Readwritesplit

Readwritesplit

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.

Overview

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.

Configuration

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.

Parameters

max_slave_connections

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 no limit.

max_slave_connections=<max. number, or % of available slaves>

For example, if you have configured MaxScale with one master and three slaves and set 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 master.

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.

max_slave_replication_lag

max_slave_replication_lag specifies how many seconds a slave is allowed to be behind the master. If the lag is bigger than the configured value a slave can't be used for routing.

This feature is disabled by default.

max_slave_replication_lag=<allowed lag in seconds>

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

use_sql_variables_in specifies where should queries, which read session variable, be routed. The syntax for use_sql_variable_in is:

use_sql_variables_in=[master|all]

The default is to use SQL variables in all servers.

When value 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 INSERT statement.

[Splitter-Service]
type=service
router=readwritesplit
servers=dbserv1, dbserv2, dbserv3
user=maxscale
password=96F99AA1315BDC3604B006F427DD9484
disable_sescmd_history=true
master_failure_mode=fail_on_write

connection_keepalive

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.

The parameter value is the interval in seconds between each keepalive ping. A keepalive ping will be sent to a backend server if the connection is idle and it has not been used within n seconds where n is greater than or equal to the value of connection_keepalive. The keepalive pings are only sent when the client executes a query.

This functionality allows the readwritesplit module to keep all backend connections alive even if they are not used. This is a common problem if the backend servers have a low wait_timeout value and the client connections live for a long time or if your workload is extremely read-heavy with writes done at lower intervals than the configured wait_timeout.

master_reconnection

Allow the master server to change mid-session. This feature was introduced in MaxScale 2.3.0 and is disabled by default.

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 changes mid-session, the connection will be closed.

If the master_reconnection parameter is enabled, the master server is allowed to change as long as the session meets the following criteria:

  • The session is already connected to the slave that was chosen to be the new master
  • No transaction is open
  • Autocommit is enabled
  • No LOAD DATA LOCAL INFILE is in progress
  • There are no queries being actively routed to the old master

When master_reconnection is enabled in conjunction with either master_failure_mode=fail_on_write or master_failure_mode=error_on_write, the session can recover from the loss of a master server. This means that when a session starts without a master server and later a slave server that it is connected to is promoted as the master, the session will come out of the read-only mode (described in detail in the master_failure_mode documentation).

slave_selection_criteria

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. LEAST_CURRENT_OPERATIONS.

The option syntax:

slave_selection_criteria=<criteria>

Where <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.

The LEAST_GLOBAL_CONNECTIONS and LEAST_ROUTER_CONNECTIONS use the connections from MariaDB MaxScale to the server, not the amount of connections reported by the server itself.

LEAST_BEHIND_MASTER and 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.

Server Weights and slave_selection_criteria

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.

For LEAST_CURRENT_OPERATIONS, the metric is number of active queries on the candidate server, for LEAST_GLOBAL_CONNECTIONS and 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 master.

Interaction Between slave_selection_criteria and max_slave_connections

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.

  • With slave_selection_criteria=LEAST_GLOBAL_CONNECTIONS and max_slave_connections=1, each session picks one slave and one master

  • With slave_selection_criteria=LEAST_CURRENT_OPERATIONS and max_slave_connections=100%, each session picks one master and as many slaves as possible

  • With slave_selection_criteria=LEAST_CURRENT_OPERATIONS each read is load balanced based on how many queries are active on a particular slave

  • With slave_selection_criteria=LEAST_GLOBAL_CONNECTIONS each read is sent to the slave with the least amount of connections

max_sescmd_history

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.

disable_sescmd_history

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

prune_sescmd_history

This option prunes the session command history when it exceeds the value configured in 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 of 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 application.

master_accept_reads

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.

# Use the master for reads
master_accept_reads=true

strict_multi_stmt

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_sp_calls

Similar to 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 strict_sp_calls.

master_failure_mode

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.

Value Description
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 that in fail_on_write or error_on_write mode, connections are accepted as long as slave servers are available.

Note: If 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 available. If master_reconnection is enabled, the session can recover if one of the slaves is promoted as the master.

retry_failed_reads

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.

delayed_retry

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 only enable delayed_retry when the possibility of duplicate statement execution is an acceptable risk.

delayed_retry_timeout

The number of seconds to wait until an error is returned to the client when delayed_retry is enabled. The default value is 10 seconds.

transaction_replay

Replay interrupted transactions. This parameter was added in MaxScale 2.3.0 and is disabled by default. Enabling this parameter implicitly enables both the delayed_retry and master_reconnection 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.

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. NOW(), @@server_id) cannot be replayed successfully but it will still be attempted.

transaction_replay_max_size

The limit on transaction size for transaction replay in bytes. Any transaction that exceeds this limit will not be replayed. The default size limit is 1 MiB. Read the configuration guide for more details on size type parameters in MaxScale.

optimistic_trx

Enable optimistic transaction execution. This parameter controls whether normal transactions (i.e. START TRANSACTION or BEGIN) are load balanced across slaves. This feature is disabled by default and enabling it implicitly enables transaction_replay, delayed_retry and master_reconnection parameters.

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.

As with 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 optimistic_trx.

causal_reads

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 be set to last_gtid.

A practical example can be given by the following set of SQL commands executed with autocommit=1.

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;

The SET command will synchronize the slave to a certain logical point in the replication stream (see MASTER_GTID_WAIT 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.

causal_reads_timeout

The timeout for the slave synchronization done by causal_reads. The default value is 10 seconds.

Routing hints

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.

Limitations

For a list of readwritesplit limitations, please read the Limitations document.

Legacy Configuration

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.

Examples

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:

  • write statements,
  • all statements within an open transaction,
  • stored procedure calls
  • user-defined function calls
  • DDL statements (DROP|CREATE|ALTER TABLE … etc.)
  • EXECUTE (prepared) statements that modify the database
  • all statements using temporary tables

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.)

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 database queries,
  • read-only queries to system, or user-defined variables,
  • SHOW statements
  • system function calls.

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:

  • SET statements
  • USE<dbname>
  • system/user-defined variable assignments embedded in read-only statements, such as SELECT (@myvar := 5)
  • PREPARE statements
  • QUIT, PING, STMT RESET, CHANGE USER, etc. commands

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 value of max_sescmd_history.

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