This documentation is for WSO2 Enterprise Service Bus version 4.7.0 . View documentation for the latest release.
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Objective: Demonstrate the session affinity based load balancing with failover capability

<definitions xmlns="http://ws.apache.org/ns/synapse">
    <sequence name="main" onError="errorHandler">
        <in>
            <send>
                <endpoint>
                    <!-- specify the session as the simple client session provided by Synapse for
                    testing purpose -->
                    <session type="simpleClientSession"/>

                    <loadbalance>
                        <endpoint>
                            <failover>
                                <endpoint>
                                    <address uri="http://localhost:9001/services/LBService1">
                                        <enableAddressing/>
                                    </address>
                                </endpoint>
                                <endpoint>
                                    <address uri="http://localhost:9002/services/LBService1">
                                        <enableAddressing/>
                                    </address>
                                </endpoint>
                            </failover>
                        </endpoint>
                        <endpoint>
                            <failover>
                                <endpoint>
                                    <address uri="http://localhost:9003/services/LBService1">
                                        <enableAddressing/>
                                    </address>
                                </endpoint>
                                <endpoint>
                                    <address uri="http://localhost:9004/services/LBService1">
                                        <enableAddressing/>
                                    </address>
                                </endpoint>
                            </failover>
                        </endpoint>
                    </loadbalance>
                </endpoint>
            </send><drop/>
        </in>
        <out>
            <!-- Send the messages where they have been sent (i.e. implicit To EPR) -->
            <send/>
        </out>
    </sequence>
    <sequence name="errorHandler">
        <makefault>
            <code value="tns:Receiver" xmlns:tns="http://www.w3.org/2003/05/soap-envelope"/>
            <reason value="COULDN'T SEND THE MESSAGE TO THE SERVER."/>
        </makefault>
        <header name="To" action="remove"/>
        <property name="RESPONSE" value="true"/>
        <send/>
    </sequence>
</definitions>

Prerequisites:

  • Start ESB with sample configuration 55: i.e. wso2esb-samples.sh -sn 55
  • Deploy the LoadbalanceFailoverService and start four sample Axis2 servers on http ports 9001, 9002, 9003 and 9004 respectively (make sure to specify unconflicting https ports).

This configuration also uses "simpleClientSession" to bind sessions as in the previous sample. But failover endpoints are specified as the child endpoints of the load balance endpoint. Therefore sessions are bound to the failover endpoints. Session information has to be replicated among the servers listed under each failover endpoint using some clustering mechanism. Therefore, if one endpoint bound to a session failed, successive requests for that session will be directed to the next endpoint in that failover group. Run the client using the following command to observe this behaviour.

ant loadbalancefailover -Dmode=session

You can see a client output as shown below.

...
[java] Request: 222 Session number: 0 Response from server: MyServer1
[java] Request: 223 Session number: 0 Response from server: MyServer1
[java] Request: 224 Session number: 1 Response from server: MyServer1
[java] Request: 225 Session number: 2 Response from server: MyServer3
[java] Request: 226 Session number: 0 Response from server: MyServer1
[java] Request: 227 Session number: 1 Response from server: MyServer1
[java] Request: 228 Session number: 2 Response from server: MyServer3
[java] Request: 229 Session number: 1 Response from server: MyServer1
[java] Request: 230 Session number: 1 Response from server: MyServer1
[java] Request: 231 Session number: 2 Response from server: MyServer3
...

Note that session 0 is always directed to MyServer1 and session 1 is directed to MyServer3. No requests are directed to MyServer2 and MyServer4 as they are kept as backups by failover endpoints. Now shutdown the server named MyServer1 while running the sample. You will observe that all successive requests for session 0 is now directed to MyServer2, which is the backup server for MyServer1's group. This is shown below, where MyServer1 was shutdown after the request 534.

 

...
[java] Request: 529 Session number: 2 Response from server: MyServer3
[java] Request: 530 Session number: 1 Response from server: MyServer1
[java] Request: 531 Session number: 0 Response from server: MyServer1
[java] Request: 532 Session number: 1 Response from server: MyServer1
[java] Request: 533 Session number: 1 Response from server: MyServer1
[java] Request: 534 Session number: 1 Response from server: MyServer1
[java] Request: 535 Session number: 0 Response from server: MyServer2
[java] Request: 536 Session number: 0 Response from server: MyServer2
[java] Request: 537 Session number: 0 Response from server: MyServer2
[java] Request: 538 Session number: 2 Response from server: MyServer3
[java] Request: 539 Session number: 0 Response from server: MyServer2
...

 

 

 

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