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UsingTheReadWriteSplittingPlugin.md

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Read-Write Splitting Plugin

The read-write splitting plugin adds functionality to switch between writer/reader instances via calls to the Connection#setReadOnly method. Upon calling setReadOnly(true), the plugin will establish a connection to a reader instance and direct subsequent queries to this instance. Future calls to setReadOnly will switch between the established writer and reader connections according to the boolean argument you supply to the setReadOnly method.

Loading the Read-Write Splitting Plugin

The read-write splitting plugin is not loaded by default. To load the plugin, include it in the wrapperPlugins connection parameter. If you would like to load the read-write splitting plugin alongside the failover and host monitoring plugins, the read-write splitting plugin must be listed before these plugins in the plugin chain. If it is not, failover exceptions will not be properly processed by the plugin. See the example below to properly load the read-write splitting plugin with these plugins.

final Properties properties = new Properties();
properties.setProperty(PropertyDefinition.PLUGINS.name, "readWriteSplitting,failover,efm");

If you would like to use the read-write splitting plugin without the failover plugin, make sure you have the readWriteSplitting plugin in the wrapperPlugins property, and that the failover plugin is not part of it.

final Properties properties = new Properties();
properties.setProperty(PropertyDefinition.PLUGINS.name, "readWriteSplitting");

The Aurora Host List Plugin is deprecated after version 2.2.3. To use the Read Write Splitting plugin without failover with versions 2.2.3 and earlier, add the Aurora Host List Plugin to the plugin list like so: "auroraHostList,readWriteSplitting".

Supplying the connection string

When using the read-write splitting plugin against Aurora clusters, you do not have to supply multiple instance URLs in the connection string. Instead, supply just the URL for the initial instance to which you're connecting. You must also include either the failover plugin or the Aurora host list plugin in your plugin chain so that the driver knows to query Aurora for its topology. See the section on loading the read-write splitting plugin for more info.

Using the Read-Write Splitting Plugin against non-Aurora clusters

The read-write splitting plugin is not currently supported for non-Aurora clusters.

Internal connection pooling

⚠️ If internal connection pools are enabled, database passwords may not be verified with every connection request. The initial connection request for each database instance in the cluster will verify the password, but subsequent requests may return a cached pool connection without re-verifying the password. This behavior is inherent to the nature of connection pools in general and not a bug with the driver. ConnectionProviderManager.releaseResources can be called to close all pools and remove all cached pool connections. See InternalConnectionPoolPasswordWarning.java for more details.

Whenever setReadOnly(true) is first called on a Connection object, the read-write plugin will internally open a new physical connection to a reader. After this first call, the physical reader connection will be cached for the given Connection. Future calls to setReadOnly on the same Connection object will not require opening a new physical connection. However, calling setReadOnly(true) for the first time on a new Connection object will require the plugin to establish another new physical connection to a reader. If your application frequently calls setReadOnly, you can enable internal connection pooling to improve performance. When enabled, the wrapper driver will maintain an internal connection pool for each instance in the cluster. This allows the read-write plugin to reuse connections that were established by setReadOnly calls on previous Connection objects.

Note: Initial connections to a cluster URL will not be pooled. The driver does not pool cluster URLs because it can be problematic to pool a URL that resolves to different instances over time. The main benefit of internal connection pools is when setReadOnly is called. When setReadOnly is called (regardless of the initial connection URL), an internal pool will be created for the writer/reader that the plugin switches to and connections for that instance can be reused in the future.

The wrapper driver currently uses Hikari to create and maintain its internal connection pools. The sample code here provides a useful example of how to enable this feature. The steps are as follows:

  1. Create an instance of HikariPooledConnectionProvider. The HikariPooledConnectionProvider constructor requires you to pass in a HikariPoolConfigurator function. Inside this function, you should create a HikariConfig, configure any desired properties on it, and return it. Note that the Hikari properties below will be set by default and will override any values you set in your function. This is done to follow desired behavior and ensure that the read-write plugin can internally establish connections to new instances.
  • jdbcUrl (including the host, port, and database)
  • exception override class name
  • username
  • password

You can optionally pass in a HikariPoolMapping function as a second parameter to the HikariPooledConnectionProvider. This allows you to decide when new connection pools should be created by defining what is included in the pool map key. A new pool will be created each time a new connection is requested with a unique key. By default, a new pool will be created for each unique instance-user combination. If you would like to define a different key system, you should pass in a HikariPoolMapping function defining this logic. A simple example is show below. Please see ReadWriteSplittingPostgresExample.java for the full example.

⚠️ If you do not include the username in your HikariPoolMapping function, connection pools may be shared between different users. As a result, an initial connection established with a privileged user may be returned to a connection request with a lower-privilege user without re-verifying credentials. This behavior is inherent to the nature of connection pools in general and not a bug with the driver. ConnectionProviderManager.releaseResources can be called to close all pools and remove all cached pool connections.

props.setProperty("somePropertyValue", "1"); // used in getPoolKey
final HikariPooledConnectionProvider connProvider =
    new HikariPooledConnectionProvider(
        ReadWriteSplittingPostgresExample::getHikariConfig,
        ReadWriteSplittingPostgresExample::getPoolKey
    );
ConnectionProviderManager.setConnectionProvider(connProvider);

private static String getPoolKey(HostSpec hostSpec, Properties props) {
  // Include the URL, user, and somePropertyValue in the connection pool key so that a new
  // connection pool will be opened for each different instance-user-somePropertyValue
  // combination.
  final String user = props.getProperty(PropertyDefinition.USER.name);
  final String somePropertyValue = props.getProperty("somePropertyValue");
  return hostSpec.getUrl() + user + somePropertyValue;
}

  1. Call ConnectionProviderManager.setConnectionProvider, passing in the HikariPooledConnectionProvider you created in step 1.

  2. By default, the read-write plugin randomly selects a reader instance the first time that setReadOnly(true) is called. If you would like the plugin to select a reader based on a different connection strategy, please see the Connection Strategies section for more information.

  3. Continue as normal: create connections and use them as needed.

  4. When you are finished using all connections, call ConnectionProviderManager.releaseResources.

⚠️ Note: You must call ConnectionProviderManager.releaseResources to close the internal connection pools when you are finished using all connections. Unless ConnectionProviderManager.releaseResources is called, the wrapper driver will keep the pools open so that they can be shared between connections.

Example

ReadWriteSplittingPostgresExample.java demonstrates how to enable and configure read-write splitting with the Aws Advanced JDBC Driver.

Connection Strategies

By default, the read-write plugin randomly selects a reader instance the first time that setReadOnly(true) is called. To balance connections to reader instances more evenly, different connection strategies can be used. The following table describes the currently available connection strategies and any relevant configuration parameters for each strategy.

To indicate which connection strategy to use, the readerHostSelectorStrategy configuration parameter can be set to one of the connection strategies in the table below. The following is an example of enabling the least connections strategy:

props.setProperty(ReadWriteSplittingPlugin.READER_HOST_SELECTOR_STRATEGY.name, "leastConnections");
Connection Strategy Configuration Parameter Description Default Value
random This strategy does not have configuration parameters. The random strategy is the default connection strategy. When switching to a reader connection, the reader instance will be chosen randomly from the available database instances. N/A
leastConnections This strategy does not have configuration parameters. The least connections strategy will select reader instances based on which database instance has the least number of currently active connections. Note that this strategy is only available when internal connection pools are enabled - if you set the connection property without enabling internal pools, an exception will be thrown. N/A
roundRobin See the following rows for configuration parameters. The round robin strategy will select a reader instance by taking turns with all available database instances in a cycle. A slight addition to the round robin strategy is the weighted round robin strategy, where more connections will be passed to reader instances based on user specified connection properties. N/A
roundRobinHostWeightPairs This parameter value must be a string type comma separated list of database host-weight pairs in the format <host>:<weight>. The host represents the database instance name, and the weight represents how many connections should be directed to the host in one cycle through all available hosts. For example, the value instance-1:1,instance-2:4 means that for every connection to instance-1, there will be four connections to instance-2.

Note: The <weight> value in the string must be an integer greater than or equal to 1.		 null
roundRobinDefaultWeight This parameter value must be an integer value in the form of a string. This parameter represents the default weight for any hosts that have not been configured with the roundRobinHostWeightPairs parameter. For example, if a connection were already established and host weights were set with roundRobinHostWeightPairs but a new reader node was added to the database, the new reader node would use the default weight.

Note: This value must be an integer greater than or equal to 1.		 1

Limitations

General plugin limitations

When a Statement or ResultSet is created, it is internally bound to the database connection established at that moment. There is no standard JDBC functionality to change the internal connection used by Statement or ResultSet objects. Consequently, even if the read-write plugin switches the internal connection, any Statements/ResultSets created before this will continue using the old database connection. This bypasses the desired functionality provided by the plugin. To prevent these scenarios, an exception will be thrown if your code uses any Statements/ResultSets created before a change in internal connection. To solve this problem, please ensure you create new Statement/ResultSet objects after switching between the writer/reader.

Session state limitations

There are many session state attributes that can change during a session, and many ways to change them. Consequently, the read-write splitting plugin has limited support for transferring session state between connections. The following attributes will be automatically transferred when switching connections:

  • autocommit value
  • transaction isolation level

All other session state attributes will be lost when switching connections between the writer/reader.

If your SQL workflow depends on session state attributes that are not mentioned above, you will need to re-configure those attributes each time that you switch between the writer/reader.