Wide-Area Forwarding

Client applications can publish and subscribe across network boundaries so persistence stores are not tied to a single location. Administrators define the following:

• Wide-Area Stores: Wide-area stores are accessed by publishers and subscribers in separate networks, connected by a wide-area network (WAN) link.

• Wide-Area Durables: Wide-area durables are accessible across a WAN link. Durables are created within a store at a particular persistence cluster when a subscriber connects to that cluster. If not a wide-area store, the durable exists only at the particular cluster. The difference with wide-area stores is that the durable's interest is propagated throughout the zone. So when a publisher publishes to a wide-area store, the message is retained by any matching durable at any persistence cluster in the zone.

• Forwarding zones: Forwarding zones delineate the scope of wide-area durables. A forwarding zone contains a set of persistence clusters, which cooperate to forward wide-area durables among all the persistence clusters in that zone.

In the following diagram, StoreS is a wide-area store and is assigned to Zone Z of one of several zone types. (Zone types are covered in the Administration guide.)

The FTL server clusters in Zone Z are: Rio, London, and Tokyo. Each cluster implements a StoreS Projection from StoreS. Application programs in Rio, London, and Tokyo can publish and subscribe to all stores in Zone Z. For example, a publisher in the London Cluster publishes a message to the StoreS Projection with Durable B and then the message is made available to Durable A (Rio Cluster) and Durable C (Tokyo Cluster), as long as they are interested subscribers. Even though the Delft Cluster implements StoreS, the cluster does not forward StoreS messages in or out of the Delft Cluster. StoreT is not a wide-area store, so it remains functionally outside Zone Z and local to the Tokyo cluster.

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