Vm end point in mule pros and cons - mule

Question

I want to use VM endpoints for achieving parallel processing in mule flows. Being a beginner with mule, I am not quite sure about the implications of doing so. I read about private flows in mule 3 , but not sure, if I can replace vm endpoints with private flows in this case and if at all there would be any advantage I can get with that. Can someone, please let me know about the pros and cons of using VM . Here is the example I wanted to use for parallel processing.
<flow name="forkAndJoinFlow">
<http:inbound-endpoint exchange-pattern="request-response" host="localhost" port="81" path="lowestprice" />
<not-filter>
<wildcard-filter pattern="*favicon*" />
</not-filter>
<request-reply>
<all>
<vm:outbound-endpoint path="shop1"/>
<vm:outbound-endpoint path="shop2"/>
</all>
<vm:inbound-endpoint path="response">
<message-properties-transformer>
<add-message-property key="MULE_CORRELATION_GROUP_SIZE" value="2" />
</message-properties-transformer>
<collection-aggregator />
</vm:inbound-endpoint>
</request-reply>
<expression-transformer evaluator="groovy" expression="java.util.Collections.min(payload)" />
<object-to-string-transformer/>
<logger level="WARN" message="#[string:Lowest price: #[payload]]" />
</flow>
<flow name="shop1Flow">
<vm:inbound-endpoint path="shop1"/>
<logger level="INFO" message="SHOP1 Flow..." />
<expression-transformer evaluator="groovy" expression="new java.lang.Double(1000.0 * Math.random()).intValue()" />
<logger level="WARN" message="#[string:Price from shop 1: #[payload]]" />
</flow>
<flow name="shop2Flow">
<vm:inbound-endpoint path="shop2" />
<logger level="INFO" message="SHOP2 Flow..." />
<expression-transformer evaluator="groovy" expression="new java.lang.Double(1000.0 * Math.random()).intValue()" />
<logger level="WARN" message="#[string:Price from shop 2: #[payload]]" />
</flow>

Answer 1

vm:endpoints are one-way and asynchronous, that means you'll not receive a response back from them.
Private flows can be synchronous as well as asynchronous, means they can return a response also if the exchange-pattern is request-response.
Also, in private flow you get all variables and headers etc versus when you push your message in vm or any other JMS, variables and inbound properties are not propagated and outbound properties of the caller flow becomes inbound in called flow

Answer 2

Private flows can be used as a replacement for your VM.
As explained private flows can be Synchronus or Asynchronous based on the processingStrategy of the flow.
I case you use a private flow in asynchronous way to acheive parallel processing, then make sure that response is posted back onto the response VM at the end of each of the private flows.
I have implemented the fork-join pattern of parallel processing using sub-flows as well. Try this.
<set-property propertyName="MULE_CORRELATION_GROUP_SIZE" value="1" />
<all enableCorrelation="IF_NOT_SET">
<async>
<set-property propertyName="MULE_CORRELATION_SEQUENCE" value="1" />
<flow-ref name="parallel-flow1"></flow-ref>
</async>
<async>
<set-property propertyName="MULE_CORRELATION_SEQUENCE" value="2" />
<flow-ref name="parallel-flow2"></flow-ref>
</async>
</all>
And the sub-flows as follows.
<sub-flow name="parallel-flow1">
....
....
<flow-ref name="join-flow" />
</sub-flow>
<sub-flow name="parallel-flow2">
....
....
<flow-ref name="join-flow" />
</sub-flow>
<sub-flow name="join-flow">
<collection-aggregator timeout="10000" failOnTimeout="true" />
<combine-collections-transformer />
....
....
</sub-flow>
You can try this with private flows.
Hope this helps.