public NodeSubscription getSubscription(String subscriptionID) {
        final NodeSubscription nodeSubscription = subscriptionsByID.get(subscriptionID);
        getLogger().trace("Got subscription by ID '{}': {}", subscriptionID, nodeSubscription);
        return nodeSubscription;
    }
Returns the subscription whose subscription ID matches the specified ID or {@code null}
if none was found. Accessing subscriptions by subscription ID is always possible no matter
if the node allows one or multiple subscriptions for the same entity. Even when users can
only subscribe once to the node a subscription ID is going to be internally created though
never returned to the user.

@param subscriptionID the ID of the subscription.
@return the subscription whose subscription ID matches the specified ID or {@code null}
        if none was found.
xmppserver/src/main/java/org/jivesoftware/openfire/pubsub/Node.java