Axis2 Clustering Support

Clustering for Stateful Web Services

This is a more complicated use case where your Web Service needs to store state in the context hierarchy. Each Web Service instance (deployed in separate nodes) will need to share state among themselves. Axis2 provides replication to support sharing of state among services.

However, if more than one node tries to update the same state in the context hierarchy, conflicts will arise and the integrity of your data will be compromised. Now your cluster will have inconsistent state. This can be avoided using a locking mechanism. However Axis2 currently does not support it yet.

If this shared state is read more frequently and updated rarely the probability of conflicts decrease. You may use Axis2 in the above use case for Stateful Web Services based on your discretion. However it's important to remember that there can be conflicts.If you have frequent writes it is not advisable to use Axis2 until we introduce locking support

Please note this warning is only applicable to the following use cases.

• Your Service is deployed in Application Scope
• You store information in the ServiceGroupContext (irrespective of your scope)

You may safely use services in "soapsession" scope provided you don't modify (or modify at all) state in ServiceGroupContext frequently. In soap-session the service context is exclusive to the client who owns the session. Therefore only that client can modify state. A conflict might arise if the same client tries to access the same service in two different nodes simultaneously which happens to modify the same state. However this is rare, but might arise due to an error in the load balancer or the client. If you use Sticky sessions, it will ensure that state will be changed in one node only by directing all requests by the same client to the same node. This is the safest way to use Axis2 clustering support for Stateful Web Services to acheive scalability.

   <cluster class="org.apache.axis2.clustering.tribes.TribesClusterManager">
     <contextManager class="org.apache.axis2.clustering.context.DefaultContextManager">
        <listener class="org.apache.axis2.clustering.context.DefaultContextManagerListener"/>
        <replication>
            <defaults>
                <exclude name="local_*"/>
                <exclude name="LOCAL_*"/>
            </defaults>
            <context class="org.apache.axis2.context.ConfigurationContext">
                <exclude name="SequencePropertyBeanMap"/>
                <exclude name="NextMsgBeanMap"/>
                <exclude name="RetransmitterBeanMap"/>
                <exclude name="StorageMapBeanMap"/>
                <exclude name="CreateSequenceBeanMap"/>
                <exclude name="ConfigContextTimeoutInterval"/>
                <exclude name="ContainerManaged"/>
            </context>
            <context class="org.apache.axis2.context.ServiceGroupContext">
                <exclude name="my.sandesha.*"/>
            </context>
            <context class="org.apache.axis2.context.ServiceContext">
                <exclude name="my.sandesha.*"/>
            </context>
        </replication>
     </contextManager>
   </cluster>

/**
 * This Service is responsible for providing the top 5
 * stocks for the day, week or quarter
 */
public class Top5StockService
{
	public String[] getTop5StocksForToday()
	{
		// If cache is null or invalid fetch it from data base
		ConfigurationContext configContext =
            MessageContext.getCurrentMessageContext().getConfigurationContext();
		
		String[]  symbols = (String[])configContext.getProperty(TOP5_TODAY);
		if (!checkValidity(configContext.getProperty(TOP5_TODAY_LOAD_TIME)))
                {
		    symbols = loadFromDatabase(TOP5_TODAY);
                    configContext.setProperty(TOP5_TODAY,symbols);
		    configContext.setProperty(TOP5_TODAY_LOAD_TIME,new java.util.Date()); 	 
                } 
		
		return symbols;
	}
	
	public String[] getTop5StocksForTheWeek()
	{
		 // If cache is null or invalid fetch it from data base
		.............
	}
	
	public String[] getTop5StocksForTheQuarter()
	{
		// If cache is null or invalid fetch it from data base
                ............
	}
}

public class ShoppingCart
{	
	public final static String SHOPPING_CART = "SHOPPING_CART";
	public final static String DISCOUNT = "DISCOUNT";
	
	public void createSession()
	{
		List<Item> cart = new ArrayList<Item>();
		ServiceContext serviceContext =
            MessageContext.getCurrentMessageContext().getServiceContext();
		serviceContext.setProperty(SHOPPING_CART, cart);
	}
	
	public void addItem(Item item)
	{
		ServiceContext serviceContext =
            MessageContext.getCurrentMessageContext().getServiceContext();
		List<Item> cart = (List<Item>)serviceContext.getProperty(SHOPPING_CART);
		cart.add(item);
	}
	
	public void removeItem(Item item)
	{
		ServiceContext serviceContext =
            MessageContext.getCurrentMessageContext().getServiceContext();
		List<Item> cart = (List<Item>)serviceContext.getProperty(SHOPPING_CART);
		cart.remove(item);
	}
	
	public double checkout()
	{
		ServiceContext serviceContext =
            MessageContext.getCurrentMessageContext().getServiceContext();
		List<Item> cart = (List<Item>)serviceContext.getProperty(SHOPPING_CART);
		
		double discount = (Double)serviceContext.getServiceGroupContext().getProperty(DISCOUNT);
		
		double total = 0;
		for (Item i : cart)
		{
			total = total + i.getPrice();
		}
		
		total = total - total * (discount/100);
		
		return total;
	}	
}