Message: Causal read timed out while in a read-only transaction, cannot retry command.
Older versions of MaxScale attempted to retry the command on the current master server which would cause the connection to be closed and a warning to be logged.
Limitations of Causal Reads
This feature does not work with prepared statements. Only SQL statements executed individually (inside a COM_QUERY packet) can be handled by the causal read mechanism.
This feature does not work with Galera or any other non-standard replication mechanisms. As Galera does not update the
gtid_slave_posvariable when events are replicated via the Galera library, the
MASTER_GTID_WAITfunction used by MaxScale to synchronize reads will wait until the timeout. With Galera this is not a serious issue as it, by nature, is a mostly-synchronous replication mechanism.
If the combination of the original SQL statement and the modifications added to it by readwritesplit exceed the maximum packet size (16777213 bytes), the causal read will not be attempted and a non-causal read is done instead.
The timeout for the slave synchronization done by
default value is 10 seconds.
The timeout is specified as documented here. If no explicit unit is provided, the value is interpreted as seconds in MaxScale 2.4. In subsequent versions a value without a unit may be rejected. Note that since the granularity of the timeout is seconds, a timeout specified in milliseconds will be rejected, even if the duration is longer than a second.
Lazy connection creation causes connections to backend servers to be opened only when they are needed. This reduces the load that is placed on the backend servers when the client connections are short. This parameter is a boolean type and is disabled by default.
By default readwritesplit opens as many connections as it can when the session
is first opened. This makes the execution of the first query faster when all
available connections are already created. When
lazy_connect is enabled, this
initial connection creation is skipped. If the client executes only read
queries, no connection to the master is made. If only write queries are made,
only the master connection is used.
router_diagnostics output for a readwritesplit service contains the
queries: Number of queries executed through this service.
route_master: Number of writes routed to master.
route_slave: Number of reads routed to slaves.
route_all: Number of session commands routed to all servers.
rw_transactions: Number of explicit read-write transactions.
ro_transactions: Number of explicit read-only transactions.
replayed_transactions: Number of replayed transactions.
server_query_statistics: Statistics for each configured and used server consisting of the following fields.
id: Name of the server
total: Total number of queries.
read: Total number of reads.
write: Total number of writes.
avg_sess_duration: Average duration of a client session to this server.
avg_sess_active_pct: Average percentage of time client sessions were active. 0% means connections were opened but never used.
avg_selects_per_session: Average number of selects per session.
The general rule with server ranks is that primary servers will be used before secondary servers. Readwritesplit is an exception to this rule. The following rules govern how readwritesplit behaves with servers that have different ranks.
Sessions will use the current master server as long as possible. This means that sessions with a secondary master will not use the primary master as long as the secondary master is available.
All slave connections will use the same rank as the master connection. Any stale connections with a different rank than the master will be discarded.
If no master connection is available and
master_reconnectionis enabled, a connection to the best master is created. If the new master has a different priority than existing connections have, the connections with a different rank will be discarded.
If open connections exist, these will be used for routing. This means that if the master is lost but the session still has slave servers with the same rank, they will remain in use.
If no open connections exist, the servers with the best rank will used.
The readwritesplit router supports routing hints. For a detailed guide on hint syntax and functionality, please read this document.
Note: Routing hints will always have the highest priority when a routing decision is made. This means that it is possible to cause inconsistencies in the session state and the actual data in the database by adding routing hints to DDL/DML statements which are then directed to slave servers. Only use routing hints when you are sure that they can cause no harm.
Examples of the readwritesplit router in use can be found in the Tutorials folder.
Readwritesplit routing decisions
Here is a small explanation which shows what kinds of queries are routed to which type of server.
Routing to Master
Routing to master is important for data consistency and because majority of writes are written to binlog and thus become replicated to slaves.
The following operations are routed to master:
- DML statements (
- DDL statements (
- All statements within an open read-write transaction
- Stored procedure calls
- User-defined function calls
- Statements that use
In addition to these, if the readwritesplit service is configured with the
max_slave_replication_lag parameter, and if all slaves suffer from too much
replication lag, then statements will be routed to the Master. (There might be
other similar configuration parameters in the future which limit the number of
statements that will be routed to slaves.)
Transaction Isolation Level Tracking
Use of the SERIALIZABLE transaction isolation level with readwritesplit is not recommended as it somewhat goes against the goals of load balancing.
session_track_system_variables=tx_isolation is configured for the MariaDB
server, readwritesplit will track the transaction isolation level and lock the
session to the master when the isolation level is set to serializable. This
retains the correctness of the isolation level which can otherwise cause
problems. Once a session is locked to the master, it will not be unlocked. To
reinstate the normal routing behavior, a new connection must be created.
For example, if transaction isolation level tracking cannot be done and an autocommit SELECT is routed to a slave, it no longer behaves in a serializable manner. This can also have an effect on the replication in the slave server.
Routing to Slaves
The ability to route some statements to slaves is important because it also decreases the load targeted to master. Moreover, it is possible to have multiple slaves to share the load in contrast to single master.
Queries which can be routed to slaves must be auto committed and belong to one of the following group:
- Read-only statements (i.e.
SELECT) that only use read-only built-in functions
- All statements within an explicit read-only transaction (
START TRANSACTION READ ONLY)
SHOW MASTER STATUS
The list of supported built-in fuctions can be found here.
Routing to every session backend
A third class of statements includes those which modify session data, such as session system variables, user-defined variables, the default database, etc. We call them session commands, and they must be replicated as they affect the future results of read and write operations. They must be executed on all servers that could execute statements on behalf of this client.
Session commands include for example:
- Commands that modify the session state (
- Text protocol
- Binary protocol prepared statements
- Other miscellaneous commands (COM_QUIT, COM_PING etc.)
NOTE: if variable assignment is embedded in a write statement it is routed
to Master only. For example,
INSERT INTO t1 values(@myvar:=5, 7) would be
routed to Master only.
The router stores all of the executed session commands so that in case of a
slave failure, a replacement slave can be chosen and the session command history
can be repeated on that new slave. This means that the router stores each
executed session command for the duration of the session. Applications that use
long-running sessions might cause MariaDB MaxScale to consume a growing amount
of memory unless the sessions are closed. This can be solved by adjusting the
Routing to previous target
In the following cases, a query is routed to the same server where the previous query was executed. If no previous target is found, the query is routed to the current master.
If a query uses the
FOUND_ROWS()function, it will be routed to the server where the last query was executed. This is done with the assumption that a query with
SQL_CALC_FOUND_ROWSwas previously executed.
COM_STMT_FETCH_ROWS will always be routed to the same server where the COM_STMT_EXECUTE was routed.
Read queries are routed to the master server in the following situations:
- Query is executed inside an open read-write transaction
- 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();
Prepares Statement Limitations
If a prepared statement targets a temporary table on the master, the slave servers will fail to execute it. This will cause all slave connections to be closed (MXS-1816).
Transaction Replay Limitations
If the results from the replacement server are not identical when the
transaction is replayed, the client connection is closed. This means that any
transaction with a server specific result (e.g.
be replayed successfully but it will still be attempted.
If a transaction reads data before updating it, the rows should be locked by
SELECT ... FOR UPDATE. This will prevent overlapping transactions when
multiple transactions are being replayed that modify the same set of rows.
If the connection to the server where the transaction is being executed is
lost when the final
COMMIT is being executed, it is impossible to know
whether the transaction was successfully committed. This means that there
is a possibility for duplicate transaction execution which can result in
data duplication in certain cases. Data duplication can happen if the
transaction consists of the following statement types:
- INSERT of rows into a table that does not have an auto-increment primary key
- A "blind update" of one or more rows e.g.
UPDATE t SET c = c + 1 WHERE id = 123
- A "blind delete" e.g.
DELETE FROM t LIMIT 100
This is not an exhaustive list and any operations that do not check the row contents before performing the operation on them might face this problem.
In all cases the problem of duplicate transaction execution can be avoided by
SELECT ... FOR UPDATE in the statement. This will guarantee that
in the case that the transaction fails when it is being committed, the row is
only modified if it matches the expected contents.
Similarly, a connection loss during
COMMIT can also result in transaction
replay failure. This happens due to the same reason as duplicate transaction
execution but the retried transaction will not be committed. This can be
considered a success case as the transaction replay detected that the results of
the two transactions are different. In these cases readwritesplit will abort the
transaction and close the client connection.
Statements that result in an implicit commit do not reset the transaction when
transaction_replay is enabled. This means that if the transaction is replayed,
the transaction will be committed twice due to the implicit commit being
present. The exception to this are the transaction management statements such as
START TRANSACTION: they are detected and will cause the
transaction to be correctly reset.
Any changes to the session state (e.g. autocommit state, SQL mode) done inside a transaction will remain in effect even if the connection to the server where the transaction is being executed fails. When readwritesplit creates a new connection to a server to replay the transaction, it will first restore the session state by executing all session commands that were executed. This means that if the session state is changed mid-transaction in a way that affects the results, transaction replay will fail.
The following partial transaction demonstrates the problem by using
SQL_MODE inside a transaction.
SET SQL_MODE=''; -- A session command BEGIN; SELECT "hello world"; -- Returns the string "hello world" SET SQL_MODE='ANSI_QUOTES'; -- A session command SELECT 'hello world'; -- Returns the string "hello world"
If this transaction has to be replayed the actual SQL that gets executed is the following.
SET SQL_MODE=''; -- Replayed session command SET SQL_MODE='ANSI_QUOTES'; -- Replayed session command BEGIN; SELECT "hello world"; -- Returns an error SELECT 'hello world'; -- Returns the string "hello world"
First the session state is restored by executing all commands that changed the state after which the actual transaction is replayed. Due to the fact that the SQL_MODE was changed mid-transaction, one of the queries will now return an error instead of the result we expected leading to a transaction replay failure.
In a service-to-service configuration (i.e. a service using another service in
targets list ), if the topmost service starts a transaction, all
lower-level readwritesplit services will also behave as if a transaction is
open. If a connection to a backend database fails during this, it can result in
unnecessary transaction replays which in turn can end up with checksum
conflicts. The recommended approach is to not use any commands inside a
transaction that would be routed to more than one node.
In older versions of MaxScale, routers were configured via the router_options parameter. This functionality was deprecated in 2.2 and was removed in 2.3.
JDBC Batched Statements
Readwritesplit does not support pipelining of JDBC batched statements. This is caused by the fact that readwritesplit executes the statements one at a time to track the state of the response.
Limitations in multi-statement handling
When a multi-statement query is executed through the readwritesplit router, it
will always be routed to the master. See
strict_multi_stmt for more
If the multi-statement query creates a temporary table, it will not be detected and reads to this table can be routed to slave servers. To prevent this, always execute the temporary table creation as an individual statement.
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
Prior to MaxScale 2.3.0, session commands that were 2²⁴ - 1 bytes or longer were not supported and caused the session to be closed.
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. In
addition, the server will not be used again for routing for the duration of the
The above-mentioned behavior for user variables can be partially controlled with
the configuration parameter
use_sql_variables_in=[master|all] (default: all)
If a SELECT query modifies a user variable when the
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
use_sql_variables_in=master. This will route all queries that use user
variables to the master.