The delivery mode for events sent to subscribers is controlled by the proxy that the subscriber passes to IceStorm. For example, if the subscriber subscribes with a oneway proxy, events will be forwarded by IceStorm as oneway messages. Subscribers can use the following proxies:
If you combine a twoway proxy with a reliability QoS of
ordered, messages will be forwarded to the subscriber in the order in which they are received. This is guaranteed because IceStorm will wait for a reply from the subscriber for each event before sending the next event.
With twoway delivery, IceStorm is informed of any failure to deliver an event by the Ice run time. For example, IceStorm may not be able to establish a connection to a subscriber, or may receive an
ObjectNotExistException when it forwards an event. Any failure to deliver an event to a subscriber (possibly after a transparent retry by the Ice run time) results in the cancellation of the corresponding subscription.
Each event is sent to the subscriber as a oneway message. If more than one event is ready to be delivered, the events are sent in a single batch. This delivery mode is more efficient than using twoway delivery. However, the subscriber cannot use active connection management without the risk of events being lost. In addition, if something goes wrong with the subscriber, such as the subscriber having destroyed its callback object without unsubscribing, or having subscribed an object with the wrong interface, IceStorm does not notice the failure and will continue to send events to the non-existent subscriber object for as long as it can maintain a connection to the subscriber’s endpoint.
With this delivery mode, IceStorm buffers events from publishers and sends them in batches to the subscriber (see
Section 28.15). This reduces network overhead and is more efficient than oneway delivery. However, as for oneway delivery, the subscriber cannot use active connection management without the risk of losing events. In addition, events can be delivered out of order if the subscriber is multi-threaded. Batch oneway delivery, while providing better throughput, increases latency because events arrive in "bursts". You can control the interval at which batched events are flushed by setting the
IceStorm.Flush.Timeout Property (see
page 1705).
With this delivery mode, events are forwarded as UDP messages, optionally with multicast semantics. Obviously, this means that events can be delivered out of order, can be lost, and can even be duplicated. In addition, IceStorm cannot detect anything about the delivery status of events. This means that if a subscriber disappears without unsubscribing, IceStorm will attempt to forward events to the subscriber indefinitely. If you use datagram delivery, you need to be careful that subscribers unsubscribe before they disappear; otherwise, stale subscriptions can accumulate in IceStorm over time, bogging down the service as it delivers more and more events to no-longer-existent subscribers.
