Title: Dynamic Datasource
<a name="DynamicDatasource-OpenEJBdynamicdatasource"></a>
# OpenEJB dynamic datasource

<a name="DynamicDatasource-Goal"></a>
## Goal

The openejb dynamic datasource api aims to allow to use multiple data
sources as one.

It can be useful for technical reasons (load balancing for example) or
functionnal reasons (filtering, aggregation, enriching...).

<a name="DynamicDatasource-TheAPI"></a>
## The API

The interface Router (*org.apache.openejb.resource.jdbc.Router*) have
only one method to get the datasource to use:

    Router.getDataSource()

The *org.apache.openejb.resource.jdbc.RoutedDataSource* wraps a classical
data source. It has to be used to declare your datasource.

You can implement all the policy you want in your Router implementation.

A class called *org.apache.openejb.resource.jdbc.AbstractRouter* is
available to ease router development.

<a name="DynamicDatasource-Knownlimitation(s)"></a>
## Known limitation(s)

You have to use the same kind of databases (same version, same
configuration...).

All database have to be created when you use the router. The way to do it
automatically can depend of your JPA provider.

<a name="DynamicDatasource-OpenJPA"></a>
### OpenJPA

OpenJPA initializes its database when the entitymanager is called for the
first time so you need to initialize all your proxied datasource before
using the other one. It can be done using an Init EJB doing a find() on
each proxied datasource.

<a name="DynamicDatasource-Hibernate"></a>
### Hibernate

Hibernate initializes the database when it starts so if you declare a
persistence unit by database all databases will be initialized at the start
up.

<a name="DynamicDatasource-Example"></a>
## Example 

<a name="DynamicDatasource-Thestory(theunittestexample)"></a>
### The story (the unit test example)

You want to use only one datasource in the code but you have a criteria to
set to choose the real database to use between three.

So in your code you want something like:

    public class RoutedEJBBean {
        @PersistenceContext(unitName = "router")
        private EntityManager em;

        // this router is not automatic, we
        // need it to select the database to use
        @Resource(name = "My Router")
        private DeterminedRouter router;

        public void persist(int id, String name, String clientDatasource) {
            router.setDataSource(clientDatasource);
            em.persist(new Person(id, name));
        }
    }


<a name="DynamicDatasource-Therouterimplementation"></a>
## The router implementation

The router will simply manage a map to store proxied datasources and a
field to store the datasource used in the current thread (ThreadLocal).

    public class DeterminedRouter implements Router {
        private String dataSourceNames; // used to store configuration (openejb.xml)
        private String defaultDataSourceName; // defautl data source name
        private Map<String, DataSource> dataSources = null; // proxied data sources
        private ThreadLocal<DataSource> currentDataSource = new ThreadLocal<DataSource>(); // the datasource to use or null

        /**
         * @param datasourceList datasource resource name, separator is a space
         */
        public void setDataSourceNames(String datasourceList) {
            dataSourceNames = datasourceList;
        }

        /**
         * lookup datasource in openejb resources
         */
        private void init() { // looking up datasources declared as proxied
            dataSources = new ConcurrentHashMap<String, DataSource>();
            for (String ds : dataSourceNames.split(" ")) {
                ContainerSystem containerSystem = SystemInstance.get().getComponent(ContainerSystem.class);

                Object o = null;
                Context ctx = containerSystem.getJNDIContext();
                try {
                    o = ctx.lookup("openejb:Resource/" + ds);
                    if (o instanceof DataSource) {
                        dataSources.put(ds, (DataSource) o);
                    }
                } catch (NamingException ignore) {
                }
            }
        }

        /**
         * @return the user selected data source if it is set
         *         or the default one
         *  @throws IllegalArgumentException if the data source is not found
         */
        public DataSource getDataSource() {
            // lazy init of routed datasources
            if (dataSources == null) {
                init();
            }

            // if no datasource is selected use the default one
            if (currentDataSource.get() == null) {
                if (dataSources.containsKey(defaultDataSourceName)) {
                    return dataSources.get(defaultDataSourceName);

                } else {
                    throw new IllegalArgumentException("you have to specify at least one datasource");
                }
            }

            // the developper set the datasource to use
            return currentDataSource.get();
        }

        /**
         *
         * @param datasourceName data source name
         */
        public void setDataSource(String datasourceName) {
            if (dataSources == null) {
                init();
            }
            if (!dataSources.containsKey(datasourceName)) {
                throw new IllegalArgumentException("data source called " + datasourceName + " can't be found.");
            }
            DataSource ds = dataSources.get(datasourceName);
            currentDataSource.set(ds);
        }

        public void setDefaultDataSourceName(String name) {
            this.defaultDataSourceName = name;
        }
    }


<a name="DynamicDatasource-Creationoftheserviceproviderfortherouter"></a>
## Creation of the service provider for the router

To be able to use your router add a file called service-jar.xml under
META-INF/<package>. For example META-INF/org.router.

This file will contain something like:

    <ServiceJar>
      <ServiceProvider id="DeterminedRouter" service="Resource"
    		   type="org.apache.openejb.resource.jdbc.Router" class-name="implementation class">
        Param defaultValue
        ParamWithNoDefaultValue
      </ServiceProvider>
    </ServiceJar>
    
## openejb.xml
    
In the openejb.xml file, you have to declare your dynamic database and in
our example it needs the proxied datasources too:
    
    <Resource id="router" type="<your implementation>" provider="<your provider>">
      Param value
    </Resource>

    <Resource id="route db" type="DataSource" provider="RoutedDataSource">
      Router router
    </Resource>

    <!–- real databases – for our example -->
    <Resource id="db1" type="DataSource">
      JdbcDriver org.hsqldb.jdbcDriver
      JdbcUrl jdbc:hsqldb:mem:db1
      UserName sa
      Password
      JtaManaged true
    </Resource>
    <Resource id="db2" type="DataSource">
      JdbcDriver org.hsqldb.jdbcDriver
      JdbcUrl jdbc:hsqldb:mem:db2
      UserName sa
      Password
      JtaManaged true
    </Resource>
    <Resource id="db3" type="DataSource">
      JdbcDriver org.hsqldb.jdbcDriver
      JdbcUrl jdbc:hsqldb:mem:db3
      UserName sa
      Password
      JtaManaged true
    </Resource>