Managing Performance/Reliability Trade-Offs

A hierarchy of reliability levels exists for P&S products, which tend to trade performance for reliability. The baseline level (often called "reliable delivery") is to publish data no more than once. Thus, subscribers who miss messages due to downtime or a network error might never receive them unless a message cache is running. Last-value and cumulative caches (which store the last piece of data, or an accumulation where appropriate) can help improve reliability without affecting performance. Reliable delivery enhances performance because the publisher doesn't have to waste time listening for complaints from subscribers. It works well for both multicast and broadcast transmission.

The next-highest reliability level uses negative acknowledgments (NAKs) from subscribers to alert the publisher that a message needs to be retransmitted. This means messages are retransmitted only when a subscriber misses one and transmits a NAK asking for a retransmit of the missed message(s). The result is that every subscriber receives every message at least once, and sometimes more than tha t. NAKs work well for multicast transmission because publishers can depend on its subscribers to act as squeaky wheels: If they don't say anything, it's because they're getting everything they need.

For even more reliability, you can move up to what Tibco calls certified delivery, which is avail able in the optional TIB/Rendezvous Pro product (which also supports distributed queueing and includes a fault-tolerant API; transactional messaging is available in a separate product, Enterprise Transaction Express). TIB/Rendezvous certified delivery requires each subscriber to positively acknowledge (ACK) receipt of each message to the publisher, although the messages are published by multicast. Positive acknowledgment is also an option for unicast publishing.

You can get even higher reliability by adding a mechanism that can keep track of all messages, approaching the type of structure provided by message queues (see "Monkey in the Middleware"). Digitally signing or encrypting messages can also add a sense of security to reliable delivery.