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Liferay Portal requires storing its data on database systems. It is possible to store custom portlet data in a separate database. But for the core features of Liferay Portal, we need to connect Liferay with a database.
 
The read/ write database
In case of transaction-centric applications, it is a good idea to separate read and write databases. In this situation, all write transactions will be executed on the write database and all read transactions will be executed on the read-only database. Using database replication mechanism, data from the write database is replicated to the read database.
 
Liferay Portal supports configuring read and write databases through portal-ext.properties:
1. add 
spring.configs = < Existing config files >, META-INF/ dynamic-data-source-spring.xml
2. Add the following properties to portal-ext.properties to configure the read database:
jdbc.read.driverClassName =< Read Database Driver Class Name >
jdbc.read.url = < Read Database JDBC URL >
jdbc.read.username = < Read Database User Name >
jdbc.read.password = < Read Database Password >
 
3. Add the following properties to portal-ext.properties to configure the write database:
jdbc.write.driverClassName =< Read Database Driver Class Name >
jdbc.write.url = < Read Database JDBC URL >
jdbc.write.username = < Read Database User Name >
jdbc.write.password = < Read Database Password >
 
Database sharding
Database sharding is the architectural solution to separate the data of same the tables in multiple database instances.
Liferay Portal can be used to host multiple portals within the same portal server using Portal Instances
 
By default, Liferay Portal stores data of all the instances in the same database.
 
Gradually, tables will grow rapidly because of the data from multiple portals. At some point in time, this will affect the performance as tables grow rapidly, and for any request internally the system will need to scan the data of all instances. We can configure multiple database shards (separate databases), and we can provide how shards should be chosen.
 
Each portal instance will be mapped to a specific shard database. By using this architectural approach, data from multiple instances will be distributed in multiple databases. By default, Liferay supports configuring three shards. But we can add more shards by changing configuration files.
 
1. Append the following property in portal-ext.properties to enable database sharding:
spring.configs = < Existing config files >, META-INF/ shard-data-source-spring.xml
2. Configure database shards by adding the following properties in portal-ext.properties:
#Shard 1
jdbc.default.driverClassName =< Database Driver Class Name for shard 1 >
jdbc.default.url = < Database JDBC URL for shard 1 >
jdbc.default.username =< Database User Name for shard 1 >
jdbc.default.password =< Database Password for shard 1 >
#Shard 2
jdbc.one.driverClassName =< Database Driver Class Name for shard 2 >
jdbc.one.url = < Database JDBC URL for shard 2 >
jdbc.one.username = < Database User Name for shard 2 >
jdbc.one.password = < Database Password for shard 2 >
#shard 3
jdbc.two.driverClassName =< Database Driver Class Name for shard 3 >
jdbc.two.url = < Database JDBC URL for shard 3 >
jdbc.two.username = < Database User Name for shard 3 >
jdbc.two.password = < Database Password for shard 3 >
 
If we want to add more than three shards, we will need to provide our own shard-data-source-spring.xml with more than three shards.
 
By default, shards will be assigned to each portal instance based on the round ribbon algorithm. Liferay also supports the manual selection algorithm.
To enable the manual shard selection algorithm, we need to add the following property in portal-ext.properties:
shard.selector = com.liferay.portal.dao.shard.ManualShardSelector