ZCML File
subscribers.zcml

<configure
    xmlns="http://namespaces.zope.org/zope"
    >
<subscriber
    handler=".bootstrap.bootStrapSubscriber"
    for="zope.app.appsetup.IDatabaseOpenedEvent"
    />
<subscriber
    handler=".http.notifyVirtualHostChanged"
    for="zope.publisher.interfaces.http.IHTTPVirtualHostChangedEvent"
    />
</configure>