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Each subscription keeps a "pointer" into the list denoting the point at which that particular subscription has reached. A particular subscription will only deliver a message if it is the next AVAILABLE entry on the queue after the pointer which it maintains which matches any selection criteria the subscription may have.

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Thread safety is maintained by using the thread-safe atomic compare-and-swap operations for maintaining queue entry state (as described above) and also for updating the pointer on the subscription. The queue is written so that many threads may be simultaneously attempting to perform deliveries simultaneously on the same messages and/or subscriptions.

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    private boolean subscriptionReadyAndHasInterest(final Subscription sub, final QueueEntry entry)
    {
        // We need to move this subscription on, past entries which are already acquired, or deleted or ones it has no
        // interest in.
        QueueEntry node = sub.getLastSeenEntry();
        while (node != null && (node.isAcquired() || node.isDeleted() || !sub.hasInterest(node)))
        {

            QueueEntry newNode = _entries.next(node);
            if (newNode != null)
            {
                sub.setLastSeenEntry(node, newNode);
                node = sub.getLastSeenEntry();
            }
            else
            {
                node = null;
                break;
            }

        }

        if (node == entry)
        {
            // If the first entry that subscription can process is the one we are trying to deliver to it, then we are
            // good
            return true;
        }
        else
        {
            return false;
        }

    } 

Here we see how the subscription is inspected to see where its pointer into the queue (the last seen entry) is in respect to the entry we are trying to deliver. We start from the subscription's current lastSeenEntry and work our way down the list passing over entries which are already acquired by other subscriptions, deleted, or which this subscription has no interest in (e.g. because the node does not meet the subscription's selection criteria); all the while we can update the lastSeenEntry to take it past the entries this subscription has now inspected. Performing this iteration we will eventually arrive at the next entry the subscription is interested in (or just fall off the end of the list). At this point either the next entry that the subscription is interested in is the entry we wish to deliver (success!) or not.