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CHAPTER 10. RELIABILITY
In general, in a broker based architecture, the reliability feature is strictly related to the "store and
forward" mechanism offered by each broker. Thanks to persistent journals, a broker can offer fault
tolerance thus avoiding message loss; of course, it is not so true when messages are stored only in
a volatile memory.
This is completely different using AMQ Interconnect, because each router neither takes ownership
of messages nor stores them in a persistent storage. In this case, the reliability feature is offered by
path redundancy which provides the possibility to reach the destination on different paths through
the router network. In normal conditions, the best path is always chosen in terms of lowest cost but,
when one or more routers go down, the topology is revisited by all remained routers and new paths
are processed in order to reach always each destination. Of course, it means that the reliability is
strictly related to the network topology the user chooses for his solution.
Because a solution based on AMQ Interconnect could be made not only by routers but by brokers
too, the reliability is improved with persistent storage on them which add not only fault tolerance but
temporal decoupling as well; without "store and forward" feature offered by brokers, the temporal
decoupling is not possible only with routers and direct peers, both senders and receivers; the
receiver must be online at same time of the sender in order to receive messages.
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Suggested rewrite:
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CHAPTER 10. RELIABILITY

In purely broker based architectures reliability features are mainly associated with the "store and forward" mechanisms offered by the brokers. Utilizing persistent journals a broker can provide a form of fault tolerance that avoids message loss; this is not the case when messages are stored only in volatile memory.

The situation is quite different with AMQ Interconnect as routers do not assume ownership
of messages and provide no persistent storage for them.

With AMQ Interconnect reliability is provided by the means of path redundancy which allows messages the option of getting to a destination via different paths through the router network.

Initially the best path based on lowest cost is chosen but if one or more routers become unavailable the topology is re-evaluated by all remaining routers and new paths are calculated in order to maintain valid routes to all destination.

Fundamentally, AMQ Interconnect reliability is strongly influenced by the specific network router topology a user implements.

As solutions based around AMQ Interconnect can include not only routers but also message brokers, you can improve overall reliability and functionality by incorporating some broker specific features.

The ability to avoid message loss can be provided by using a broker's persistent storage mechanism.

In addition to improved reliability brokers can provide temporal decoupling; basically without brokers in the network both senders and receivers must be on-line simultaneously for messages to flow between them, thus the sender and receiver are tightly coupled. By using a broker's “store and forward” mechanism senders can send messages when the receiver is not on-line, the broker can take responsibility for delivering the messages once the receiver is available, this loosens the coupling between the sender and receiver.
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