Q:

Mule 3: Controlling whether a flow is allowed to be executed

My current situation:
I currently have a Mule ESB application with three flows which process messages originating from two different sources, these three flows are tied together using a VM queue.
Flow #1:
Inbound (Endpoint #1) -> (Perform message processing and transformations) -> Outbound (Endpoint #3)
Flow #2:
Inbound (Endpoint #2) ->  (Perform message processing and transformations) -> Outbound (Endpoint #3)
Flow #3
Inbound (Endpoint #3) -> (Perform message processing and transformations, do stuff) -> Outbound
Problem/ Issue:
Now what I want to do is introduce a fourth flow, Flow #4, that gets state information from an inbound endpoint and based off this information be able to prevent Flow #3 from ever being executed/ prevent it from processing its inbound messages.
In other words, what I'd ideally like is to have Flow #4 run at the startup of the ESB application (like all flows seem to automatically do), and based on the state information it gets from its inbound message, prevent/ allow or enable/ disable Flow #3 from ever processing messages from Endpoint #3.  
The following are what I ideally require:
Requirements:

Must be able to accomplish solely through the mule flow XML, no additional POJO/ custom Java objects.
Flow #4 must be performed at startup of the ESB application and only the first inbound message needs to ever be processed.
Ideally, I don't want Flow #3 to have a composite inbound source or have to evaluate every inbound message the state of some global variable.

What's the best way to accomplish what I want to do?
If there is no real good solution, then if I must omit #3 requirement, then what's the best way to accomplish such a global variable that is shared between two independent flows that aren't tied together by some outbound -> inbound endpoint in the XML config?  I've tried using session properties, but they require that the flows be tied together as either subflows or by an endpoint.
Thanks.

A:

Use a global property and a few MEL expressions to make this happen:
<global-property name="gate_open" value="true" />

<flow name="gated-flow">
    <vm:inbound-endpoint path="gated.in" />
    <expression-filter expression="#[app.registry.gate_open]" />
    ...
</flow>

<flow name="gate-controller">
    <vm:inbound-endpoint path="gate.in"  />
    <expression-component>
      app.registry.gate_open = false
    </expression-component>
</flow>

Sending any message to vm://gate.in will close the gate and gated-flow will stop processing the messages it receives.
You can use any protocol you want instead of VM.

